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International audience; In order to understand the effect of human practices on microbial mats organisation, the study aimed to investigate the biodiversity within microbial mats from exploited and abandoned salterns. Despite several attempts, archaeal 16S rRNA gene fragment sequences were not obtained, indicating that microbial mats were probably dominated by Bacteria with very low abundance of Archaea (< 1%). Thus, the study compared the bacterial and meiofaunal diversity of microbial mats from abandoned and exploited salterns. The higher salinity (101 ± 3.7 psu vs. 51.1 ± 0.7 psu; Welch t-test p < 0.05) of the exploited site maintained lower bacterial diversity in comparison to the abandoned site where the salinity gradient was no longer maintained. However, the microbial mats exhibited similar bacterial class composition while the eukaryotic diversity was significantly higher in the exploited saltern. The abandoned saltern was dominated by sulfate-reducing bacteria and Nematoda, while the exploited saltern was characterized by the presence of halophilic bacteria belonging to Marinobacter, Salinivibrio and Rhodohalobacter genera, and the larger abundance of Hypotrichia (ciliates). Such bacterial and eukaryotic diversity difference might be explained by human actions for salt recovery in exploited salterns such as scraping the surface of microbial mat and increasing salinity renewing the microbial mat each year. Such action decreases the bacterial diversity changing the food web structure that favour the presence of a larger diversity of eukaryotic organisms. Our study provides new insights on microbial mat communities inhabiting salterns, especially the consequences of abandoning saltern exploitation.

International audience; Digestion-resistant bacteria (DRB) refer to the ecological bacterial group that can be ingested, but not digested by protistan grazers, thus forming a specific type of bacteria-protist association. To test the hypothesis that the environment affects the assembly of DRB in protists, a mixotrophic ciliate, Paramecium bursaria, and a heterotrophic ciliate, Euplotes vannus, were reared at different temperatures, light conditions, and concentration gradients of antibiotic oxytetracycline and heavy metals. Community profiling indicated that the composition of DRB in both species varied significantly across the manipulated conditions, except for in P. bursaria under light/dark treatments. Clone library analysis of bacterial 16S rRNA genes showed that DRB were diverse. Pseudomonas became more abundant during the warmer treatment of P. bursaria, whereas the dominance of Pseudoalteromonas weakened and Vibrio became more abundant in E. vannus at a higher temperature. During the treatment of diel light:dark cycles, Aestuariibacter and Alteromonas were selected for in E. vannus but not Pseudoalteromonas, which was highly represented in the all-light and alldark treatments. In contrast, P. bursaria consistently hosted Nevskia, Curvibacter, and Asticcacaulis under all light conditions. There were many bacterial species co-resistant to oxytetracycline and to protistan digestion, in which Sphingomonas, Alteromonas, Aestuariibacter, Puniceicoccaceae (Verrucomicrobia), Pseudomonas, and Sulfitobacter were frequently abundant. Flectobacillus and Aestuariibacter were major lead-resistant bacteria associated with the studied protists. Acinetobacter and Hydrogenophaga were abundant in the P. bursaria treated with a high dose of mercury. Aestuariibacter was found as a dominant group of DRB in E. vannus across all cadmium treatments. In summary, this study demonstrates for the first time that environmental stress selects for bacterial populations associated with protists and that there are diverse bacterial species that not only are resistant to pollution stresses but can also survive protistan predation. This work highlights that bacteria-protists associations need to be taken into account in understanding ecological and environmental issues, such as resilience of bacterial community and function, microbial co-occurrence, and quantity and distribution of antibiotic resistant bacteria and genes.

International audience; Estuarine intertidal flats strong biological productivity is mainly based on the activity of benthic microalgae communities or microphytobenthos (MPB), mostly dominated by diatoms. Epipelon is a major MPB growth form comprising motile species, which perform repeated "vertical migration" patterns in the upper sediment layers according to tidal and diurnal cycles with upward migration at the beginning of the daylight emersion and downward migration before immersion starts. Although this fascinating behavior has been extensively studied for more than a century, many of its features remain uncharacterized. Epipelon migratory rhythms are believed to be driven by an endogenous internal clock of unknown nature in combination with diverse environmental stimuli. Among the environmental stimuli impacting on MPB vertical migration, light is probably the most important. Rhythmic changes in surface abundance of natural MPB assemblages were therefore continuously assessed at high frequency by Imaging-PAM fluorimetry in fresh sediment sampled at different seasons, comprising 85 migration profiles from 40 sediment samplings over 2 years, and exposed to different light conditions without any other environmental stimuli (i.e., no tidal-like water flow and stable optimal temperature). In particular, we manipulated (i) the 24-h natural photoperiod MPB that was acclimated to in order to disentangle the tight link between the diurnal and tidal rhythmicity of epipelon migration, and (ii) the light spectrum in order to potentially influence MPB accumulation at the surface of sediment. We found that the migration rhythmicity mapped onto the tidal cycle but that it was modulated, and even overridden, by the diurnal cycle and by the irradiance level during daytime periods with a positive phototactic upward migration up to a certain threshold (in our conditions, 120 µmol photons m −2 s −1 of white light). Also, we found blue wavelengths (465 nm) triggered MPB surface accumulation, as compared to other wavelengths (white, green, and red) in patterns that were intensity-dependent and species-dependent. In particular, we foundtwo species, Navicula spartinetensis and Gyrosigma fasciola, which strongly migrate up under blue light and could potentially be used as model species for further studying the light-responses of intertidal MPB.

