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Advancing understanding of phytoplankton primary production dynamics in the Seine estuary
Advancing understanding of phytoplankton primary production dynamics in the Seine estuary
Introduction
Our main understanding of phytoplanktonic primary production heavily relies on algorithms based on measurements of chlorophyll a concentration. While effective in stable and homogeneous zones such as the open ocean, this method fails to capture primary production variations in dynamic areas such as coasts, rivers, or estuaries. Chlorophyll measurement provides a static view of primary producers, masking the intricate processes influencing spatial and temporal variability in biomass and growth rates of photosynthetic organisms at fine scales. High-frequency measurements are thus needed. However, traditional methods used to assess primary production values (carbon isotope incorporation or the evolution of oxygen production) require relatively long incubation times, yielding low-frequency data that do not capture short-term variations in photosynthetic processes. Since the 1970s-80s, new methods based on chlorophyll fluorescence of photosynthetic cells (PAM technology) have emerged. These methods offer flexibility, sensitivity, non-invasiveness, and the ability to estimate photosynthetic parameters at high frequencies. However, they do not directly measure fixed carbon. Only the combination of these methods allows determination of the number of electron moles needed to fix one mole of carbon (ϕec), transforming high-frequency fluorescence data into carbon units. However, the relationship between these two processes might be influenced by environmental factors. This was the topic of the PROUESSE project,
In the context previously described in the introduction, one of my research projects, focusing on the phytoplanktonic compartment, aimed to investigate the relationship between cell fluorescence and carbon incorporation across several diatom species in response to daily light gradients.
The findings indicated that, for the individual species studied, light did not have a significant effect, and there was no significant variation in the number of electron moles required to fix one mole of carbon (ϕec). This study enabled us to explore these relationships in natural settings without the need to standardize light intensities during each sampling event.
The results of this study were published in the journal 'Photosynthesis Research' in 2018. Access to the paper
Following previous findings, we also examined the short-timescale temporal dynamics of phytoplanktonic photosynthetic parameters over tidal cycles at fixed stations along the salinity gradient of the Seine estuary.
The study revealed that small-scale temporal variability (i.e., 5 minutes) in photosynthetic parameters can exceed tidal-scale variability (i.e., 6 hours), reinforcing the postulate that high-frequency measurements are needed in dynamic ecosystems such as estuaries, as low-frequency measurements can easily result in over- or underestimation. Additionally, the findings highlighted the importance of light and suspended particle dynamics in controlling phytoplankton productivity and the adaptability of phytoplankton to variable and often stressful environmental conditions.
The results were published in the journal 'Marine Environmental Research' in 2017. Access to the paper
Then, we examined the short-timescale spatial dynamics of phytoplanktonic primary production by coupling carbon isotope incorporation and chlorophyll fluorescence across the salinity gradient of the Seine estuary over one year.
This study established a precise statistical relationship between biological and physico-chemical parameters, allowing for a fine-scale estimation of phytoplanktonic primary production. It also accounted for variations in light attenuation with depth and turbidity on both temporal and vertical scales, enhancing the accuracy of the annual primary production estimates for the saline estuary.
The results were published in the journal 'Estuaries and Coasts' in 2018. Access to the paper
The annual bacterial and eukaryotic community dynamics along the Seine macrotidal estuary was also studied through a molecular survey across the salinity gradient over the same year.
This study represents a milestone in the understanding of macrotidal estuary dynamics and the Seine ecosystem, through the identification of putative markers of ecosystem functioning. It identifies seasons and biotic associations as main drivers of the Seine estuary microbiota and reveals the importance of a core microbiota throughout the year.
The results were published in the journal 'Environmental Microbiome' in 2025. Access to the paper
Advancing understanding of the toxigenic Pseudo-nitzschia australis blooms in the English Channel
Advancing understanding of the toxigenic Pseudo-nitzschia australis blooms in the English Channel
Pseudo-nitzschia australis is a toxic pennate diatom isolated from the English Channel, capable of producing domoic acid, a toxin that bioaccumulates in its predators (e.g., scallops) and can reach dangerous levels for human health.
I had the opportunity to participate in a study examining how temperature and light influence the growth, physiology, and production of domoic acid in this species under semi-continuous culture.
The findings indicated that domoic acid production increased with higher temperatures and light intensities, correlating with faster growth rates. These results suggest that domoic acid production in this species is not solely triggered by nutritional stress or unfavorable growth conditions. Understanding these dynamics is crucial for comprehending toxin production within the bloom dynamics of this species.
Results of this study were published in the journal 'Harmful Algae' in 2014. Access to the paper
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