Carbon (C) and nitrogen (N) are essential nutrients for coral–Symbiodiniaceae associations, yet global change can disrupt C and N acquisition by corals, affecting their resilience under stress. We investigated how two octocorals (Sarcophyton glaucum, Lobophytum sp.) and two hexacorals (Stylophora pistillata, Turbinaria reniformis) assimilate nitrogen from three 15N-labelled sources—dissolved free amino acids (DFAAs), Synechococcus (picoplankton), and Artemia salina nauplii (microplankton)—supplied at 1 µM N, under control (26 °C) and heat stress (30 °C) conditions. Corals were also incubated with natural pico-nanoplankton assemblages, with concentrations measured via flow cytometry. In addition, we measured the rates of photosynthesis and respiration, to estimate the relative contribution of autotrophy to the corals’ respiratory needs. Across all species, Synechococcus was the most efficiently assimilated N source, with uptake increasing under heat stress. Estimates of heterotrophic carbon assimilation (using C:N ratio) coupled with respiratory measurements showed that Synechococcus can provide 30–70

Microbial dormancy plays an important role in the persistence, dispersal, and functioning of microbial communities in moderate to extreme environments. The activity or inactivity of microbial communities also has implications for rates of biogeochemical transformations and thus elemental stocks and redox conditions. Microbial communities inhabiting glacier surface environments encounter harsh and variable environmental conditions including nutrient limitation, low temperatures, and light availability across various micro-habitats including cryoconite and the bare ice surface. The metabolic states of cells within these microhabitats and in relation to their environment is fundamental to the functioning of the ecosystem and has implications for ecosystem resilience, responses to environmental change, and biogeochemical cycling. This report describes an expedition to Brøggerhalvøya, north-west Svalbard, carried out in July 2025, within the framework of the ERC SIESTA project. A major objective of the project is to resolve microbial activity and dormancy on an individual cell basis, to characterise the adaptive and functional traits of active and dormant fractions of the native glacier microbial population, and to link microbial metabolic states to broader ecological and biogeochemical dynamics. Here we report the site characteristics, the samples collected, the analyses undertaken, and the future analyses planned. Two small valley glaciers near to Ny-Ålesund were selected for investigation during this summer campaign: Midtre Lovénbreen and Austre Brøggerbreen. The data collected in the field, combined with subsequent laboratory analyses, will provide insights into the spectrum of dormancy and activity in situ among glacier microbial communities, and the taxa and functions associated with active and inactive fractions of the communities. These findings will contribute to a deeper understanding of the impacts and role of both short- and long-term microbial dormancy in glacial environments.

Microbial dormancy plays an important role in the persistence, dispersal, and functioning of microbial communities in moderate to extreme environments. The activity or inactivity of microbial communities also has implications for rates of biogeochemical transformations and thus elemental stocks and redox conditions. Microbial communities inhabiting glacier surface environments encounter harsh and variable environmental conditions including nutrient limitation, low temperatures, and light availability across various micro-habitats including cryoconite and the bare ice surface. The metabolic states of cells within these microhabitats and in relation to their environment is fundamental to the functioning of the ecosystem and has implications for ecosystem resilience, responses to environmental change, and biogeochemical cycling. This report describes an expedition to Brøggerhalvøya, north-west Svalbard, carried out in July 2025, within the framework of the ERC SIESTA project. A major objective of the project is to resolve microbial activity and dormancy on an individual cell basis, to characterise the adaptive and functional traits of active and dormant fractions of the native glacier microbial population, and to link microbial metabolic states to broader ecological and biogeochemical dynamics. Here we report the site characteristics, the samples collected, the analyses undertaken, and the future analyses planned. Two small valley glaciers near to Ny-Ålesund were selected for investigation during this summer campaign: Midtre Lovénbreen and Austre Brøggerbreen. The data collected in the field, combined with subsequent laboratory analyses, will provide insights into the spectrum of dormancy and activity in situ among glacier microbial communities, and the taxa and functions associated with active and inactive fractions of the communities. These findings will contribute to a deeper understanding of the impacts and role of both short- and long-term microbial dormancy in glacial environments.

