1. Plenary conferences

What do we know about algal species diversity at the global scale?”

Line Le Gall (Muséum National d’Histoire Naturelle, Paris, France) 

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Cyanobacterial blooms and cyanotoxins

Annick Méjean (Université Paris Denis-Diderot, France)

Cyanobacteria are photosynthetic micro-organisms of an extraordinary morphological and genetic diversity. Among the eldest prokaryotes on Earth, the oxygen of our modern atmosphere is believed to have originated from cyanobacterial oxygenic photosynthesis. Cyanobacteria colonise aquatic (fresh water, brackish water, or seawater) and terrestrial niches worldwide, and have been described in virtually every possible environment.

Their presence remains most of the time unnoticed by man in water resources. However, blooming episodes occur frequently during spring and summer at the surface of water bodies, more particularly in cases of eutrophisation due to environmental pollution and with increasing global warming.

Cyanobacterial proliferation has an economical impact on wastewater treatment because massive accumulation can damage the recycling systems and may also impede the recreational usage of lakes and ponds. Moreover, the presence of cyanobacteria may also endanger animal and human health when cyanobacteria species involved synthesize toxic molecules. Of various chemical natures, the cyanotoxins target various different organs as a such as the liver, the skin or the nervous system. For humans, intoxications due to cyanobacteria following bathing or water ingestion generally cause symptoms such as diarrhoea, respiratory difficulty, cutaneous or ocular irritations. The major families of cyanotoxins include hepatotoxins (microcystins and nodularins), cytotoxins (cylindrospermopsins), neurotoxins (anatoxins, anatoxin-a(S) or guanitoxin, and saxitoxins) and dermatotoxins.

We will see that a wide variety of cyanobacterial species produces cyanotoxins and are found in different continents of the world making toxic algal bloom as a worldwide phenomenon. The biosynthesis of two major cyanotoxins will be presented and different methods of detection of toxic cyanobacteria and cyanotoxins in environnemental samples will be described.

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Environmental controls over mosses and lichens contribution to the nitrogen cycle in extra-tropical forests.

Romain Darnajoux (Princeton University, NJ, USA)

Mosses and lichens, colloquially referred to as cryptogams, are non-vascular photoautotrophs that live at biotic and abiotic interfaces of vegetation, rocks, and soils to function as the “skin” of the Earth. Their associated microbiomes are estimated to contribute about half of terrestrial new nitrogen (N) input through biological N fixation, as well as 5-10 % of land N2O emissions through undetermined N loss pathways. However, our understanding of the biotic and abiotic controls over these crucial metabolisms is insufficient given the importance of cryptogams’ holobiont for ecosystem succession and resilience, particularly under global change.

In this talk, I will illustrate how cryptogamic covers can be used to investigate the environmental controls over crucial enzymatic activities and their ecosystem functions from organism to global scale. Using hundreds of samples of lichens and mosses collected along large latitudinal gradients in Asia and America, my collaborators and I provide the first large-scale demonstration of the contribution of a previously overlooked enzymatic pathway, the vanadium nitrogenase, and uncover several of its environmental controls by trace metal availability and N demand. I will further document the segregation of biological N fixation and N2O production metabolisms and discuss the role of exogenous N availability in controlling this dichotomization.

Overall, my research highlights the use of lichens and mosses as highly relevant model organisms to study the role and controls of environmental microbiomes at the outmost layer of the critical zone. Moreover, the ability of cryptogam-associated microbiomes to conduct both N input and N removal processes could contribute to safeguarding terrestrial ecosystems from N imbalance and help buffer the heterogeneity in N input from natural and anthropic sources. I will finally discuss the implications of these findings for the contributions of cryptogamic covers to ecosystem services in the pre-industrial era, in the present, and the near future.

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2. Oral presentations

2a. Phycology (Exèdre Dick Annegarn)

How historic material analysis can impact our knowledge on ecological preferences 

Bart Van de Vijver1,2

1 Meise Botanic Garden, Research Department, Nieuwelaan 38, 1860 Meise, Belgium

2 University of Antwerp, Department of Biology, ECOSPHERE, Universiteitsplein 1, 2610 WIlrijk, Belgium

Many of the diatom names we use today are based on research done and descriptions made in the 19th century by famous diatom scientists such as Kützing, Grunow and Van Heurck. Most of their material which formed the basis of this pioneering research, is preserved in major herbaria and museums around the world such as the Meise Botanic Garden, the Natural History Museum in London or the Vienna Naturhistorisches Museum. In many cases the original authors added short written descriptions and, in some cases, often tiny drawings as well. These descriptions and drawings were then interpreted and used by later authors, sometimes strongly deviating from the original idea of the 19th century authors.

As each species also represents its own ecological preferences, it is clear that a good ecological analysis based on diatom species can only be performed when the original taxonomic identity of that species is correct. In this presentation, several examples are discussed of renewed taxonomic research of some well-known species and the consequences that new knowledge may have on ecological work, showing that the updated ideas can have a major impact, leading to clear ecological changes.

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Functional characterization of high-chlorophyll fluorescence 101 (HCF101), a chloroplastic Fe-S cluster transfer protein in the green microalga Chlamydomonas reinhardtii

Antoine Kairis1,2, Anna Caccamo1, Nicolas Rouhier2, Claire Remacle1

1 Genetics and physiology of microalgae, InBios/Phytosystems Research Unit, University of Liege, Belgium
2 Interactions Arbres-Microorganismes, Université de Lorraine-INRAE , France

Iron-sulfur (Fe-S) proteins play a vital role in numerous chloroplastic processes, including photosynthesis and amino acid metabolism. The synthesis, transfer, and incorporation of Fe-S clusters into target proteins are orchestrated by the SUF (sulfur mobilization) machinery, involving approximately 20 proteins. The final step of Fe-S cluster transfer and insertion is executed by various proteins, including high-chlorophyll fluorescence 101 (HCF101), a P-loop NTPase. In Arabidopsis, hcf101 mutants exhibit a significant impact on photosystem I (PSI), consistent with the protein ability to bind a [4Fe-4S] cluster into a monomer in vitro. Three cysteines were initially proposed as essential for cluster binding, but two of them are not conserved in Chlamydomonas. To address this discrepancy, Chlamydomonas HCF101 was expressed as a recombinant protein in Escherichia coli and purified to homogeneity, confirming its ability to bind a [4Fe-4S] cluster but in a dimeric form. This suggests that CrHCF101 more closely resembles homologs such as the Arabidopsis cytosolic NBP35, bacterial MrpORP, or human mitochondrial Ind1.
To gain further insights into HCF101 role in Chlamydomonas, a hcf101 mutant is currently under investigation, alongside a complemented strain. The mutant exhibited reduced maximal quantum yield of PSII in the dark (Fv/Fm), quantum yield of PSII in light-adapted cells (φPSII), and relative electron transfer rate of PSII (rETR). Additionally, the PSI/PSII ratio was decreased, and immunoblotting experiments indicated a significant impact on the abundance of [4Fe-4S] cluster-binding subunits of PSI. Growth measurements revealed that the doubling time was unaffected in minimal medium but significantly impacted in acetate medium, highlighting the photosynthetic deficiency in the hcf101 mutant strain when a carbon source is available. These findings collectively confirm the role of HCF101 in the maturation of [4Fe-4S] subunits of PSI in Chlamydomonas, and emphasize the need of further investigating the Fe-S cluster ligation mode.
AK is fellow research of FNRS

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Drivers of Acclimation in Synechococcus Populations Exposed to Environmental Stress

Arunima Sikder1, Frederik De Laender1.