International audience; The ecology of leptocephali remains poorly known but they appear to feed on marine snow that can vary spatially and temporally according to the food web dynamics. This study provided new information about the position of leptocephali within the functional structure of microbial plankton and other food web components of the western South Pacific (WSP) region at a large geographic scale including the New Caledonia, Fiji, and Samoa islands. The hydrographic structure varied with latitude, and nutrient levels were generally low but somewhat variable. Stable isotopic signatures were examined in relation to the 3 current zones of the eastward flowing South Equatorial Countercurrent (SECC; north), the westward South Equatorial Current (SEC; mid-latitudes), and the eastward South Tropical Countercurrent (STCC; south), and all zones were found to be primarily based on a heterotrophic planktonic functioning that were co-limited by inorganic nitrogen and phosphorus, with biomasses of planktonic groups varying with depth. Isotopic signatures of leptocephali were compared to the signatures of other mesozoplankton, micronekton, and Trichodesmium components of the food web, and in relation to the signatures of particulate organic matter (POM) that varied among the 3 collection depths. The isotopic signatures of six taxa of leptocephali, other taxonomic groups and POM showed interesting variability according to latitude and among some stations. The presence of Trichodesmium at the surface in the STCC zone influenced the isotopic signatures of POM and thus the signatures of leptocephali. The signatures of leptocephali were therefore linked with the overall food web and were consistent with the larvae feeding on marine snow components of POM. The two apparent groups of leptocephali with different isotopic signatures that have also been observed in other oceanic areas may be explained by feeding behavior at different depths or on different types of marine snow.

This paper is a practical contribution to two important debates raised by the implementation of marine ecosystem based management: (i) which ecosystem data can be provided by a fisheries survey optimised for ecosystem monitoring; and (ii) how to combine/select potential indicators to derive useful information on marine ecosystem status and dynamics? A suite of 143 potential indicators, including spatial indices, representing small pelagic fish and their biotic and abiotic environments are presented. Indicators were routinely derived from the PELGAS integrated ecosystem survey conducted in spring in the Bay of Biscay (BoB). The general patterns over time in this suite of 5–16 years, non-stationary time series are characterised using a methodology based on min–max autocorrelation factors (MAF), to select the most continuous indicators within, and across, several ecosystem components: hydrology, phytoplankton, mesozooplankton, small pelagic fish and megafauna. Potential interactions between selected indicators and external forcing variables, including climate and fishing, were assessed. The results confirm the importance of river discharges, bottom temperature, chlorophyll-a and mesozooplankton biomass in the dynamics of the BoB pelagic ecosystem. Small pelagic fish species appear to have followed distinct trajectories during the last 15 years. A marked decrease in anchovy and sardine mean weights at ages 1 and 2 over the last 15 years was highlighted; potentially caused by density-dependent competition. The quasi-absence of significant correlation between selected survey indicators and climate and fishing pressure proxies suggests a moderate exploitation rate of small pelagic fish resources, and confirms the so far limited effects of large-scale climate forcing on the BoB pelagic ecosystem. Perspectives for the assessment of marine ecosystem status on the basis of suites of indicators derived from integrated ecosystem surveys are discussed.

International audience; To assess one of the dimensions of mesozooplankton functional diversity, this study quantifies energy density during the springtime in the Bay of Biscay considering both taxonomic and size-classes diversity. Energy density among copepods species (Centropages typicus, Anomalocera patersoni, Calanus helgolandicus, and Labidocera wollastoni), as well as anchovy eggs (Engraulis encrasicolus) ranges from 0.5 to 6.7 kJ/g on a taxonomic basis. Considering size-classes, energy density varies from 0.74 to 1.26 kJ/g. C. helgolandicus exhibits with average energy density estimates generally higher in the plume of the Gironde estuary. In contrast, no spatial coherence is found in the variability of mesozooplankton energy density by size-classes. Our results show that the mesozooplanktonic resource is not homogeneous in terms of quality in the Bay of Biscay. During spring, some species and some geographical areas seems thus to be more profitable to predators than others. We argue that the energy density is a key functional trait of mesozooplankton, but the assessment of mesozooplankton quality should be preferentially based on taxonomy rather than on size-classes. We conclude that interspecific and spatial variability of energy density among the mesozooplankton community can have important implications on fish population dynamics.

In intertidal sediments, circadian oscillations (i.e., tidal and diel rhythms) and/or depth may affect prokaryotic activity. However, it is difficult to distinguish the effect of each single force on active community changes in these natural and complex intertidal ecosystems. Therefore, we developed a tidal mesocosm to control the tidal rhythm and test whether diel fluctuation or sediment depth influence active prokaryotes in the top 10 cm of sediment. Day- and nighttime emersions were compared as they are expected to display contrasting conditions through microphytobenthic activity in five different sediment layers. A multiple factor analysis revealed that bacterial and archaeal 16S ribosomal RNA (rRNA) transcript diversity assessed by pyrosequencing was similar between day and night emersions. Potentially active benthic Bacteria were highly diverse and influenced by chlorophyll a and phosphate concentrations. While in oxic and suboxic sediments, Thaumarchaeota Marine Group I (MGI) was the most active archaeal phylum, suggesting the importance of the nitrogen cycle in muddy sediments, in anoxic sediments, the mysterious archaeal C3 group dominated the community. This work highlighted that active prokaryotes organize themselves vertically within sediments independently of diel fluctuations suggesting adaptation to physicochemical-specific conditions associated with sediment depth.