Recent studies have shown that diazotrophs can form aggregates sinking at velocities up to 400 m d-1, challenging the long-standing assumption that these organisms are confined to the surface ocean, and suggesting an understimated role in carbon (C) sequestration in warm oligotrophic waters. Yet, the extent to which their biomass escapes remineralization in the mesopelagic zone remains poorly constrained. Here, we experimentally simulated the aggregation and sinking of the filamentous diazotroph Trichodesmium erythraeum using roller tanks, following organic matter distribution over a 10-day period-equivalent to a 1000 m descent at a sinking velocity of ~100 m d-1. Our results show that 33% of organic C and 36% of N remained in the particulate fraction at the end of the experiment, indicating that microbial remineralization was incomplete and relatively proportional. Remineralization was most intense during the first 3 to 5 days of descent (0-500 m), after which potential C flux declined. We also estimated that a substantial portion of bacterial biomass was incorporated into the aggregates and may contribute to the C export, revealing a dual role for bacteria as both recyclers and exporters of organic matter. Given the widespread distribution and high productivity of Trichodesmium erythraeum in the expanding (sub)tropical ocean, our results highlight the need to include its fate in global biogeochemical models.

Here we investigated the bioavailability of black carbon (BC)-derived dissolved organic matter (DOM) for a natural mixed community of marine heterotrophic prokaryotes. We ran an in vitro biodegradation experiment that took place over 3 months and exposed a community of organisms collected in the northwestern Mediterranean Sea (Bay of Marseille, France) to three different soluble fractions of BC prepared in the laboratory from various fossil fuel combustion particulates: standard diesel (DREF), oxidized diesel (DREF-OX), and natural samples of ship soot (DSHIP). Over the course of the three months, we observed significant decreases in the concentrations of dissolved organic carbon (DOC; from 9 to 21 %), dissolved BC (DBC; from 22 to 38 %) and dissolved polycyclic aromatic hydrocarbons (d-PAH; from 24 to 64 %) along with variability in the growth dynamics and activity of the heterotrophic prokaryotic community. The heterotrophic prokaryotic community exposed to DREF-OX treatment showed the highest values of respiration and production and the highest cell abundance, associated with the highest decrease in DOC (21 %) and d-PAH (64 %) concentrations. In the DREF and DSHIP treatments, prokaryotic activity was oriented towards anabolism. DREF treatment led to the highest decrease in DBC concentration (38 %). DSHIP treatment, which presented a substantially different d-PAH and dissolved metals content to the other two treatments, showed the lowest decreases in DOC, DBC and d-PAH concentrations, as well as the lowest prokaryotic activity and biomasses. Our results indicate that BC-derived DOM, including the most condensed fraction of this material, is partly bioavailable and therefore likely to be assimilated by marine prokaryotes. The origin of BC/soot deposited at the ocean surface turns out to be a key parameter that dictates the efficiency of biodegradation of its dissolved fraction by heterotrophic prokaryotes.

To gain insight into the impact of bottom-up changes in the plankton community on planktivorous fish in the context of the decline of small pelagic fisheries in the Northwestern Mediterranean Sea, we have conducted an extensive year-long study. The investigation combined biochemical analyses (proteins, carbohydrates, and lipids) with C and N stable isotope analyses (SIA) to simultaneously study phytoplankton, zooplankton, and eight planktivorous fish species (Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita, Sprattus sprattus, Cepola macrophthalma, Chromis chromis, Boops boops, and Spicara maena). This study is the first to analyze both stable isotope and biochemical compositions in coastal particulate organic matter (POM) size classes (0.7-2.7 mu m, 2.7-20 mu m, and 20-200 mu m), zooplankton size classes (200-300 mu m, 300-500 mu m, 500-1000 mu m, 1000-2000 mu m, and >2000 mu m), and taxonomic groups. We demonstrated that: (1) POM stable isotope compositions varied based on its spatial origin, the taxonomic composition of its biota, and its biochemical content; (2) delta N-15 values increased with zooplankton size classes and groups, indicating different trophic levels; (3) Phytoplankton exhibited a lipid-rich composition (similar to 55%), while zooplankton and fish muscles were protein-rich (similar to 61% and similar to 66%, respectively). Bayesian stable isotope mixing models revealed that, on average: (1) POM from oceanic waters contributed the most to the POM in the bay (>51%), with a dominance of pico-POM (similar to 43%); (2) The 200-1000 mu m zooplankton primarily consumed nano-POM, the 1000-2000 mu m zooplankton mostly consumed micro-POM (similar to 64%), and the >2000 mu m zooplankton also mostly consumed micro-POM; (3) Mesozooplankton (200-2000 mu m) constituted the main portion (similar to 42%) of the diet for planktivorous fish species, while macrozooplankton organisms (>2000 mu m) were the primary food resource (similar to 43%) for both B. boops and S. sprattus. Our study underscores the complexity of the pelagic food web and highlights the bottom-up transfer of organic matter from the smallest phytoplankton size fractions to planktivorous fish.