1 Université de Namur

Understanding the multifaceted consequences of climate change is currently a major challenge in science. Globally, the oceanic ecosystems are undergoing warming temperatures, acidification, along with increased pollution with rising sea levels.
The ecological concept of acclimation provides insights on how organisms adjust to such environmental shifts. Acclimation is the physiological response of an organism via reversible phenotypic changes when temporarily exposed to altered environments. Through this work, we delve deeper into the ecological dynamics that occur during acclimation: what eco-environmental factors drive such phenotypic changes and how?
We test this hypothesis on an important primary producer of the oceans: the pico-phytoplankton Synechococcus spp. Populations of this plankton in the oceans support the higher trophic levels via nutrient cycling, carbon fixation, oxygen production; thereby maintaining ecosystem balance. Understanding its population ecology, especially in the backdrop of relevant environmental issues serves as a bottom-up approach for the comprehensive assessment of climate change on oceans.
We performed microcosm experiments with this versatile model organism by acclimating it to environmental stress (high temperature and pollution) vs. control conditions for a period of ten days. To factor-in density effects, each acclimating culture had two setups - density controlled growth and uncontrolled growth. Population densities and a set of seven phenotypic traits were tracked via flow-cytometry every 24 hours.
Statistical analyses show that environmental stress primarily alters growth rates and density dependence in Synechococcus spp. populations under stress. The altered population densities then drive phenotypic shifts (more strongly for cell dimensions than for pigment content), enabling the populations to acclimate.
Previous research has not considered the potential effects of density during acclimation. We reveal that phenotypic shifts during acclimation don’t result directly from environmental stress, rather, from an interactive complex of the environment and density related factors.

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The bacterioplankton community composition and a host genotype dependent occurrence of taxa shape the Daphnia magna gut bacterial community.

Martijn Callens1, Luc De Meester2,3, Koenraad Muylaert1, Shinjini Mukherjee2, Ellen Decaestecker1

1 Aquatic Biology, KU Leuven; 2 Freshwater Ecology, Evolution and Conservation, KU Leuven; 3 IGB Berlin

The assembly of host-associated bacterial communities is influenced by a multitude of biotic and abiotic factors. It is essential to gain insight in the impact and relative strength of these factors if we want to be able to predict the effects of environmental change on the assembly of host-associated bacterial communities, or deliberately modify them. The environmental pool of bacteria, from which the host is colonized, and the genetic background of the host are both considered to be important in determining the composition of host-associated bacterial communities. We experimentally assessed the relative importance of these two factors and their interaction on the composition of Daphnia magna gut bacterial communities. Bacterioplankton originating from natural ponds or a laboratory culture were used to inoculate germ-free Daphnia of different genotypes. We found that the composition of the environmental bacterial community has a major influence on the Daphnia gut bacterial community, both reflected by the presence or absence of specific taxa as well as by a correlation between abundances in the environment and on the host. Our data also indicate a consistent effect of host genotype on the occurrence of specific bacterial taxa in the gut of Daphnia over different environments.

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Nitrate assimilation in the coral-algal symbiosis : understanding the patterns of expression of the enzyme nitrate reductase in Symbiodiniaceae.

Chloé Stévenne1, Jean-Christophe Plumier1, Stéphane Roberty1

1 University of Liège

Coral holobionts consist of complex associations between a coral host, microalgae (Symbiodiniaceae) and a microbiome. The oligotrophic nature of coral reef environments implies that corals rely heavily on the nutrient exchanges with their photosynthetic endosymbionts to function. In addition to providing translocated photosynthates, the algae account for most of the uptake of dissolved inorganic nitrogen. Coral hosts are unable to reduce nitrate as they lack the necessary enzymes, whereas Symbiodiniaceae have been shown to express the enzyme nitrate reductase (NR). However, the evidence supporting the active reduction of nitrate by the symbiotic algae during symbiosis is scarce and equivocal. Rigorous studies on nitrate assimilation by Symbiodiniaceae are lacking yet are essential for understanding coral holobiont functioning. Using western-blotting and qRT-PCR, we studied the expression of the enzyme nitrate reductase in free-living Symbiodiniaceae under varying conditions to decipher the mechanisms underlying its regulation. We show that the expression and regulation of NR is a dynamic and reversible process impacted by NO3- and NH4+ concentrations. In particular, we demonstrate that NR is actively degraded in the presence of NH4+ and that the synthesis of NR is light-dependent. Additionally, qRT-PCR assays suggest that NR expression is regulated at the post-transcriptional stage as the NR-coding gene is expressed uniformly across conditions. Because NR has never been identified in in hospite coral symbionts, we next investigated the expression of NR in Symbiodiniaceae during symbiosis. For this purpose, different coral species were depleted in nitrogen before they were exposed to an enrichment in NO3-. We demonstrate, for the first time, that Symbiodiniaceae express NR during symbiosis, and that this expression is dependent on NO3- concentrations. This study deciphers the functioning of an essential enzyme involved in coral nutrition, shedding a new light and raising new questions on the process of inorganic nitrogen assimilation by coral holobionts.

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Endosymbiosis and kleptoplasty in the evolution of complex algae

Mick Van Vlierberghe1, Denis Baurain1

1 ULiege – InBioS

Complex algae form a network of photosynthetic organisms scattered across the eukaryotic tree. However, the exact mechanisms by which their plastids were acquired remain unclear. Contrary to expectations of secondary endosymbiotic models, a sizable part of plastid-targeted genes are not from the same origin as the plastid itself. For example, for CASH lineages (bearing complex red plastids), scenarios like the ancestral cryptic serial endosymbioses or the shopping bag model provide a possible interpretation of gene mosaicism but focus on the endosymbiotic mechanism. Similarly, while the Rhodoplex hypothesis fits the single phylogenetic origin of those plastids, it does not explain gene mosaicism. Moreover, it implies multiple tertiary (or quaternary) endosymbiotic events, whereas most complex algae do not bear nucleomorphs and only have three or four membranes surrounding their plastids. To overcome the inconsistencies of those models, kleptoplasty was suggested as an additional mechanism for explaining plastid spread, not only in CASH lineages but also in euglenids. In line with the shopping bag model, this hypothesis posits multiple transient interactions with preys of diverse origins but also proposes a rationale for the selective force driving the progressive accumulation of plastid-targeted genes: to maintain functional kleptoplastids for increasingly longer periods of time, before ultimately reducing them into fully integrated plastids. In such a scenario, the phylogenetic diversity of nucleus-encoded plastid-targeted genes would be higher than predicted with the endosymbiotic models, where genes originate mostly from a single source, the algal symbiont. To test the idea, we designed and implemented an automated phylogenetic pipeline for detecting transfers in single-gene trees in a discovery-driven and taxonomy-aware fashion. By transforming individual transfer events to organism-centric alpha-diversities, this approach allowed us to quantify the relative contribution of endosymbiosis and kleptoplasty in the origin of complex algae.

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The immediate and rapid consequences of asymmetric genome merging on gene expression.

Lucas Prost-Boxoen1,2, Quinten Bafort1,2, Antoine Van de Vloet1,2, Yunn Thet Paing1, Griet Casteleyn2, Olivier De Clerck2, Yves Van de Peer1,3,4

1 Department of Plant Biotechnology and Bioinformatics, Ghent University and VIB-UGent Center for Plant Systems Biology, Ghent, Belgium
2 Department of Biology, Ghent University, Ghent, Belgium
3 College of Horticulture, Academy for Advanced Interdisciplinary Studies, Nanjing Agricultural University, Nanjing, China
4 Department of Biochemistry, Genetics and Microbiology, University of Pretoria, Pretoria, South Africa

Polyploidy is a pivotal mechanism for evolution and speciation, especially within plant species. Although the comprehensive implications of genome doubling and merging on cellular biology remain partially concealed, marked influences on gene expression have been evidenced. Notably, the additivity hypothesis often fails to represent the complex impact of polyploidization on gene expression. To further explore the genome-wide transcriptional dynamics consequent to polyploidization, we conducted an RNA-Seq gene expression analysis on a nascent triploid strain of the microalgal model species, Chlamydomonas reinhardtii. The expression profile of this strain was compared with its haploid and diploid progenitors to scrutinize the immediate impact of genome merging and dosage on parental legacy. Furthermore, it was contrasted with five independently evolving lines at generations 225 and 425 to investigate the evolution of gene expression patterns subsequent to genome merging. The newly formed triploid and its parental strains exhibited substantial differential gene expression, with a mere 10% of the genes demonstrating additive expression levels. A significant two-thirds of the genes revealed expression level dominance toward one parent, yet a genome-wide parental dominance was notably absent. Despite potential artefactual effects attributed to the cell cycle, certain genes evidenced rapid shifts in expression levels. Conclusively, genome merging caused profound effects on gene expression within the triploid, without adopting genome-wide dominance from either parent despite asymmetric genome inheritance.