International audience; Evaluations of the functioning of benthic marine food webs could be improved by quantifying organic matter fluxes from the meiofauna to higher trophic levels. In this study, we measured the simultaneous ingestion of meiofauna and macrofauna by common dwellers of a tropical intertidal mudflat on the coast of Amazonia. The meiofauna and macrofauna (tanaid) communities of a tropical intertidal mudflat of French Guiana were separately enriched with 15N and 13C, respectively. The enriched preys were then used as tracers during feeding experiments with common predators of different sizes and feeding mechanisms: a Portunidae crab (Callinectes bocourti), a Penaeidae shrimp (Farfantepenaeus subtilis), and a Gobiidae fish (Gobionellus oceanicus). In feeding experiments with all predators except crabs, feeding rates increased with the availability of meiofauna and macrofauna food sources. The ability of consumers to ingest their food selectively was evaluated by calculating the differences in the ratio of macrofauna to meiofauna between the (1) ingested material and (2) that available in the environment. Larger predators showed a higher degree of preferential macrofauna ingestion than smaller predators, consistent with the optimal foraging theory. For large predators, the meiofauna would be important only during early life or in the absence of large food items.

International audience; Mesozooplankton can be considered the most important secondary producers in marine food webs because they hold an intermediate position between the phytoplankton assemblage and the upper trophic levels. They also are a robust indicator of climatic and hydrological conditions. We conducted an analysis of the interannual variability of the spring mesozooplankton assemblage, as sampled by the PELGAS fisheries survey in the southern part of the Bay of Biscay (Northeast Atlantic Ocean) between 2003 and 2013. We examined hydrology and trophic drivers to explain the variability. Our results revealed that the subsurface temperature, the subsurface salinity, the biomasses of subsurface pico-, nano-, and microphytoplankton, and the copepod assemblage exhibited a recurrent spatial pattern that was driven mainly by freshwater and nutrient inputs from the main rivers. The mesozooplankton assemblage was dominated by copepods (82%), composed of coastal, neritic, and oceanic copepod genera that paralleled the various hydrological fronts converging in the southern Bay of Biscay. The copepod community displayed high temporal-variability; there were three periods of abundant adult copepods throughout the southern Bay of Biscay. The copepod community was structured primarily around the drive for resource control, especially by the microphytoplankton biomass (24.3% of the total variability), and to a lesser extent by hydrological features (13.7% of the total variability).

International audience; Coccolithophore blooms occur regularly from April to June in the Bay of Biscay where they have been observed for many years from ocean-colour imagery thanks to the ability of their calcite plates to scatter light. They are easily depicted on interpolated images of non-algal Suspended Particulate Matter (SPM) derived from satellite reflectance data in May, at the time of the PELGAS (PELagique GAScogne) annual surveys. Over the springs 2012 to 2015, the PELGAS surveys provided in-situ data on the turbidity throughout the water column and on the hydrological environment prevailing during the blooms in the vicinity of the continental shelf break. The satellite-derived SPM in the area of coccolithophore blooms was closely related to measured turbidity in the euphotic zone. The deep maxima of coccolithophore-derived biomass were well observed in the turbidity signal throughout the water column. Despite the recurrence of SPM patterns due to coccolithophores in the well-mixed waters over the shelf break in spring, the variability of the satellite-derived SPM and the turbidity profiles was particularly high during the four years studied. The year 2013 showed persistent blooms from mid-April to the end of May over large areas inside and outside the shelf waters, whereas no bloom was observed in 2014, neither on SPM images nor through turbidity profiles. The presence or absence of coccolithophore blooms during the years 2012-2015 seem related to the level of stratification of the water column, with more intense blooms occurring during mixed conditions. These results should improve classification of the PELGAS surveys from those occurring in a coccolith-rich environment, synonymous with mixed and cold waters, to those occurring in a coccolith-poor environment where surface waters are warmer and more stratified.

International audience; Microphytobenthos communities of different sediment types were investigated in intertidal flats of the coastal area around La Rochelle (Pertuis Charentais, France) in July 2014 and January 2015. Biotic variables of biomass, abundance and species composition of microphytobenthos were evaluated together with environmental variables, including irradiance, sediment temperature, grains size, pore water salinity, pH, nutrients, organic matter, water content and heavy metal concentrations in the sediment. The relationships between biotic and environmental parameters showed that: (1) the microphytobenthos biomass and community structures showed significant differences among different sediment types; (2) variation in the microphytobenthos communities were significantly correlated with environmental variables, especially with the heavy metals, grain size, irradiance, pore water salinity, organic matter and concentration of PO4 3+ and Si(OH)4; (3) the species number and richness were both significantly correlated with organic matter, and (4) the species Entomoneis corrugate, Navicula aitchelbee and Gyrosigma cf.limosum were positively correlated to heavy metals, and so were N. phylleptosoma, Surirella brebissonii to Si(OH)4 and NH4 + concentrations, and G. acuminatum and S. brebissonii to PO4 3+ and NH4 +. It is suggested that the spatial variation in biodiversity of microphytobenthos may reflect environmental quality status in coastal intertidal ecosystems.