The recent development of biological sensors has extended marine plankton studies from conducting laboratory bench work to in vivo and real-time observations. Flow cytometry (FCM) has shed new light on marine microorganisms since the 1980s through its single-cell approach and robust detection of the smallest cells. FCM records valuable optical properties of light scattering and fluorescence from cells passing in a single file in front of a narrow-collimated light source, recording tens of thousands of cells within a few minutes. Depending on the instrument settings, the sampling strategy, and the automation level, it resolves the spatial and temporal distribution of microbial marine prokaryotes and eukaryotes. Cells are usually classified and grouped on cytograms by experts and are still lacking standards, reducing data sharing capacities. Therefore, the need to make FCM data sets FAIR (Findability, Accessibility, Interoperability, and Reusability of digital assets) is becoming critical. In this paper, we present a consensus vocabulary for the 13 most common marine microbial groups observed with FCM using blue and red-light excitation. The authors designed a common layout on two-dimensional log-transformed cytograms reinforced by a decision tree that facilitates the characterization of groups. The proposed vocabulary aims at standardising data analysis and definitions, to promote harmonisation and comparison of data between users and instruments. This represents a much-needed step towards FAIRification of flow cytometric data collected in various marine environments.

Diazotrophs are often limited by iron (Fe) availability in the oligotrophic ocean. The Western Tropical South Pacific (WTSP) ocean has been suggested as an intense N2 fixation area due to Fe fertilizations through shallow hydrothermal activity. Yet, the Fe demand of diazotrophs in their natural habitat, where they cohabit with other microbial organisms also requiring Fe, remains unknown. Here we develop and apply a method consisting of coupling 55Fe uptake experiments with cell-sorting by flow cytometry, and provide group-specific rates of in situ Fe uptake by the microbial community in the WTSP, in addition to bulk and size fractionation rates. We reveal that the diazotrophs Crocosphaera watsonii and Trichodesmium contribute substantially to the bulk in situ Fe uptake (~33% on average over the studied area), despite being numerically less abundant compared to the rest of the planktonic community. Trichodesmium had the highest cell-specific Fe uptake rates, followed by C. watsonii, picoeukaryotes, Prochlorococcus, Synechococcus and finally heterotrophic bacteria. Calculated Fe:C quotas were higher (by 2 to 52-fold) for both studied diazotrophs compared to those of the non-diazotrophic plankton, reflecting their high intrinsic Fe demand. This translates into a diazotroph biogeographical distribution that appears to be influenced by ambient dissolved Fe concentrations in the WTSP. Despite having low cell-specific uptake rates, Prochlorococcus and heterotrophic bacteria were largely the main contributors to the bulk Fe uptake (~23% and ~12%, respectively). Overall, this group-specific approach increases our ability to examine the ecophysiological role of functional groups, including those of less abundant and/or less active microbes.