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Up and down again, genome downsizing in synthetic Chlamydomonas reinhardtii polyploids

Antoine Van de Vloet1,2, Lucas Prost-Boxoen1,2, Quinten Bafort1,2, Yunn Thet Paing1, Griet Casteleyn2, Ewout Crombez1, Lucile Jomat3, Stephane D. Lemaire3,4, Olivier De Clerck2, Yves Van de Peer1,5,6

1 Department of Plant Biotechnology and Bioinformatics, Ghent University and VIB-UGent Center for Plant Systems Biology, Ghent, Belgium
2 Department of Biology, Ghent University, Ghent, Belgium
3 Institut de Biologie Physico-Chimique, UMR 8226, CNRS, Sorbonne Université, 75005 Paris, France
4 Institut de Biologie Paris-Seine, UMR 7238, CNRS, Sorbonne Universite, 75005 Paris, France
5 College of Horticulture, Academy for Advanced Interdisciplinary Studies, Nanjing Agricultural University, Nanjing, China
6 Department of Biochemistry, Genetics and Microbiology, University of Pretoria, Pretoria, South Africa

Whole genome duplications (WGDs) are ubiquitous within flowering plants and are associated with evolutionary diversification and speciation. More than 35% of current Angiosperm species are polyploid and all sequenced Angiosperm genomes show clear evidence of ancient WGD. Bursts in Angiosperm diversification can be traced back to these ancient WGDs and 15% of speciation events in Angiosperms is accompanied by a ploidy increase. Although these observations suggest that WGD acts as an important evolutionary driver within the flowering plants, the initial cost of WGD is high and the number of ancient WGD events within a lineage does not scale with contemporary chromosome numbers and genome size data. This suggests that WGD is often followed by massive genome downsizing, eventually leading to ‘diploidization’. Past efforts in studying the diploidization process have mainly focused on studying naturally occurring (paleo)polyploids, and experimental data covering genome dynamics over many generations after WGD is scarce. In this study, we increase the ploidy level of Chlamydomonas reinhardtii and investigate how stable different ploidy levels are over time. For this we used a method based on antibiotic resistance genes, complementation mating and flow cytometry. Using this approach, we have successfully produced diploids, triploids, and tetraploids, starting from wild-type haploid strains. We show that diploids retain their ploidy level in a stable way for at least 400 generations, but that triploids and tetraploids lose genomic content shortly after the initial ploidy increase. All replicate triploid and tetraploid populations appear to stabilize at roughly the same genome size, with slightly higher genomic content than diploids. Future efforts in whole genome sequencing will aid in explaining this widespread trend and which genomic regions are retained and why.

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The usage of reproducible bioinformatic for genomics and taxonomy of Cyanobacteria

Lequeux Alina, Vaz Marcelo, Beets Kim, Savora Haifaa, Baurain Denis, Wilmotte Annick, Cornet Luc

University of Liege

Microbial culture collections play a pivotal role in advancing our understanding of biodiversity, offering well-characterized biological resources for both fundamental and applied scientific research. To meet the evolving demands of taxonomy and species delineation, the Belgian Coordinated Collections of Microorganisms (BCCM) has embarked on a journey to enhance its genomic capabilities. The result of this endeavor is the GEN-ERA toolbox, a user-friendly, publicly accessible resource designed to empower researchers, regardless of their bioinformatics expertise.

The GEN-ERA toolbox simplifies the entire genomics process, from genome retrieval and quality assessment, including the estimation of genomic contamination, to phylogenetic tree construction. It also provides tools for comparative genomics, such as average nucleotide identity comparisons and metabolic modeling. Built upon Singularity containers and Nextflow, this toolbox ensures reproducibility and accessibility, making it accessible to a wide range of users.

To showcase the practical utility of the GEN-ERA toolbox, we utilized it to study the Cyanobacterial genus Laspinema. Our analysis unveiled the presence of five distinct species within this genus, with two awaiting formal description from strains housed in the BCCM/ULC collection. Furthermore, we delved into a genomic comparison to unravel the mechanisms of cold adaptation within this genus, particularly among the Antarctic species. Our comparative genomic analysis uncovered a wealth of previously uncharted genetic material in Antarctic cyanobacteria, shedding light on their unique metabolic adaptations.

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Insights into the biological and chemical diversity of cyanobacteria from the BCCM/ULC collection

Maria Christodoulou, Marcelo Vaz, Kim Beets, Haifaa Savora, Luc Cornet, Annick Wilmotte

BCCM/ULC Cyanobacteria collection, University of Liège, Liège, Belgium

Cyanobacteria represent an ancient group of morphologically diverse photosynthetic bacteria. Their long and complex evolutionary history is considered to have contributed to the successful colonization of a wide range of habitats from polar to temperate and tropical regions.
The BCCM/ULC public culture collection hosts more than 400 cyanobacterial strains, of which approximately 140 derive from polar, subpolar or alpine environments. The aim of the collection is to preserve the deposited biological material, distribute it to interested parties for fundamental and applied research, valorise it by performing research and provide services and training linked to the isolation, preservation, and identification of Cyanobacteria as well as training on new bioinformatic tools.
All deposited strains are studied by applying a polyphasic approach workflow, which includes a combination of morphological (microscopy), molecular (16S rRNA gene and ITS region) and ecological data. As part of an ongoing effort to discover new molecules with potential pharmaceutical applications, the strains will be evaluated for their antibacterial and/or antifungal activities. Furthermore, whole-genome sequencing as well as comparative genomics are applied to study taxonomically interesting morphotypes and bioactive metabolite-producing strains.
Several strains are the reference (or ‘type’) for newly described taxa. These include Plectolyngbya hodgsonii, Shackletoniella antarctica, Timaviella circinata and T. karstica, Parakomarekiella sesnandensis, and Petrachloros mirabilis isolated from Antarctica and other extreme environments and Johannesbaptistia floridana, Brasilonema fioreae, Leptochromothrix valpauliae, Vermifilum ionodolium, Neolyngbya biscaynensis, and Affixifilum floridanum described from subtropical and tropical habitats. Recently, 30 more strains of filamentous rock-inhabiting cyanobacteria from Finland have been deposited to the collection. These include the new species Pseudanabaena epilithica and P. suomiensis as well as many new taxonomically interesting morphotypes.