In tropical South America, the mudflats of the Amazonian coast are unique because of their large size and unrivaled migration dynamics. On Guiana’s coast, macrofaunal communities are believed to be well-adapted to these dynamic conditions. In this study, the benthic macrofauna was sampled in April 2012 in the Awala-Yalimapo region of western French Guiana and at two sites in Suriname: Warappa Kreek and Bigi Pan. These sites are found 800, 920, and 1140 km from the Amazon delta, respectively. The richness, diversity, and densities of the macrofaunal communities in these mudflats are here described for the first time. Only 38 operational taxonomic units (OTUs) were recorded, among which two species were common and widely distributed: the tanaid crustacean Halmyrapseudes spaansi and the polychaete Sigambra grubii; the former represented 84% of all individuals collected, with densities reaching up to 73,000 individuals m−2. Most of the OTUs consisted of relatively small individuals (

International audience; Temperature is an important factor for fish. Yet, little is known about its effects on the feeding behavior of fish and the subsequent consequences of these behavioral changes on the spatial distribution of resources. We analyzed the differences in the feeding behavior of two sizes of juvenile Liza aurata at two water temperatures (i.e. 10°C and 20°C), using laboratory mesocosms. We also examined whether temperature-induced changes in feeding behavior of the smaller size of L. aurata would affect the spatial distribution of the microphytobenthos (MPB) biomass, an important resource in coastal systems. Both the number of feeding events and the swimming velocity during feeding in juvenile L. aurata were higher at 20°C than at 10°C, independently of the fish size. The time spent feeding did not vary between 10°C and 20°C, while the distance covered during feeding was significantly smaller at 20°C than at 10°C. Grazing did not affect the mean MPB biomass, but did increase its spatial variance at the smaller scale (i.e. a few centimeters) at 20°C. A high number of feeding events, a high swimming velocity during feeding and a small distance covered during feeding in 20°C-acclimated L. aurata most likely represented an adaptation to an increase in metabolism, as well as to the need to reduce the energy costs of feeding at 20°C. Results also indicated that changes in feeding behavior of the 20°C-acclimated L. aurata were responsible for the increase in small-scale spatial variability in the MPB biomass. We suggested that the enhanced spatial patchiness due to grazing by fish at 20°C might yield a local increase in the mean MPB biomass, probably increasing photosynthetic efficiency of cells and algal growth that counterbalance the negative effect of algal removal by fish, and resulting in the lack of an overall significant effect on the mean.

International audience; Grey mullets are unique among temperate-region fish species in their ability to feed on mudflat biofilm. In this study, we examined mullet feeding strategies on biofilm and associated meiofauna by using a diet study and stable isotope analysis to explore functional interactions between mullets and tidal flats. A stomach vacuity investigation showed that mullets did not import any materials from subtidal areas into the mudflat but exported mud, biofilm, and associated meiofauna. The results of mullet stomach content and fecal analyses, when compared to the availability of tidal flat resources, showed evidence of mullets' ability to ingest and assimilate biofilm and to concentrate major meiofauna grazers such as nematodes, copepods and, secondarily, foraminifers and ostracods. Isotopic ratios confirmed diet investigations, and as recently shown in salt marsh habitats, mullets exhibited an intermediate trophic position, supporting the hypothesis that they can assimilate both biofilm and major meiofauna grazers. The function of the tidal flat as a feeding habitat for gray mullets and the role of mullets as the main export pathway of biofilm from tidal flat ecosystems are discussed.

International audience; In intertidal marine sediments, characterized by rapidly fluctuating and often extreme lightconditions, primary production is frequently dominated by diatoms. We performed a comparativeanalysis of photophysiological traits in 15 marine benthic diatom species belonging to the fourmajor morphological growth forms (epipelon (EPL), motile epipsammon (EPM-M) and non-motileepipsammon (EPM-NM) and tychoplankton (TYCHO)) found in these sediments. Our analysesrevealed a clear relationship between growth form and photoprotective capacity, and identified fastregulatory physiological photoprotective traits (that is, non-photochemical quenching (NPQ) and thexanthophyll cycle (XC)) as key traits defining the functional light response of these diatoms. EPMNMand motile EPL showed the highest and lowest NPQ, respectively, with EPM-M showingintermediate values. Like EPL, TYCHO had low NPQ, irrespective of whether they were grown inbenthic or planktonic conditions, reflecting an adaptation to a low light environment. Our resultsthus provide the first experimental evidence for the existence of a trade-off between behavioural(motility) and physiological photoprotective mechanisms (NPQ and the XC) in the four majorintertidal benthic diatoms growth forms using unialgal cultures. Remarkably, although motilityis restricted to the raphid pennate diatom clade, raphid pennate species, which have adopted anon-motile epipsammic or a tychoplanktonic life style, display the physiological photoprotectiveresponse typical of these growth forms. This observation underscores the importance of growthform and not phylogenetic relatedness as the prime determinant shaping the physiologicalphotoprotective capacity of benthic diatoms.