In the dark ocean, the balance between the heterotrophic carbon demand and the supply of sinking carbon through the biological carbon pump remains poorly constrained. In situ tracking of the dynamics of microbial degradation processes occurring on the gravitational sinking particles is still challenging. Our particle sinking simulator system (PASS) intends to mimic as closely as possible the in situ variations in pressure and temperature experienced by gravitational sinking particles. Here, we used the PASS to simultaneously track geochemical and microbial changes that occurred during the sinking through the mesopelagic zone of laboratory-grown Emiliania huxleyi aggregates amended by a natural microbial community sampled at 105 m depth in the North Atlantic Ocean. The impact of pressure on the prokaryotic degradation of POC and dissolution of E. huxleyi-derived calcite was not marked compared to atmospheric pressure. In contrast, using global O-2 consumption monitored in real-time inside the high-pressure bottles using planar optodes via a sapphire window, a reduction of respiration rate was recorded in surface-originated community assemblages under increasing pressure conditions. Moreover, using a 16S rRNA metabarcoding survey, we demonstrated a drastic difference in transcriptionally active prokaryotes associated with particles, incubated either at atmospheric pressure or under linearly increasing hydrostatic pressure conditions. The increase in hydrostatic pressure reduced both the phylogenetic diversity and the species richness. The incubation at atmospheric pressure, however, promoted an opportunistic community of "fast " degraders from the surface (Saccharospirillaceae, Hyphomonadaceae, and Pseudoalteromonadaceae), known to be associated with surface phytoplankton blooms. In contrast, the incubation under increasing pressure condition incubations revealed an increase in the particle colonizer families Flavobacteriaceae and Rhodobacteraceae, and also Colwelliaceae, which are known to be adapted to high hydrostatic pressure. Altogether, our results underline the need to perform biodegradation experiments of particles in conditions that mimic pressure and temperature encountered during their sinking along the water column to be ecologically relevant.

Abstract. The surface ocean receives important amounts of organic carbon from atmospheric deposition. The degree of bioavailability of this source of organic carbon will determine its impact on the marine carbon cycle. In this study, the potential availability of dissolved organic carbon (DOC) leached from both desert dust and anthropogenic aerosols to marine heterotrophic bacteria was investigated. The experimental design was based on 16-days incubation, in the dark, of a marine bacterial inoculum into artificial seawater amended with water-soluble Saharan dust (D-treatment) and anthropogenic (A-treatment) aerosols, so that the initial DOC concentration leachate from aerosols is 36 µM C. Glucose-amended (G) and non-amended (control) treatments were run in parallel. Over the incubation period, an increase in bacterial abundance (BA) and bacterial production (BP) was observed first in the G-treatment, followed then by D and finally A treatments, with bacterial growth rates significantly higher in the G and D treatments than the A treatment. Following this growth, maxima of BP reached were similar in D (879 ± 64 ng C L−1 h−1; n = 3) and G (648 ± 156 ng C L−1 h−1; n = 3) treatments and were significantly higher than in A-treatment (124 ± 39 ng C L−1 h−1; n = 2). The DOC consumed over the incubation period was similar in A (9 ± 4 µM; n = 2) and D (9 ± 2 µM; n = 3) treatments and was significantly lower than that consumed in the G-treatment (22 ± 3 µM). Nevertheless, the bacterial growth efficiency (BGE) in the D treatment (14.2 ± 5.5 %; n = 3) compared well with the G treatment (7.6 ± 2 %; n = 3), suggesting that the metabolic use of the labile DOC fraction in both conditions was energetically equivalent. In contrast, the BGE in the A-treatment was lower (1.7 ± 0.1 %; n = 2), suggesting that the most part of used labile DOC was catabolized. The results obtained in this study highlight the potential of aerosol organic matter to sustain the metabolism of marine heterotrophs and stress the need to include this external source of organic carbon into biogeochemical models, for a better constraining of the carbon budget.

The surface ocean receives important amounts of organic carbon from atmospheric deposition. The degree of bioavailability of this source of organic carbon will determine its impact on the marine carbon cycle. In this study, the potential availability of dissolved organic carbon (DOC) leached from both desert dust and anthropogenic aerosols to marine heterotrophic bacteria was investigated. The experimental design was based on 16 d incubations, in the dark, of a marine bacterial inoculum into artificial seawater amended with water-soluble Saharan dust (D treatment) and anthropogenic (A treatment) aerosols, so that the initial DOC concentration was similar between treatments. Glucose-amended (G) and non-amended (control) treatments were run in parallel. Over the incubation period, an increase in bacterial abundance (BA) and bacterial production (BP) was observed first in the G treatment, followed then by the D and finally A treatments, with bacterial growth rates significantly higher in the G and D treatments than the A treatment. Following this growth, maxima of BP reached were similar in the D (879 ± 64 ng C L−1 h−1; n=3) and G (648 ± 156 ng C L−1 h−1; n=3) treatments and were significantly higher than in the A treatment (124 ng C L−1 h−1; n=2). The DOC consumed over the incubation period was similar in the A (9 µM; n=2) and D (9 ± 2 µM; n=3) treatments and was significantly lower than in the G treatment (22 ± 3 µM; n=3). Nevertheless, the bacterial growth efficiency (BGE) in the D treatment (14.2 ± 5.5 %; n=3) compared well with the G treatment (7.6 ± 2 %; n=3), suggesting that the metabolic use of the labile DOC fraction in both conditions was energetically equivalent. In contrast, the BGE in the A treatment was lower (1.7 %; n=2), suggesting that most of the used labile DOC was catabolized. The results obtained in this study highlight the potential of aerosol organic matter to sustain the metabolism of marine heterotrophs and stress the need to include this external source of organic carbon in biogeochemical models for a better constraining of the carbon budget.

