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Understanding the dynamics of exopolysaccharides in the Seine Estuary waters
Understanding the dynamics of exopolysaccharides in the Seine Estuary waters
Introduction
In pelagic ecosystems, the available carbon pool significantly arises from the primary production of microalgae. This carbon is present both as cellular biomass and as secreted exopolysaccharides (EPS). EPS represent a substantial carbon reserve and are a critical component of the carbon budget. These polymers can coagulate to form larger particles known as transparent exopolymeric substances (TEP), which play a significant role in the aggregation of organic matter and the formation of marine snow. Additionally, EPS play crucial ecological roles in the biogeochemical cycling of nutrients, providing substrates for microbial food webs, influencing sedimentation processes, and contributing to the overall health of ecosystems.
Despite their importance, the dynamics of EPS and TEP are still not well understood in estuarine waters. The intricate interactions between these substances, the physical and chemical variations in estuarine waters, and the succession of microalgae communities remain to be fully elucidated. In this context, part of my research aims to explore the complex relationships and processes governing EPS dynamics in the Seine estuary waters.
In the context previously described, one of my research aimed to elucidate the temporal and spatial variations of EPS and TEP across different haline zones and contrasting seasons.
Our findings revealed that EPS dynamics were strongly influenced by hydrodynamics, particularly tide-driven processes at daily scales and seasonally linked to winter conditions. Our results suggested that between 9% and 33% of the soluble EPS pool in the water column originated from phytoplankton excretion, while between 0.4% and 1.6% originated from microphytobenthos. The majority thus appeared to derive from detrital carbon pools in the maximum turbidity zone and sediment. This study underscores the critical role of high-frequency measurements in understanding carbon flux dynamics in estuarine ecosystems.
The results of this study were published in the journal 'Marine Environmental Research' in 2017. Access to the paper
Building upon previous findings, our study investigated the dynamic patterns of TEP and EPS across the salinity gradient of the Seine estuary over a year, concurrent with changes in phytoplankton communities.
We observed that the phytoplankton community, dominated by marine diatoms, exhibited varying species richness correlated with salinity fluctuations. EPS production coincided with the phytoplankton spring bloom, with specific species associated with soluble and bound EPS. In contrast, TEP distribution was influenced by hydrodynamics, maximum turbidity zone formation, and sediment resuspension, likely originating from stressed or dying phytoplankton or sedimentary remobilization. These findings underscore the dynamic role of these carbon pools in estuarine ecosystems, influencing the fate of both organic and inorganic matter.
This research was published in the journal 'Estuarine, Coastal and Shelf Science' in 2018. Access to the paper
Understanding the role of exopolysaccharides as a carbon source for denitrifying bacteria.
Understanding the role of exopolysaccharides as a carbon source for denitrifying bacteria.
Introdution
Among bacteria involved in nitrate reduction processes, some rely on the use of organic carbon, which provides the energy necessary for converting inorganic nitrogen. Exopolysaccharides, as a readily usable source of organic carbon, could play an important role in sustaining these processes and associated ecosystem services. Therefore, a part of my research has aimed to investigate how EPS impacts denitrifying bacteria in wetlands.
My first interaction with denitrifying bacteria was through a study aimed at elucidating the spatial and temporal variations in nitrate reduction and ammonium production rates in the Seine Estuary.
The study revealed that overall microbial activity had a strong correlation with chlorophyll a and EPS fractions. Consequently, the presence of microalgae emerged as a crucial driver of nitrate reduction rates in riparian sediments, fueling benthic nitrate-reducing activity and enhancing denitrification processes in estuarine ecosystems.
This research was published in the journal 'Estuarine, Coastal and Shelf Science' in 2021. Access to the paper
Building on previous findings regarding the role of microalgae and EPS in nitrate reduction, one of my research focused on the impact of microphytobenthos and their EPS secretions as sources of organic carbon on nitrate reduction processes in intertidal sediments.
The findings confirmed that microphytobenthos, particularly through their secretion of labile EPS, significantly increased nitrate reduction rates, thereby affirming their role as a key carbon substrate for nitrate reduction. Moreover, these carbon sources were found to promote complete denitrification, converting nitrogen to its gaseous form, which is ecologically beneficial.
The results of this study were published in the journal 'Estuarine, Coastal and Shelf Science' in 2022. Access to the paper
To complement the previous findings, we also investigated the influence of different wetland plant types on nitrate reduction processes in intertidal sediments.
Our study revealed that, unlike microphytobenthos which initially caused a significant increase in nitrate reduction rates but was rapidly depleted due to its labile nature, organic carbon derived from wetland plants exhibited a delayed yet sustained enhancement in nitrate reduction rates. These results underscore the importance of both labile and recalcitrant carbon sources in sediments to effectively support the nitrogen biogeochemical cycle.
The results were published in the journal 'Wetlands' in 2023. Access to the paper
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