Dinophysis is the most harmful toxic phytoplankton on the French coast in terms of its impact on local economy and public health. In Arcachon Bay, Dinophysis spp. have periodically affected shellfish industry for the last ten years; the most important events are analysed in detail in this paper. Regular monitoring revealed that these events originated outside Arcachon Bay in the open ocean. Data from 14 surveys and two coastal networks showed that Dinophysis was primarily found in the vicinity of Capbreton, 100 km south of the mouth of Arcachon Bay. The Dinophysis distribution on the continental shelf was determined during two surveys in 2005 and 2008: the highest concentrations were located along the coast and reached 18 000 cells.L − 1 . Analysis of available current data revealed that strong westerlies lead to northward currents of up to 19 cm.s − 1 . These marine meteorological conditions were frequently observed just prior to Dinophysis events and lead us to suggest that northward currents transport Dinophysis from the Capbreton area to Arcachon Bay.

International audience; Parasitological data are increasingly used to provide information on host populations, trophic interactions and free-living biodiversity. Here, parasitic helminths have been researched in Sardina pilchardus and Engraulis encrasicolus from the Bay of Biscay (North-East Atlantic). In addition, a review of helminths infecting E. encrasicolus has been provided. Surprisingly, our field study reveals the occurrence of only three species of Ascaridoidea (Anisakis simplex sensu stricto, Anisakis pegreffi and Hysterothylacium aduncum) in both fish species despite the high diversity of helminth parasites (mostly heteroxenous trophically-transmitted) recorded throughout their distribution range, i.e. 39 and 15 taxa, respectively, for S. pilchardus and E. encrasicolus: among them, 12 shared taxa. This low diversity of helminth parasites in the Bay of Biscay may be indicative of a low free-living biodiversity. Moreover, similarities in the helminth parasites of S. pilchardus and E. encrasicolus, recorded both in our field study and in their entire geographical distribution, suggest ecological and feeding similarities between these two planktivorous fish. However, the higher prevalence and mean intensity of Ascaridoidea in E. encrasicolus (vs. S. pilchardus) in the Bay of Biscay also highlighted differences, which were considered the result of its higher trophic plasticity.

International audience; Coral reef and atoll lagoons are among the most diversified marine ecosystems but also the most affected by the combined effects of climate change and human activities. The Iles Eparses (Scattered Islands) in the Western Indian Ocean have been little affected by human pressure and can be considered to be "pristine" ecosystems. Metazooplankton plays a major role in the functioning and productivity of aquatic ecosystems, and this study was undertaken: (i) to determine the spatial abundance, distribution and species composition of metazooplankton, (ii) to assess the effect of metazooplankton grazing on pico- and nanophytoplankton and (iii) to analyze the trophic positions of metazooplankton by using the stable isotope signatures of a wide variety of taxa and particulate organic matter from the Iles Eparses and Mayotte. Tromelin Island (which is not located in the Mozambique Channel) had the lowest metazooplankton abundance with no cyanobacteria Trichodesmium spp. or mollusks (pteropods) presence, and with δ(15)N signatures of organisms that were higher than for the islands in the Mozambique Channel. Trichodesmium spp. was found in the Mozambique Channel and the plankton food web was probably based preferentially on these cyanobacteria with lower δ(15)N signatures indicating direct or indirect trophic transfer of diazotrophic nitrogen to metazooplankton. Three of the islands were distinct: Europa had the highest proportion of copepods, with oithonids being dominant, which is typical of rich mangrove systems, while Juan de Nova and Mayotte seemed to be the sites most affected by human activity with a high abundance of appendicularians and distinct particulate organic matter ∂(13)C signatures. Grazing experiments showed that food could be a limiting factor for metazooplankton in the Iles Eparses. However, the effect of metazooplankton grazing on phytoplankton appeared to be very low (0.01-2.32% of the total phytoplankton per day).

International audience; Population dynamics and feeding habits of the testate amoebae Nebela tincta and Hyalosphenia papilio were studied along a short "fen" to "bog" gradient in a Sphagnum-dominated mire (Jura, France). Samples were collected in living "top segments" (0-3cm) and early declining "bottom segments" (3-6cm) of Sphagnum fallax peat. Observations of digestive vacuole content and stable isotope analyses ((13)C and (15)N) were used to establish the feeding behavior of both testate amoeba species. Owing to their vertical distribution, the feeding habit of H. papilio was described from top segments, and that of N. tincta from bottom segments. Among identified food sources, those most frequently ingested by N. tincta were spores and mycelia of fungi (55%), microalgae (25%) and cyanobacteria (8.5%). For H. papilio, the most frequently ingested prey were ciliates (55%) and microalgae (35%). Nonmetric Multidimensional Scaling analysis clearly demonstrated that the two species did not have the same feeding habit along the "fen-bog" gradient, and furthermore that a significant spatial split exists in the feeding behavior of H. papilio. Additionally, isotope analyses suggested that H. papilio and N. tincta did not have the same trophic position in the microbial food web, probably resulting from their different feeding strategies.