Sterilization is essential for discriminating biotic responses from abiotic reactions in laboratory experiments investigating biogeochemical processes of complex natural samples. However, the conventional methods used to effectively sterilize materials or culture media do not allow sterilizing complex natural samples while maintaining biogeochemical balances. The aim of this study was to develop a low-cost and easy-to-use method to obtain geochemically unmodified and sterilized samples from complex lacustrine or coastal marine ecosystems. In preliminary assays, the impact of several sterilization methods (autoclaving, chemical poisoning, microwave, UV irradiation) on the trace metals balances was studied using borosilicate glass (BG), fluorinated ethylene-propylene (FEP) or polyethylene terephthalate (PET) bottles. Unlike other methods, UV sterilization had minor effects on the distribution of dissolved trace metals. Additional tests using complex lacustrine and coastal marine samples under 10 g/L sediments were performed using a homemade UV sterilization chamber designed to simultaneously irradiate a large number samples. Results showed: •very reproducible UV tests in BG and FEP bottles•faster sterilization using FEP bottles than using BG bottles•low variations of dissolved trace metals concentrations, except for Al, Cu, Fe and Zn.

Linking atmospheric deposition to marine carbon and nutrient cycle is hampered by the lack of data on atmospheric fluxes of organic matter. To fill this gap, this study reports the first quantification of atmospheric fluxes of soluble organic carbon (SOC), nitrogen (SON) and phosphate (SOP) to the NW Mediterranean Sea. Simultaneous measurements of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphate (DOP) in the surface mixed layer (SML) allowed estimating the potential contribution of atmospheric fluxes to marine DOC, DON and DOP inventories. We found an annual atmospheric flux of 59 mmol Cm-2 year(-1) for SOC, 16.4 mmol N m(-2) year(-1) for SON and 23.6 mu mol P m(-2) year(-1) for SOP, with proportions of SON and SOP to total soluble nitrogen and phosphate of 40% and 25%, respectively. Assuming these annual fluxes valid for the entire western Mediterranean basin, atmospheric fluxes would be higher than DOC, DON and DOP fluxes from the Rhone River by a factor of 6, 17 and 2, for C, N and P, respectively. Inventories of DOC, DON and DOP in the surface mixed layer displayed similar trends over the study period with maximum values at the end of the stratification period. DOP contributed by 85 +/- 11% to total dissolved phosphate (TDP) pool and exhibited a labile fraction (LOOP) of 27 +/- 19%. The contribution of atmospheric deposition to the DOC, DON and DOP pools in the SML, estimated for the stratification period, was low for C (3%) and P (4.5%) and moderate for N (12%). The labile fraction of atmospheric SOP (LSOP) was quantified throughout the sampling period and showed a high variability ranging from 0 to 97%. Atmospheric fluxes of LSOP contributed by 7% to marine LDOP pool and could sustain up to 8% of the heterotrophic prokaryotic phosphate demand in the SML of the NW Mediterranean Sea during the stratification period. The results obtained in this study stress the need to include atmospheric fluxes of organic matter in marine biogeochemical models to achieve a more complete picture of carbon and nutrient cycle in the Mediterranean Sea.

Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, ∼ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated; ∼ 7–15 % of the total fixed N (net N2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 ± 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 ± 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred ( ∼ 65–70 %), followed by heterotrophic bacteria ( ∼ 23–34 %). The DDN transfer in bacteria was higher (34 ± 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 ± 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium ( ∼ 5–9 % of the DDN transfer) than when it was dominated by UCYN-B ( ∼ 28 ± 13 % of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium.