BCCM/ULC is supported by the Belgian Science Policy Office

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2b. Plant Ecology and evolution (Salle Trifac 3)

Environmental quality assessment through lichen biomonitoring in various urban contexts across Wallonia

Hugo Counoy1, Marie Cors2, Virginie Hutsemekers2, Laurence Delahaye2, Laure Turcati3, Yannick Agnan1

1 Earth and Life Institute (UCLouvain), 2 ECO-IMPACT, 3 Sorbonne Université

Urban environments have a significant impact on species diversity mainly resulting from pollution, urban heat island, and ecological discontinuity. Studying the presence and absence of biological indicators in these areas provides an opportunity to assess the environmental quality. Lichens, a symbiotic association between algae and fungi, are widely used as bioindicators, offering valuable insights complementary to physico-chemical sensors.
This project aimed to compare lichen diversity along an urbanization gradient in Wallonia, from rural to residential and industrial areas. To achieve this goal, we selected five cities with increasing levels of anthropogenic pressure: (1) Bertrix, Dourbes, and Vielsalm (as background cities, n=4); (2) Namur (as residential city; n=20); and (3) Liège (as an industry-impacted city, n=16). In each city, we selected several sampling sites based on the city extent and the tree availability. We then determined the frequency of each lichen species on the trunks of four lime trees (Tilia spp.), between 100 and 150 cm above the ground, following the standard European protocol (prEN 16413).
In total, 78 lichen species were observed on the 48 inventoried sites. The average lichen diversity decreased along the anthropogenic pressure gradient: Dourbes (30 ± 7.8), Vielsalm (24 ± 4.7), Bertrix (21.8 ± 1.5), Namur (19.8 ± 4.9), and Liège (17.4 ± 6.6). In addition to lichen diversity, the lichen communities differed significantly between cities, with nitrophilous-dominant species in agricultural environments and acidophilous-dominant species near forest areas. Notably, eight strict or facultative saxicolous species were observed in Liège, likely linked to dust pollution. Thus, coupling both lichens abundance and ecology provide a better understanding on the impact of urbanization on lichen biodiversity and environmental assessment in contrasted Walloon cities. This study also highlighted the presence of relatively uncommon species in Wallonia in remote areas, such as Anaptychia ciliaris, Phaeophyscia endophoenicea, or Parmelina pastillifera.

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Mind your fossils: the importance of fossil data integration for dating and biogeographical analyses in the real-case scenario of a hard-to-interpret fossil

Federico Fabriani1

1 Ghent University

Molecular dating is a powerful tool used in evolutionary studies since it allows to infer the past history of organisms. Fossils are the most important source of chronological data used in molecular dating and they often also provide morphological and geographical data which can be included in dating models. Dating strategies determine the type and the way fossil and molecular data are integrated into dating analyses but the effect of the choice of dating strategy on the results of molecular dating and subsequent analyses, like biogeographical hypothesis testing, is still poorly understood. This is especially true in real-case scenarios, in particular for a group with a single hard-to-interpret fossil. To address this issue, we built 12 dating models representing the 3 most widely-used dating strategies (i.e. node dating, tip dating and total-evidence dating) and inferred divergence time in the Duguetieae tribe (Annonaceae), a plant group with amphi-Atlantic disjunct distribution and a single character-poor fossil. We also tested alternative biogeographical hypotheses to explain the current distribution of the tribe. We assessed the performance of each dating model and compared the results of dating and biogeographical analyses of the 12 models. Our results show that different strategies lead to considerably different date estimates and can support contrasting biogeographical hypotheses. It is particularly relevant that the widely-applied node dating strategy showed the largest variation in the results depending on the choice of molecular clock and fossil interpretation. On the other hand, the total-evidence dating strategy outperformed the other strategies while allowing the most comprehensive integration of fossil data. We recommend to always critically evaluate the choice of dating strategy and dating model parameters before performing molecular dating.

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Flower production decreases with warmer and more humid atmospheric conditions in a Western Amazonian forest

Jason Vleminckx1, James A. Hogan2, Margaret Metz3, Liza Comita4, Simon Queenborough5, S Joseph Wright6, Renato Valencia7, Milton Zambrano8, Nancy Garwood9

1 Université Libre de Bruxelles, 2 University of Florida, Gainesville, FL, USA, 3 Lewis & Clark College, Portland, OR 97219, USA, 4 Yale University, New Haven, CT 06511, USA 5 Yale University, New Haven, CT 06511, USA, 6 Smithsonian Tropical Research Institute, Apartado 0843–03092, Balboa, Republic of Panama, 7 Escuela de Ciencias Biológicas, Pontificia Universidad Católica del Ecuador, Quito, Ecuador, 8 Escuela de Ciencias Biológicas, Pontificia Universidad Católica del Ecuador, Quito, Ecuador, 9 Southern Illinois University, Carbondale, IL 62901, USA

● Climate models predict that everwet Western Amazonian forests will face warmer and wetter atmospheric conditions, and increased cloud cover. It remains unclear how these changes will impact plant reproductive performance, such as flowering, which plays a central role in sustaining food webs and forest regeneration. Warmer and wetter nights may cause reduced flower production, via increased dark respiration rates or alteration in the reliability of flowering cue-based processes. Additionally, more persistent cloud cover should reduce the amounts of solar irradiance, which could limit flower production.
● We tested whether inter-annual variation in flower production has changed in response to fluctuations in irradiance, rainfall, temperature, and relative humidity over 18 years in an everwet forest in Ecuador.
● Analyses of 184 plant species showed that flower production declines as nighttime temperature and relative humidity increases, suggesting that warmer nights and greater atmospheric water saturation negatively impact reproduction. Species varied in their flowering responses to climatic variables but this variation that was not explained by life form or phylogeny.
● Our results shed light on how plant communities will respond to climatic changes in this everwet region, in which climate change impacts have been poorly studied compared to more seasonal Neotropical areas.

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New insights into the phylogeography of a threatened tropical African tree, Pericopsis elata (Fabaceae)

Surabhi Ranavat1, Saskia Sergeant1, Dieu-Merci Assumani1,2,3, Nils Bourland4, Claire Micheneau1, Jean-François Gillet5, Jean-Louis Doucet5, Olivier Hardy1

1 Evolution Biologique et Ecologie, Faculté des Sciences, Université Libre de Bruxelles (ULB), Av. F.D. Roosevelt 50, 1050, Brussels, Belgium, 2 Faculté de Gestion des Ressources Naturelles Renouvelables, Université de Kisangani, B.P. 2012, Av. Kitima 3, Kisangani, Democratic Republic of the Congo, 3 Institut National pour l’Etude et la Recherche Agronomiques (INERA - Yangambi), Yangambi, Democratic Republic of the Congo, 4 Royal Museum for Central Africa, Service of Wood Biology, 13 Leuvensesteenweg, Tervuren, 3080, Belgium, 5 Unité de Gestion des Ressources Forestières et des Milieux Naturels, Université de Liège, Gembloux Agro-Bio Tech, Gembloux, Belgium

Climatic oscillations in the Pleistocene have had a great impact in shaping the biodiversity and distribution patterns of tree species across tropical Africa. Genetic data can be used to infer the signatures of these oscillations, giving an insight into the past demographic events. Here, we examine the phylogeography of Pericopsis elata, a highly exploited tree distributed in Western and Central African tropics. It is a peculiar tree as it has a mixed-mating system, a phenomenon rare in tropical trees. Based on previous analysis, P. elata showed the presence of three different clades corresponding to biogeographical sub-centres Upper Guinea (UG), Lower Guinea (LG), and Congolia (C). Focussing on the LG (Cameroon and Republic of Congo) and C (Democratic Republic of Congo) groups, we investigated the genetic diversity and inbreeding using microsatellite markers. The C group had homogenous genetic diversity and the inbreeding coefficient (FIS, adult) was 0.15, but the LG group had a steep westward decay of diversity with FIS (adult) increasing from 0.13 in the east to 0.5 in the west, indicating higher inbreeding in this gene pool. We also dated the crown age of P. elata using a plastome-based phylogeny, which was estimated to be around 216 kya (95% HPD = 238-194 kya). This was further used to date the intraspecies phylogeny, where the divergence date of LG+C was 98 kya at which point an individual from Republic of Congo diverged from the rest of the gene pool, and the divergence date of LG and C was around 64.4 kya. This indicates that there may have been a refugium in the Republic of Congo, from where P. elata has had a recent post-glacial westward expansion into Cameroon with founder effects, possibly facilitated by inbreeding.