The trophic fate of benthic bacteria in an intertidal mudflat (Brouage, Marennes-Oleron, France) was evaluated in situ, and environmental parameters that potentially influence the consumption of bacteria by the most abundant organisms of the meio- and macrofauna were identified. Variations in grazing rates were estimated using 15N pre-enriched bacteria at different temporal and spatial scales on a cross-shore transect over the mudflat. Grazing incubations were performed in microcosms with freshly collected grazers. Environmental factors varied more by season than by day or sampling station. Bacterial uptake by grazers did not appear to be strongly influenced by abiotic factors and was not linked to bacterial abundance. Algal abundance was negatively correlated with bacterivory in both the nematode community and the foraminifer Ammonia tepida, suggesting that bacteria constitute an alternative resource that is consumed when algae are less abundant. Bacteria were mainly ingested by the mudsnail Hydrobia ulvae and secondarily by nematodes with grazing rates of copepods and A. tepida being considerably lower. The estimated grazing in the upper and middle part of the mudflat represented 7% and 28% of bacterial production, respectively. In the lower mudflat, daily grazing never represented >3% of bacterial production throughout the year. Consequently, grazing appears to be a minor factor in the regulation of bacterial production. Bacterivory did not vary clearly according to season; consequently, the fate of bacteria in this benthic food web is poorly structured by season.

International audience; The majority of sediment dweller foraminifera are deposit feeders. They use their pseudopodia to gather sediment with associated algae, organic detritus and bacteria. Uptake of bacteria by foraminifera have been observed but rarely quantified. We measured uptake of bacteria by the common foraminifera Ammonia tepida using 15N pre-enriched bacteria as tracers. In intertidal flats, seasonal, tidal and circadian cycles induce strong variations in environmental parameters. Grazing experiments were performed in order to measure effects of abiotic (temperature, salinity and irradiance) and biotic (bacterial and algal abundances) factors on uptake rates of bacteria. In mean conditions, A. tepida grazed 78 pgC ind-1 h-1 during the first eight hours of incubation, after which this uptake rate decreased. Uptake of bacteria was optimal at 30°C, decreased with salinity and was unaffected by light. Above 7 × 108 bacteria ml wt sed-1, uptake of bacteria remained unchanged when bacterial abundance increased. Algal abundance strongly affected algal uptake but did not affect uptake of bacteria. As uptake of bacteria represented 8 to 19% of microbes (algae plus bacteria) uptake, Ammonia seemed to be mainly dependant on algal resource.

Carpentier EA 7316; International audience; Leptocephali are the poorly-understood transparent larvae of elopomorph fishes that live in the ocean surface layer throughout the world’s tropical and subtropical oceans. Their feeding ecology has been difficult to understand because they appear to primarily feed on particulate organic material (POM), which contains few identifiable objects, and there have been few studies on their diets or trophic positions. This study presents the first results on the trophic position of c.a. 50 leptocephali belonging to 7 families of leptocephali that were compared to 30 taxa of other marine animals and to POM samples. To that end, the carbon and nitrogen stable isotope ratios were analyzed on specimens of leptocephali, various taxa of mesozooplankon, cephalopods, fishes, and POM collected west of the Mascarene Plateau in the western Indian Ocean. Nitrogen and carbon isotopic ratio analyses indicated that the 12 taxa of DNA barcoded leptocephali (≥15 species) could be separated into 2 groups of species with either higher (Group 1: 9 taxa of 7 families, 25–91 mm) or lower (Group 2: 3 taxa of 2 families, 43–275 mm) δ15N ranges. Group 2 exclusively included species that reach much larger sizes of > 150–200 mm (Nemichthys and Avocettina, 3 species of Ariosoma-type), whereas Group 1 included Anguilla bicolor bicolor, Serrivomeridae, Muraenidae, Congridae (3 species), Chlopsidae, Ophichthidae (2 species), and Thalassenchelys. Differences in feeding depths, the types of POM ingested by preference or because of different jaw morphology, or the transport of larvae from other regions with different isotopic signatures are possible reasons for the differences between the two groups. The isotopic signatures of 14 taxa of copepods had higher but slightly overlapping δ15N and δ13C signatures compared to leptocephali and most crustaceans and other mezozooplankton, cephalopods and mesopelagic fish taxa had even higher values. The δ15N and δ13C signatures and composition of POM were variable spatially and with depth and may have been influenced by particulates originating from the shallow banks of the Mascarene Plateau. The two apparent isotopic groups of leptocephali should be examined in relation to their consumption of POM, which can include various proportions of prokaryotes, phytoplankton, protozoans, discarded appendicularian houses and other materials, by conducting further studies in different regions and using a variety of techniques.

