Research

Microbial ecology of seagrass meadows

A central topic of our research is the role of seagrass and sediment microbiomes in seagrass meadow ecosystems in the Baltic Sea. Seagrass meadows are extremely valuable ecosystems worldwide that provide nursing grounds for juvenile fish, stabilise sediments and sequester large amounts of carbon. In the Baltic Sea, seagrass meadows have unfortunately experienced reductions in depth limits and areal cover in the last decades which is likely due to a complex interplay of factors related to eutrophication, overfishing and other anthropogenic disturbances. In order to conserve and restore these ecosystems it is essential to understand the interaction between seagrass and the microbes that colonize its leaves, roots and seeds as these can influence for example fouling of the leaves, nutrient uptake from sediments and germination of seeds. Within the BMBF-funded SeaStore consortium, we study how seagrass and sediment microbiomes influence restoration success. The role of the seagrass seed microbiome is investigated by DBU stipend holder Anne Brauer.

Check out the SeaStore project website

Check out the NDR documentary about the SeaStore project

Check out our paper on Zostera marina leaf microbiomes

Check out our paper on DNA extraction bias in seagrass-associated microbiota

Particle microbiomes in marine phytoplankton blooms

As part of the DGF funded POMPU consortium, we investigate how the microbiomes of marine particles influence the degradation of phytoplankton-derived polysaccharides in the B03 subproject. Marine phytoplankton blooms are explosions of primary productivity, resulting in fresh organic carbon vital to marine ecosystems and substantial for global carbon budgets. The fate of phytoplankton-derived carbon is dictated by its microbial consumers, which determine what proportion of fixed carbon is outgassed as CO2 or assimilated into marine food webs. However, a significant proportion of this carbon is never consumed, and sinks to the ocean floor as particulate organic matter to be buried and thereby locked away from the global carbon cycle (the biological carbon pump). Thus, particle-associated microbial communities, or particle microbiomes, are key to understanding marine carbon cycling. In bloom situations, particles mainly consist of living and dead algal cells and their respective attached bacteria which form biofilms accessing the exudates and structural components of the algae. The identity and function of both the algal and bacterial partners in marine particles are decisive for the fate of marine carbon. Understanding how these particle-associated microbiomes assemble is essential for understanding how they contribute to degradation of marine organic matter such as algal polysaccharides.

Check out our paper on sulfate-reducing bacteria in particle microbiomes

Check out our paper on particle-associated bacteria as gatekeepers of marine polysaccharide degradation

Check out our paper on alpha-glucans from bacterial necromass

Mixotrophic bacterivory in lakes

In the DFG and FWF-funded project LakeMix we study how mixotrophic protists impact microbial foodwebs in changing lake ecosystems in collaboration with Dr. Robert Ptacnik at the WasserCluster Lunz. Mixotrophic protists, that combine photosynthesis with phagotrophy, are common in marine and freshwater ecosystems. Yet their role in food web dynamics, including trophic interactions, is not well understood. Accumulating evidence from marine environments supports considerable quantitative importance of mixotrophs as bacterivores. Nevertheless, models of the pelagic food web still largely rely on the traditional dichotomy of autotrophic and heterotrophic protists. In lakes, mixotrophic bacterivory has rarely been quantified, and although putatively mixotrophic taxa are numerically abundant there is limited knowledge on how these taxa impact bacterial communities. While it seems obvious that light must play a pivotal role for the potential importance of mixotrophy, experimental evidence on the effect of light on microbial food webs is scarce although light regimes are predicted to change as a result of, for example, altered stratification patterns due to global warming. In LakeMix, we aim to fill these voids by assessing the quantitative and qualitative impact of mixotrophic bacterivory in lakes in the context of climate-related changes.

Check out our paper on top-down structuring of freshwater bacteria by mixotrophic flagellates

Check out our paper on responses of mixotrophic protists to light

Role of dispersal for maintenance of diversity in lake plankton communities

I am involved in the Austrian Research Foundation funded project "MesoSperse" led by Dr. Robert Ptacnik at the WasserCluster in Lunz am See, Austria, where we are investigating the effect of ecological connectivity eukaryotic and bacterial microbial diversity. I am using high-thoughput sequencing techniques to analyse planktonic microbial communities in experimental lake communities (mesocosms) and in natural lakes across the European alps. I am interested in how ecological connectivity interacts with the local environment to determine community assembly, and how interactions between phytoplankton and bacteria are modulated under different connectivity regimes.

Check out our paper on spatial insurance against a heatwave in aquatic communities

Priming effects in stream biofilms

One example of an algal-bacterial interaction with potential consequences for ecosystem carbon cycling is the so-called priming effect which can cause bacteria to degrade more refractory organic matter when they are exposed to labile organic matter (for example from algae). This phenomenon has been intensively researched and debated in soils for several decades due to its importance for soil carbon storage and consequently for global climate. However, it has only recently been recognized as a potentially important factor also in aquatic ecosystems (Guenet et al. 2010). In my project "PRIMA" (EU, FP7, Marie Curie IEF), I worked together with Professor Tom Battin and PhD student Karoline Wagner at the Department of Limnology and Oceanography, University of Vienna to:

Quantify potential priming effects mediated by biofilms in the hyporheic and benthic zones of streams.
Uncover the underlying mechanisms of priming effects.

To address these topics we used a suite of approaches and methods, including:

Microcosm design and experimentation.
Stable isotope labeling techniques.
Extracellular enzyme activity assays.
High-throughput sequencing of microbial communities.

Five papers are already out from this project (see publications) as well as a review and metaanalysis on aquatic priming effects!

Bacterial biofilms on kelp

Kelp forests are hotspots of productivity and biodiversity in temperate coastal areas around the world. Despite the ecological and economical importance of kelp, it is not well understood how kelp produced carbon subsidizes life within and beyond the kelp forest. Bacteria living on and around kelps have the ability to utilize kelp-derived dissolved and particulate organic matter and thereby provide a basis for the food chain. Before my PhD work at the University of Bergen, there was very little knowledge about these bacterial communities. With the help of my advisors Professor Lise Øvreås and Dr. Kjersti Sjøtun, I have:

The full text of my PhD-thesis can downloaded from here.

I finished my PhD in 2011, but I hope to return to this topic in the future as many questions about the role of kelp-associated bacteria remain unanswered.

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