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Phylogenomics, evolution, and conservation of Asimina (Annonaceae)

Luiz Henrique M. Fonseca1, Kate Goodrich2, Pieter Aselman1, Lars Chatrou1

1 Ghent University, 2 Widener University (USA)

Asimina Adans. and Deeringothamnus Small are the only temperate genera in Annonaceae, an otherwise tropical plant family containing fruit crops such as sugar apple and soursop. The two genera have eleven recognized species distributed in eastern USA. Species limits in Asimina and Deeringothamnus were determined using traditional morphological traits. The centre of diversity is peninsular Florida, where up to ten species can be found in their native habitats. Flower morphology is remarkably variable in Asimina and Deeringothamnus, with differences in colour, size, odour, and the shape and thickness of their petals. Mixed morphologies are common in Asimina and much evidence of hybridization between most of the species has been documented, with some putative hybrids being formally described. Species with intermediate floral morphology and scent composition are regularly observed in the field. The potential hybrid origin of some species suggests a complex evolutionary history for Asimina-Deeringothamnus and increases the level of difficulty for determining evolutionary relationships. Hybridization also blurs species boundaries making them difficult to recognize the taxa. Here we inferred the phylogeny of Asimina and Deeringothamnus using 695 low-copy nuclear gene. To obtain DNA sequences from these genes, we combined two available bait kits (i.e., Angiosperms353 and Annonaceae) creating the new Annonaceae799 bait kit. We also evaluated the role of incomplete lineage sorting, hybridization, and introgression on the phylogeny of the clade. Overall, we recovered a monophyletic Deeringothamnus nested inside the clade containing all Asimina species. Putative hybridizations/intrigressions were also inferred for the clade, such as the hybrid origin of Asimina pygmaea. Hybrids hypothesized by morphology were also confirmed using molecular data. Phylogenetic results using plastome data also confirmed the intricate evolutionary history of the clade, with considerable incongruence between plastome and nuclear trees.

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3. Posters

3a. Phycology

Evaluation of the feasibility of removal of Microcystis blooms using flocculation combined with dissolved air flotation

Narasinga Rao & Koenraad Muylaert

KU Leuven University

As toxic cyanobacterial blooms interfere with the use of lakes for recreation or drinking water production, there is a need for methods to mitigate such blooms. While a reduction in nutrient levels is generally the most sustainable mitigation method, this is not always achievable. Therefore, rapid intervention methods are required. Flocculation combined with sedimentation has been proposed as a method for rapid removal of cyanobacteria blooms. Here we propose flocculation combined with dissolved air flotation (DAF) as an alternative method. In DAF, tiny (100 µm) air bubbles are generated by depressurising air-saturated water and these bubbles concentrate flocs into a float layer than can be skimmed of. This method exploits the natural tendency of cyanobacteria to float and removes toxins as well as nutrients embedded in the biomass from the ecosystem. The use of the non-toxic flocculant pDMAEMA combined with DAF was effective in removing Microcystis cells from suspension a doses of only a few ppm. When optimising the method, we noted that the flocculation time could be reduced to only a few minutes. This encouraged us to introduce the flocculant into the pressurised water stream, a method known as PosiDAF. PosiDAF allowed us to further reduce the pDMAEMA dose to below 1 ppm.

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Study of the interdependence of photoprotection mechanisms in model microalgae

Alain Gervasi, Patrick Meyer, Pierre Cardol

ULiège

While light is essential for the fixation of carbon dioxide by photosynthetic organisms, too intense light levels can be harmful. The oxidative stress caused by the absorption of an excess of photons can lead to the damage of the macromolecules that compose the photosynthetic apparatus, thus causing a decrease in photosynthetic yield.

Among the strategies developed by photosynthetic organisms to protect themselves from high light, the ability to move away from it (photophobia) and the thermal dissipation at the level of the light-collecting antennae are the two main strategies described independently in the literature.

The purpose of this project is to investigate the articulation between these two photoprotection strategies in two model species of unicellular and motile microalgae : Chlamydomonas reinhardtii (a green alga for which a large collection of mutants exists) and Euglena gracilis (a photosynthetic eukaryotic microalga of great biotechnological interest). In addition, the impact of environmental factors such as light intensity, temperature, medium composition and CO2 concentration will be studied to assess their effect on the dynamic.

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Microbiome mediated tolerance to biotic stressors: a case study of the interaction between a toxic cyanobacterium and an oomycete-like infection in Daphnia magna

Shira Houwenhuyse1*, Lore Bulteel1*, Isabel Vanoverberghe1, Anna Krzynowek2, Naina Goel1,3, Manon Coone1, Silke Van den Wyngaert4, Arne Sinnesael1, Robby Stoks5 & Ellen Decaestecker1

*Shira Houwenhuyse and Lore Bulteel contributed equally to this work.
1 Laboratory of Aquatic Biology, Department of Biology, University of Leuven- KU Leuven, Campus KULAK, E. Sabbelaan 53, 8500 Kortrijk, Belgium.
2 Laboratory of Molecular Bacteriology, Department of Microbiology, Immunology and Transplantation, Rega Institute for Medical Research - KU Leuven, Herestraat 49, PO Box 1037, 3000 Leuven, Belgium.
3 Faculty of Bioscience Engineering, Department of Animal Sciences and Aquatic Ecology, University of Ghent - UGent, Oostende, Belgium.
4 Department of Biology, University of Turku, Vesilinnantie 5, 20014 Turku, Finland.
5 Evolutionary Stress Ecology and Ecotoxicology, University of Leuven – KU Leuven, Charles Debériotstraat 32, 3000, Leuven, Belgium

Organisms are increasingly facing multiple, potentially interacting stressors in natural populations. The ability of populations coping with combined stressors depends on their tolerance to individual stressors and how stressors interact, which may not be correctly captured in controlled laboratory settings. One largely unexplored reason for this is that the microbial communities in laboratory settings often differ from the natural environment, which could result in different stressor responses and interaction patterns. In this study, we investigated the impact of single and combined exposure to a toxic cyanobacterium and an oomycete-like parasite on the performance of three Daphnia magna genotypes. Daphnia individuals were first sterilized and then experimentally given a natural or a laboratory-derived microbial inoculum. Survival, reproduction and body size were monitored for three weeks and gut microbiomes were sampled and characterized at the end of the experiment. Our study confirmed that natural and laboratory microbial inocula and gut microbiomes are differently structured with natural microbiomes being more diverse than laboratory microbiomes. Our results showed that exposure to the stressors reduced D. magna performance compared to the control. An antagonistic interaction between the two biotic stressors was revealed with respect to D. magna survival, when Daphnia individuals were exposed to the laboratory microbial inoculum. This effect was consistent across all three genotypes. In Daphnia exposed to a natural microbial inoculum this antagonistic interaction could not be detected and the genotype x exposure interaction was genotype dependent. Our results indicate that host-stressor interactions depend on the microbial inoculum and that the gut microbiome has potentially a strong role in this, thereby providing an unexplored dimension to multiple-stressor research.

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In vivo evaluation of bioenergetic parameters in heat-stressed Cassiopea

Edmee Royen, Pierre Cardol

University of Liege

The symbiotic partnership between cnidarians and dinoflagellates from the family Symbiodiniaceae constitutes the basis of remarkably diverse ecosystems. This tight association displays a complex energetic metabolism, involving respiration from both partners and photosynthesis from the dinoflagellate symbionts. Despite the major importance of these two processes, their interplay and regulations remain poorly studied. Abiotic factors can unsettle the symbiotic balance, leading to the collapse of the partnership and threatening the survival of entire ecosystems. Among them, the rise in sea water temperature is getting more and more concerning as global warming takes place.
To address this topic, our first approach consisted in subjecting Cassiopea, the model organism for photosynthetic jellyfish, to a mild hyperthermic stress for 28 days. Despite an increase of 6°C of the water temperature, the stressed jellyfish kept on growing, their symbiont density stayed constant and their photosynthetic capacity was barely impacted. The measurement of the level of pigments in Symbiodinium cells hints at an adaptation of the photosynthetic apparatus to heat stress, with the slight increase of the amount of total pigments per cell. Despite the minor impact of heat stress on the photosynthesis of Cassiopea, an increased respiration along with a rise in bell pulsation rate were observed in the hstressed population.
To understand the factors that ensure photosynthetic stability at cellular scale, we are repeating this long-lasting heat stress experiment, collecting samples to carry out bottom-up proteomics. The production of reactive oxygen species is also under investigation, along with the fatty acid composition of both symbiotic partners.
All in all, our work aims at taking advantage of the biophysical and spectroscopic techniques in the context of the mutualistic partnership between cnidarians and dinoflagellates, as well as its disruption. Our results show the tolerance of Cassiopea jellyfish and their dinoflagellate symbionts to a long-lasting hyperthermic stress.