International audience; The strong biological production of estuarine intertidal flats is mainly supported by benthic diatoms in temperate areas. Their photosynthetic productivity is largely driven by changes in light intensity and temperature at the surface of sediment flats during emersion. The impact of an increase in salinity of the upper-layer sediment pore-water during emersion, which is often coupled with high light (HL), has been less studied. Furthermore, benthic diatoms show several growth forms which inhabit specific sediment types where the pore-water salinity can differentially vary due to the degree of cohesion of sediment grains. So far, no study explored if the main growth forms of benthic diatoms (i.e. epipelon, epipsammon and tychoplankton) show different photophysiological response to a combine high salinity-HL stress. Based on field monitoring, we compared the photophysiology (photosynthetic efficiency and photoprotection) of three representatives of the main growth forms during a short high salinity coupled with a moderate HL stress and stable optimal temperature, i.e. experimental conditions reproducing Spring environmental conditions in intertidal flats by the Atlantic French coast. Our results show that all growth forms reacted to HL exposure alone, as expected. While the epipelon representative was relatively insensitive to high salinity alone and combined with HL, the tychoplankton representative was highly sensitive to both, and the epipsammon representative was sensitive mainly to the stress combination. These specific responses fitted well with i) their natural habitat (i.e. more or less cohesive sediment) for which light climate and changes in salinity are different, ii) their growth form (i.e. motile, immotile or amphibious) which determines their probability to be confronted to a combined high salinity-HL stress. Hence, the negative effect of high salinity on photosynthetic efficiency of benthic diatoms appears to be mostly restricted to epipsammon and tychoplankton, and in field conditions, its effect probably remains negligible compared to HL stress.

International audience; Small pelagic fish represent an essential link between lower and upper trophic levels in marine pelagic ecosystems and often support important fisheries. In the Bay of Biscay in the north-east Atlantic, no obvious controlling factors have yet been described that explain observed fluctuations in European sardine Sardina pilchardus and European anchovy Engraulis encrasicolus stocks, in contrast to other systems. The aim of this study was therefore to investigate to which extent these fluctuations could be trophodynamically mediated. The trophic ecology of both fish species was characterised over three contrasting periods (spring 2010 and 2011 and autumn 2011) in the area, in relation to potential variation in the abundance and composition of the mesozooplankton resource. Stable isotope analyses of carbon (δ13C) and nitrogen (δ15N) were performed on potential mesozoplanktonic prey items and in the muscle of adult fish, as well as in the liver whenever available, and mixing models were applied. In both springs, the mesozooplankton resource was abundant but qualitatively different. During this period of the year, results based on muscle isotope values in particular showed that S. pilchardus and E. encrasicolus likely do not compete strongly for food. On the medium term, E. encrasicolus always presented a greater trophic plasticity than S. pilchardus, both in terms of feeding areas and in the size of the mesozooplanktonic prey consumed. In autumn, mesozooplankton abundances were lower, and it was likely that S. pilchardus and E. encrasicolus share food resources during this period. No clear links between the variation in the mesozooplanktonic resource and the trophic segregation maintained between adults of both fish species in spring could be made. Although a certain potential exists for trophodynamically mediated fluctuations of both species under specific abiotic conditions (i.e. due to the existing trophic segregation in spring in particular), the overall results suggest that fluctuations in abundance of both fish species are probably not directly linked to their trophic ecology in the Bay of Biscay, at least at the level of adult individuals.

International audience; In ecological systems, it is necessary to describe the trophic niches of species and their segregation or overlap to understand the distribution of species in the community. In oceanic systems, the community structure of top predators such as seabird communities has been well documented with many studies in several biogeographical areas. But for coastal habitats, very few investigations on the trophic structure have been carried out in avian communities. In this study, the trophic resource partitioning was investigated on eight of the most abundant species of a shorebird community on the central Atlantic coast of France. Our work comprised a comprehensive sample of birds with different ecomorphogical patterns and data on their main prey to encompass potential sources of overlap and segregation in this community. We examined the stable carbon (δ13C) and nitrogen (δ15N) isotopic composition of blood to investigate the trophic structure (1) on a temporal scale by comparing migration and wintering periods; (2) on a spatial scale through inter-site comparisons; and (3) on the community level within groups of phylogenetically related species. Diets appeared different in several cases between periods, between sites and between juveniles and adults for the same sites. A clear trophic partitioning was established with four functional groups of predators in winter inside the community. The Grey Plover, the Bar-tailed Godwit, the Curlew and a majority of the dunlins were worms-eater mainly feeding on Nereis diversicolor or Nephtys hombergii Two species were predominantly deposit-suspensivorous mollusc-eaters, including the Red Knot and the Black-tailed Godwit feeding mainly on Macoma balthica. The Oystercatcher fed mainly on suspensivorous molluscs like Cerastodrema edule and two species including the Redshank and some dunlins adopted opportunistic behaviours feeding on mudflat and/or in marshes.