In the eutrophic Ghar El Melh Lagoon (GML, Tunisia), the distribution of heterotrophic prokaryotes, pico- and nanophytoplankton was studied at five stations in November 2012 at the single cell level, along with environmental factors. Flow cytometry analysis of ultraplankton (<10 μm) resolved (i) two heterotrophic prokaryote groups, low and high nucleic acid contents (LNA and HNA, respectively), and (ii) eight to nine ultraphytoplankton groups (cryptophyte-like cells, two nanoeukaryote subgroups, two picoeukaryote subgroups and three Synechococcus-like cells subgroups). Prochlorococcus was not detected. According to redundancy analysis (RDA), a significant difference was found in the distribution of the ultraplankton between stations (F = 2.61, p < 0.05); maximum proliferations of heterotrophic prokaryotes were observed in the inner parts of the lagoon at stations 3, 4 and 5 affected by urban, agricultural and industrial discharges. Ultraphytoplankton concentrations were the highest near the outlet of the lagoon at stations 1 and 2 influenced by freshwater outflow and oligotrophic Mediterranean water inflow, respectively. At station 1, the large ultraphytoplankton concentration derives from the high abundance of cryptophyte-like cells favoured by the freshwater outflow whereas at station 2, the input of oligotrophic Mediterranean water enhanced the abundance of Synechococcus and picoeukaryotes at the expense of nanoeukaryotes. Two trophic regimes were thus differentiated in GML.

Surface colonization in seawater first corresponds to the selection of specific microbial biofilm communities. By coupling flow cytometry, microscopy and high throughput sequencing (HTS, 454 pyrosequencing) with artificial surfaces and environmental analyses, we intend to identify the contribution of biofilm community drivers at two contrasted French sites, one temperate and eutrophic (Lorient, Atlantic coast) and the other at a mesotrophic but highly contaminated bay (Toulon, North-Western Mediterranean Sea). Microbial communities were shaped by high temperatures, salinity and lead at Toulon by but nutrients and DOC at Lorient. Coatings including pyrithione exhibited a significant decrease of their microbial densities except for nanoeukaryotes. Clustering of communities was mainly based on the surface type and secondly the site, whereas seasons appeared of less importance. The in-depth HTS revealed that γ- and α-proteobacteria, but also Bacteroidetes, dominated highly diversified bacterial communities with a relative low β-diversity. Sensitivity to biocides released by the tested antifouling coatings could be noticed at different taxonomic levels: the percentage of Bacteroidetes overall decreased with the presence of pyrithione, whereas the α/γ-proteobacteria ratio decreased at Toulon when increased at Lorient. Small diatom cells (Amphora and Navicula spp.) dominated on all surfaces, whereas site-specific sub-dominant taxa appeared clearly more sensitive to biocides. This overall approach exhibited the critical significance of surface characteristics in biofilm community shaping.

After the exponential growth phase, variability in the scattering efficiency of phytoplankton cells over their complete life cycle is not well characterised. Bulk measurements are impacted by senescent cells and detritrus. Thus the analysis of the evolution of the optical properties thanks to their morphological and/or intra-cellular variations remains poorly studied. Using the Cytosense flow cytometer (CytoBuoy b.v., NL), the temporal course of the forward and sideward efficiencies of two phytoplankton species (Thalassiosira pseudonana and Chlamydomonas concordia) were analyzed during a complete life-cycle. These two species differ considerably from a morphological point of view. Over the whole experiment, the forward and sideward efficiencies of Thalassiosira pseudonana were, on average, respectively 2.2 and 1.6 times higher than the efficiencies of Chlamydomonas concordia. Large intra-species variability of the efficiencies were observed over the life cycle of the considered species. It highlights the importance of considering the optical properties of phytoplankton cells as a function of the population growth stage of the considered species. Furthermore, flow cytometry measurements were combined with radiative transfer simulations and biogeochemical and optical measurements. Results showed that the real refractive index of the chloroplast is a key parameter driving the sideward signal and that a simplistic two-layered model (cytoplasm-chloroplast) seems particularly appropriate to represent the phytoplankton cells.