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Rapid temporal adaptation structures Daphnia magna’s tolerance to toxic cyanobacteria.

Emma Gouwy1, Maxime Fajgenblat2,3, Manon Coone1, Alice Boudry1,2, Rafaela Almeida2, Luc De Meester2,4,5,6 & Ellen Decaestecker1

1 Laboratory of Aquatic Biology, Interdisciplinary Research Facility Life Sciences, KU Leuven, KULAK, Campus Kortrijk, Kortrijk, Belgium
2 Laboratory of Freshwater Ecology, Evolution and Conservation, KU Leuven, Leuven, Belgium
3 Data Science Institute, I‐BioStat, Hasselt University, Diepenbeek, Belgium
4 Leibniz Institute für Gewasserökologie und Binnenfischerei (IGB), Berlin, Germany
5 Institute of Biology, Freie Universität Berlin, Berlin, Germany
6 Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany

Cyanobacterial blooms exert substantial pressure on aquatic systems globally, posing threats to ecosystem functioning, biodiversity and public health. While zooplankton grazers, such as Daphnia magna, play an important role in suppressing these blooms, the production of cyanobacterial toxins adversely impacts their fitness, ultimately affecting their ability to maintain a clear water state. Daphnia has been shown to adapt to a deteriorating food quality throughout the growth season as an increasing fraction of the phytoplankton community is represented by cyanobacteria during summer months. In our study, we investigate short-term temporal dynamics underlying the interactions between Daphnia magna and the common cyanobacteria Microcystis sp. Over two consecutive years, we collected D. magna clones and Microcystis strains from a single pond at two time points in the growth season (April vs. May/June). We assessed the survival of both early and late D. magna clones when exposed to Microcystis strains from either the same or a different time point within the growth season. Our findings reveal that D. magna survival is primarily determined by whether they were exposed to Microcystis from the same time point, with strong statistical support for contemporal combinations featuring a considerably higher survival compared to those involving different time points. This pattern was consistent across both years. In addition to these temporal dynamics, we also identified important effects of D. magna genotype and Microcystis genotype identity on survival, with different D. magna clones displaying varying sensitivity and different Microcystis strains displaying varying toxicity. Our results suggest the existence of strong and fast-paced red queen dynamics underlying the tolerance of zooplankton grazers to cyanobacteria.

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Functional effects of Polysiphonia sp. epiphytism on the farmed Kappaphycus alvarezii (Doty) Liao: Competition for the resource, parasitism or both?

Gilles Lepoint*1, Gaëtan Tsiresy*2,3, Mélanie Deydier1, Frédéric Pascal4, Bruno Frédérich5, Igor Eeckhaut6

1 Laboratory of Trophic and Isotopic Ecology, FOCUS, University of Liège, Liège, Belgium
2 Polyaquaculture Research Unit, Institut Halieutique et des Sciences Marines, University of Toliara, Toliara, Madagascar
3 Institut Supérieur de Technologie Régional de la Côte-Est (ISTRCE), Fénérive-Est, University of Toamasina, Toamasina, Madagascar
4 Ocean Farmers, Toliara, Madagascar
5 Laboratory of Evolutionary Ecology, FOCUS, University of Liège, Liège, Belgium
6 Marine Biology and Biomimetics, University of Mons, Mons, Belgium

Seaweed farming for the production of carrageenan is a growing economic activity. Like everywhere in the marine environment, farmed algae such as Kappaphycus alvarezii can host algal organisms as epiphytes. Epiphytes ensure important functions in natural ecosystems, but these organisms can have negative impacts on their hosts and, in aquaculture, be considered a plague responsible for significant economic losses. The mechanisms by which epiphytes act functionally on their hosts are multiple: shading effects, competition for nutrients or parasitism. Parasitism is characterised by the epiphyte diverting a proportion of the host’s resources. The objective of our work was to assess the impact of the epiphytes Polysiphonia sp. on the N and C acquisition of its farmed host Kappaphycus alvarezii using two isotopic experiments with 13C and 15N as tracers. Our results demonstrated a double cumulative action: epiphytes could be capable of quickly outcompeting their hosts in terms of nutrient acquisition because of their better efficiency in C and N uptake, while also functionally qualifying as true parasites, as they divert some of the N resources acquired by their host. In terms of biocontrol, we suggest that the choice of nutrient-rich areas to practice Kappaphycus farming is likely to favour the epiphytes rather than their hosts, considering their relative needs and abilities to incorporate nutrients.

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Molecular assessment of Ulva (Ulvales, Chlorophyta) diversity in the tropical Western Indian Ocean

Laura Leaerts1, Luna van der Loos2, Olivier De Clerck3, Frederik Leliaert4

1 Meise Botanic Garden, 2 Ghent University

Ulva (Ulvales, Chlorophyta) is a widely distributed genus of multicellular green algae, mainly found in marine and estuarine habitats worldwide, with some species also occurring in freshwater habitats. Our understanding of the diversity of Ulva species worldwide remains limited, particularly when it comes to tropical regions. In this study, we employed a combined approach of morphological and molecular methods to identify 174 herbarium specimens of Ulva (including Enteromorpha) from the tropical Indian Ocean, including Tanzania, Kenya, Mauritius, Mozambique, Sri Lanka, Thailand and Madagascar, present in the herbarium of Meise Botanic Garden (collection of Eric Coppejans). Ulva diversity was investigated utilized DNA sequence data of three commonly used markers: plastid encoded tufA and rbcL, and the nuclear encoded rDNA internal transcribed spacer (ITS). Prior to our research, 15 species had been reported from the study area based on morphological characteristics, as documented in AlgaeBase. However, our analyses based on DNA evidence confirmed only two of these species, U. lactuca and U. pulchra. Six species, U. ohnoi, U. chauguleii, U. pennata, U. meridionalis, U. tepida, U. spumosa and U. limnetica are recorded for the first time from the study area. Seven clades could not be associated with species names and possibly represent new species. Our research underscores the importance of herbarium collections as valuable resources for documenting seaweed diversity and conducting phylogenetic research. Our study also emphasizes the importance DNA sequence data in the assessment of Ulva diversity, and indicates there is a substantial amount of yet undiscovered diversity in the tropical Indian Ocean.

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Diatom biodiversity on loggerhead sea turtles and their environment in the Adriatic Sea

Margaux Pottiez1,2, Sunčica Bosak3, Ralitsa Zidarova4, Bart Van de Vijver1,2

1 Meise Botanic Garden, Research Department
2 University of Antwerp, Department of Biology – ECOSPHERE
3 University of Zagreb, Department of Biology
4 Institute of Oceanology ‒ Bulgarian Academy of Sciences, Department of Marine Biology and Ecology

Over the last decade, interest in exploring diatom communities residing on the surface of marine vertebrates has increased immensely, with recent evidence supporting the existence of close associations between diatoms and sea turtles. Despite this, the question remains to what extent these communities include opportunistic species and whether the flora changes depending on the environment of the host. In order to bridge this knowledge gap, diatom communities on the carapace of 12 loggerhead sea turtles from the Adriatic Sea were sampled from rescue centres on the Verudela peninsula in Pula and the island Lošinj (Croatia). Additionally, 15 samples were taken from the sediment and 3 from the recovery tanks where the turtles were housed.
A highly diverse community of 271 taxa belonging to 66 genera was discovered in the samples. The diversity was significantly higher in the environmental samples compared to the carapace samples, indicating a need for specialised adaptations in order to survive on the carapace. Detrended Correspondence Analysis divided the samples into three distinct groups: the environment, turtles sampled within a few days after rescue and turtles that were house for at least one month together with the recovery tanks. In addition, the same sea turtle that was sampled both immediately after rescue and more than a month later, showed two distinct diatom communities, largely overlapping with the turtle samples within a few days or the recovery tank samples respectively. This illustrates a swift species turnover within epizoic diatom communities on loggerheads when introduced to a new environment. During this process, existing species are shed while new ones are incorporated from the surroundings. This highlights the remarkable ability of diatom communities to promptly acclimate to changing environments by changing the species composition, underscoring their capacity for swift environmental modification.

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New taxa of terrestrial rock-inhabiting cyanobacteria from Finland

Maria Christodoulou1,2 & Kaarina Sivonen1

1 Cyanobacteria Research Group, Department of Microbiology, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland, 2 BCCM/ULC Cyanobacteria collection, University of Liège, Liège, Belgium

Cyanobacteria represent a group of photosynthetic bacteria found in almost all habitats on Earth including those, which are considered hostile to life from our anthropocentric point of view and are known as ‘extreme’ environments. Caves, cave-like environments, and rocks represent examples of such ecosystems. Cyanobacteria thriving in aquatic or soil habitats have been extensively studied. However, the number of studies dealing with terrestrial rock-inhabiting cyanobacteria is limited. The aim of this work is to investigate the diversity of epilithic filamentous cyanobacteria from non-studied terrestrial habitats in Finland.
Environmental samples were collected using sterile tools from different locations in Southern Finland including the historically and culturally important Suomenlinna Fortress (UNESCO). Strains were isolated and studied by applying a polyphasic approach workflow, which included morphological (light microscopy), molecular (16S rRNA, 16S-23S ITS, rbcLX, rpoC1) and ecological data.
Thirty strains of filamentous cyanobacteria were isolated for the first time from wet and dry rocks as well as other sciophilic habitats in Finland. Nine of these strains belong to the order Nostocales and were morphologically close to Nostoc, Calothrix and Roholtiella whereas the remaining strains resembled Leptolyngbya, Pseudanabaena Phormidesmis, Phormidium and Microcoleus. Phylogenetic analysis based on 16S rRNA indicated the existence of at least 3 new cyanobacterial lineages and at least 12 new species. These findings contribute to expanding cyanobacterial diversity and highlight the importance of studying terrestrial epilithic cyanobacteria.
This work was supported by a scholarship by the Vilho, Yrjö and Kalle Väisälä Foundation of the Finnish Academy of Science and Letters to MC and a NordForsk grant (NordAqua; 82845) to KS.

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Hemicellulose valorization for biofuel production from microalgae grown in heterotrophy

Pablo Perez Saura1, Pierre Cardol1, Claire Remacle1

1 Laboratory of Genetics and Physiology of Microalgae, InBios/Phytosystems research unit, University of Liège, Chemin de la Vallée 4, 4000 Liège, Belgium.

Biofuel demand is increasing yearly because they are considered a sustainable alternative to fossil fuels (Martinez-Villarreal et al., 2022). Due to their lipid-rich biomass content, many microalgal species are good candidates for biodiesel and biokerosene production. Heterotrophic growth is an alternative to photoautotrophic growth for algal biomass and high-added value bio compounds production (Perez Saura et al., 2022). After cellulose, hemicellulose is the second most abundant material found in plants. Hemicellulose hydrolysis mainly liberates xylose, glucose, and acetate in variable proportions depending on the lignocellulosic material and hydrolytic process. Here, we selected three microalgal species capable of growing heterotrophically and showing interesting features for biofuel production: Galdieria sulphuraria, Euglena gracilis, and Auxenochlorella protothecoides. We analyzed their capacity to grow in the presence of the three carbon sources mentioned above and characterized their biomass content. Although G. sulphuraria did not show high fatty acids content (5-15% w/w), it is interesting for biofuel production because of its ability to reach high biomass productivity using xylose as a unique carbon source (±1.5 gDW/L/d). Unfortunately, its growth was inhibited by the presence of acetate. E. gracilis could only assimilate acetate with low biomass productivity (±0.5 gDW/L/d) and fatty acid content (±6% w/w). A. protothecoides showed relatively low biomass productivity (0.4-0.6 gDW/L/d) but high fatty acid content (22-32% w/w) in the presence of glucose, acetate, or a mix of the three sugars. Interestingly, despite it could not grow when xylose alone was added, quantitative analysis of sugar depletion showed that xylose concentration was decreasing in the medium as long as other carbon sources were assimilated by the microalga. With this comparative study, we discuss the strengths and weaknesses of each strain grown heterotrophically for their potential use as feedstock for biofuel production from lignocellulosic material.
Funded by Belgian ADV_BIO grant.
References :
Martinez-Villarreal et al. (https://doi.org/10.1016/j.biombioe.2022.106555)
Perez Saura, P. et al. (https://doi.org/10.3389/fpls.2022.978246)

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3b. Lichens and mosses

Disentangling climate change from air pollution effects on epiphytic bryophytes
Virginie Hutsemékers1, Lea Mouton2, Hannah Westenbohm2, Flavien Collart3, Alain Vanderpoorten2

1 Belgian Interregional Environment Agency, Brussels, Belgium
2 University of Liège, Institute of Botany, Liège, Belgium
3 Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland

At the interface between atmosphere and vegetation, epiphytic floras have been largely used as indicators of air quality. The recovery of epiphytes from high levels of SO2 pollution has resulted in major range changes, whose interpretation has, however, been challenged by concomitant variation in other pollutants as well as climate change. Here, we combine historical and contemporary information on epiphytic bryophyte species distributions, climatic conditions, and pollution loads since the 1980s in southern Belgium to disentangle the relative impact of climate change and air pollution on temporal shifts in species composition. The relationship between the temporal variation of species composition, climatic conditions, SO2, NO2, O3, and fine particle concentrations, was analyzed by variation partitioning. The temporal shift in species composition was such, that it was, on average, more than twice larger than the change in species composition observed today among communities scattered across the study area. The main driver, contributing to 38% of this temporal shift in species composition, was the variation of air quality. Climate change alone did not contribute to the substantial compositional shifts in epiphytic bryophyte communities in the course of the last 40 years. As a consequence of the substantial drop of N and S loads over the last decades, present-day variations of epiphytic floras were, however, better explained by the spatial variation of climatic conditions than by extant pollution loads. The lack of any signature of recolonization delays of formerly polluted areas in the composition of modern floras suggests that epiphytic bryophytes efficiently disperse at the landscape scale. We suggest that a monitoring of epiphyte communities at 10-year intervals would be desirable to assess the impact of raising pollution sources, and specially pesticides, whose impact on bryophytes remains poorly documented.

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Deep environmental sequencing combining UMI-PCR and MinION in Arthoniales: basidiomycete yeasts absent?

Luca Borgato1,2, Pieter Asselman1, Damien Ertz2,3, Annemieke Verbeken1

1 Ghent University, 2 Meise Botanic Garden, 3 Fédération Wallonie-Bruxelles

Cystobasidiomycetes discovered by Spribille et al. (2016) in the cortex of lichens might play a role in the cortex architecture and may even be responsible for the appearance of foliose and fruticose thalli in the Lecanoromycetes. Other studies discovered also a high diversity of these yeasts, but others found very few lichens colonized by them, suggesting that they are less ubiquitous. In the present study, we tested the presence and distribution of these basidiomycete yeasts in the Arthoniales, using an array of taxa with various thallus types, from corticate and ecorticate crustose thalli to fruticose species. A preliminary study has been carried out on a limited number of samples. DNA was extracted using the CTAB protocol, ITS loci were amplified with UMI primers and sequenced using MinION techniques. The Ligation Sequencing Amplicons Native Barcoding kit was used to build the library. It proved to be an excellent technique as it allowed to obtain a very deep sequencing with high quality sequences. Not only the mycobiont and the photobiont were obtained, but also sequences from other taxa, such as plants, phytopathogenic fungi or Cystobasidiomycetes, although sometimes in small amount. However, the group of Cystobasidiomycetes highlighted by Spribille et al. (2016) were not detected in the Arthoniales, independently from the thallus growth form or the presence of a cortex. Nevertheless, corticate Arthoniales have a different cortex type than foliose and fruticose thalli of the Lecanoromycetes studied by Spribille et al. (2016). In conclusion, Cystobasidiomycetes may well have played a role in the cortex or thallus architecture in Lecanoromycetes, but not in Arthoniales. However, to better support this conclusion a larger number of samples are needed, notably in order to cover more lichenized lineages.

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The origin of oceanic island flora: where do Macaronesian bryophytes come from?

Sébastien Mirolo1, Alice Ledent1, Jairo Patiño2, Alain Vanderpoorten1

1 University of Liège[WA1]  ; 2. University of La Laguna, Tenerife

Oceanic islands are among the world's richest ecosystems, but their diversity comes entirely from continental sources. A puzzling question in ecology is to determine the origin of this specific flora. For Macaronesian plant species (Canary Islands, Azores, Madeira), the classic view is that of a European, Mediterranean origin. But what about bryophytes? Our study aims to explore this question and understand the different paths taken by these highly dispersable organisms to colonize these archipelagos.

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3c. Plant Ecology and Evolution

Small is beautiful, and so is big: monitoring tree drought stress responses

Flor Hiergens1, Kathy Steppe1

1 Ghent University

‘Small is beautiful, and so is big.’ Besides being big and beautiful, trees provide numerous functions including carbon storage and atmospheric cooling, and therefore contribute to the mitigation of global warming. However, that same global warming is exercising increasing pressure on these growth-associated tree functions through an increase in extreme drought events, both in frequency and duration.
To fully understand the ecophysiological response of trees, and by extension forest systems, to these increasing drought events, there is a strong need for automated, continuous, and high-resolution monitoring of the tree water-status that can serve as an early-warning system of tree drought-stress. One method that is put forward to serve this purpose consists of monitoring the stem diameter variation, from which the tree water deficit (TWD) can be calculated. A state of TWD occurs when the stem diameter falls below its highest previous pre-dawn value and results from the daily shrinkage and swelling mechanism displayed by the tree stem and therefore is, regardless of drought, a daily recurring phenomenon. However, if a state of TWD persists for a full day or across multiple successive days, it means that even after closure of the stomata due to the absence of photosynthetically active radiation (PAR), the internal tree water reserves cannot be fully replenished, which clearly indicates drought-stress.
Despite the already demonstrated successes of the method, high quality diameter data on pendunculate oak (Quercus robur L.), gathered during the extreme dry summer of 2022 in the experimental forest of Ghent University (Aelmoeseneie, Gontrode, Belgium), failed to identify drought-stress during phases of strong growth. A plausible explanation is that the negative effect on diameter swelling, by an incomplete replenishment of the tree water reserves, is compensated for by turgor-driven growth due to cell division and expansion. In contrast to the TWD data, measurements of sap flow density did indicate drought-stress during these same phases of strong radial stem growth.
These results highlight that, despite the valuable insights garnered from TWD analysis, it should be employed judiciously. Sole reliance on TWD data appeared insufficient to fully decipher the physiological responses of the oak trees to extreme drought events. We, therefore, propose the integration of complementary measurements using sap flow sensors and/or tree water potential assessments.

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High phenotypic plasticity and growth performance in outcrossed seedlings of Pericopsis elata (Fabaceae)

Jean Pierre Ngongo*,1,2, Chadrack Kafuti3, Hans Beeckman4, Olivier J. Hardy1, Nils Bourland4

1 Université Libre de Bruxelles. Faculty of Sciences. Evolutionary Biology and Ecology. CP 160/12. Avenue F. D. Roosevelt, 50. BE-1050 Brussels, Belgium
2 Faculty of Agronomic Sciences, Department of Natural and Renewable Resources Management. University of Kindu, B-122 Kindu, DRC
3 UGCT-Woodlab-UGent, Laboratory of Wood Technology, Ghent University, Coupure Links 653, 9000 Gent, Belgium
4 Service of Wood Biology, Royal Museum for Central Africa, Leuvensesteenweg 13, 3080 Tervuren, Belgium

Logging in the Congo Basin is geared towards selecting a minority of species with a high commercial value. The long-term conservation of these species has become a major challenge for ecologists and forest managers. The silvicultural approach is one of the best ways of promoting the long-term conservation of these species. This study focused on Pericopsis elata, a heliophilous species found in the semi-deciduous humid forests of tropical Africa, which is deficient in natural regeneration. Highly exploited for its high-quality timber, it is listed in CITES appendix II (CoP18 #17) and is recorded as Endangered A2cd on the IUCN Red List. With a selfing rate of 54%, it may be affected by inbreeding depression and loss potential to adapt to changing environmental conditions. Several studies have assessed the impact of inbreeding in terms of fecundity and survival. However, little is known about the effect of inbreeding on growth potential and phenotypic plasticity. A silvicultural trial of 648 P. elata seedlings was carried out in the DR. Congo for 4 years cultivated in a density gradient. Paternity analyses using microsatellite markers were carried out to identify the matrimonial origin of the seedlings. We found a significant difference between the growth of inbred and outbred plants. Analysis of leaf characteristics revealed a higher level of phenotypic plasticity in outbred seedlings than inbred seedlings, reflecting their possible better adaptation to environmental conditions.
Our presentation will show how controlling planting density and selecting outbred seeds can improve the growth and adaptation of P. elata seedlings to changing environmental conditions.

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The limits of photosynthesis in the Far-red : adaptation of light-harvesting complexes in Euglena gracilis

Feller Tom1, Miranda-Astudillo Héctor2, Vega de Luna Félix1, Javaux Emmanuelle3, Cardol Pierre1

1 Genetic & physiology of microalgae, Uliege, Belgium
2 Biología Molecular y Biotecnología, UNAM, Mexico
3 Early Life Traces & Evolution-Astrobiology, Uliege, Belgium

Starlight represents an unlimited and efficient source of energy that is abundantly used by life on Earth, at the basis of trophic chains, and could also play a key role in the development and sustainability of other biospheres elsewhere in the Universe. For instance, potentially habitable exoplanets orbiting nearby very low-mass red dwarf star like TRAPPIST-1 are exposed to solar light enriched in infrared components. In the frame of the PORTAL project, we aim to test the limits of photosynthesis in the infra-red range and identify putative mechanisms underlying acclimation to far red light in complex eukaryotic algae.
In this study we characterized the light-harvesting complexes (LHC) from the excavate Euglena gracilis, a secondary photosynthetic unicellular eukaryote that arises from an endosymbiosis between a green alga and an ancient phagotroph euglenozoan species. After exposition to far-red/near infra-red light (720-940nm) or to very low white light, the absorption spectrum of the excavate Euglena gracilis shows an additional peak at 695 nm. By separating components of the photosynthetic machinery under native form by electrophoresis, we identified a 250 kDa complex which has red-shifted absorption and fluorescence spectra. It is composed of 5 LHCE antenna proteins unique to Euglena gracilis, and contains chlorophyll a but no chlorophyll b. This LHCE antenna complex may rapidly (a few min) dissociate or reassociate to Photosystem II, in a process called state transition, in response to far-red enriched light.
This newly discovered far red light-harvesting antenna's role will be investigated to understand if LHCE complex sustains a growth rate and photosynthesis under far red light.