International audience; Resuspension thresholds in terms of friction velocity were experimentally quantified for the prokaryotes, protists and for the first time, viruses of intertidal mudflat biofilms. Differences in resuspension thresholds could be related to the type, behaviour and size of microorganisms and their association with particles. Free microorganisms (viruses, bacteria and some nanoflagellates) were resuspended by weak flow at friction velocities lower than 2 cm s− 1. Chlorophyll a, some nanoflagellates and attached bacteria were resuspended together with the bed's muddy sediment, which required friction velocities larger than 3 cm s− 1. Diatoms smaller than 60 μm were resuspended at velocities between 3 and 5 cm s− 1, while those larger than 60 μm were resuspended at higher friction velocities (5.5 to 6.5 cm s− 1).The thresholds of resuspension also depended on the micro-scale position of microorganisms in the sediment (horizontal and vertical distributions). In the field, the vertical distribution of chlorophyll a (a proxy of microphytobenthos) was skewed, with a maximum in the first 2 mm of sediment. Along the neap–spring tidal cycle, chlorophyll a revealed an increase in MPB biomass in the first 2 mm of the sediment, in relation to light increases with exposure durations. The horizontal distribution of chlorophyll a could be inferred from erosion experiments. During the initial phase of biofilm growth, the distribution of chlorophyll a seemed horizontally homogeneous, and was uniformly eroded at the beginning of the increase in chlorophyll a. From these results, we can make a hypothesis: in the subsequent phase of biofilm growth until the maximum of emersion duration, the eroded quantity of chlorophyll a was larger than expected based from chlorophyll a vertical distribution, suggesting that biofilm horizontal distribution became patchy and enriched chlorophyll a was preferentially eroded. When emersion duration and biofilm growth decreased, the trend was reversed, and eroded quantity of chlorophyll a was lower than expected from chlorophyll a vertical distribution, suggesting that areas with low chlorophyll a were preferentially eroded. Such erosion patterns when biofilm growth decreased probably resulted from the bulldozing activity of a surficial sediment bioturbator, the gastropod Peringia ulvae. Our study did not directly prove this horizontal distribution but it should be further discussed. This distribution needs to be studied to acquire real evidence of patchy distributions.

International audience; In intertidal mudflats, during low tide exposure, microphytobenthos (MPB) migrate vertically through the surface sediment and form, with the heterotrophic bacteria, a transient biofilm. Inside this biofilm, multiple interactions exist between MPB and bacteria. These micro-organisms secrete a wide range of extracellular polymeric substances (EPS), which are major components of the biofilm matrix. In this study, we used a tidal mesocosm experiment in order to decipher the interactions of the MPB-EPS-bacteria complex within the biofilm. We tried to determine if the EPS could control bacterial activities and/or production and/or richness according to the age of the biofilm and to the immersion/emersion period. The dynamics of biomasses of MPB and prokaryotes, the bacterial production, the hydrolysis of predominating organic constituents in the dissolved organic carbon (DOC) pool (i.e., carbohydrates and polypeptides), and the bacterial structure were studied in relation to the different EPS fractions (carbohydrates and proteins: colloidal and bound) dynamics during 8 days. Our experiment had emphasized the influence of the environmental conditions (light, immersion/emersion) on the interactions within the biofilm and also on the effects on biofilm ageing. Bacterial production was always inhibited by the bound EPS-carbohydrate, especially during low tide. Our results suggest that the concentration and composition of EPS had a major role in the bacterial/MPB interactions: these interactions can be either positive or negative in order to regulate the productive phases of MPB and bacteria.

International audience; European sardine (Sardina pilchardus) and European anchovy (Engraulis encrasicolus) are two species of economical and ecological significance in the Bay of Biscay (north-east Atlantic). However, the trophic ecology of both species is still poorly known in the area, and more generally, few studies have considered the potential trophic overlap between sardines and anchovies worldwide. This study aims to highlight the trophic links between the mesozooplankton and adults of these two pelagic fish in the Bay of Biscay, through carbon and nitrogen stable isotopes analysis (SIA). Mesozooplankton and individuals of sardines and anchovies were collected during one season (spring 2010), over spatially contrasted stations within the study area. First, the potential effect of preservation (ethanol vs. freezing) and of delipidation (by cyclohexane) on mesozooplankton δ13C and δ15N values was assessed. Results demonstrated the necessity to correct for the preservation effect and for lipid contents in mesozooplankton for further analyses of sardines' and anchovies' diet through SIA. Next, this study highlighted the interest of working on identified mesozooplanktonic organisms instead of undetermined assemblages when unravelling food sources of planktivorous fish using stable isotopes. The inter-specific variability of isotope values within a planktonic assemblage was effectively high, probably depending on the various feeding behaviours that can occur among mesozooplankton species. Intra-specific variability was also significant and related to the spatial variations of baseline signatures in the area. To investigate the foraging areas and potential diet overlap of S. pilchardus and E. encrasicolus, mixing models (SIAR) were applied. Both fish species appeared to feed mainly in the neritic waters of the Bay of Biscay in spring and to select mainly small- to medium-sized copepods (e.g. Acartia sp., Temora sp.). However, E. encrasicolus showed a greater trophic plasticity by foraging more offshore and on a wider range of prey sizes, while S. pilchardus seemed more limited to coastal areas and the mesozooplanktonic species of these waters for feeding.

Relationships between bacteria, microphytobenthos and extracellular polymeric substances (EPS) that make up microbial biofilms over bare mudflats were investigated at an hourly frequency during two 14-day spring– neap cycles in winter and summer 2008. Bacterial abundance and total chl a concentration were lower in summer (0.78 × 108 ± SD 0.39 × 108 cell.m−2 and 59.0 ± SD 10.42 mgchla.m−2) than in winter (3.7 × 108 ± SD 1.9 × 108 cell.m−2 and 106.64 ± SD 11.29 mgchla.m−2), coinciding with a high abundance of the gastropod Peringia ulvae in summer, which subsequently impacted 1st-cm chl a concentration by intense grazing. Bound and colloidal EPS carbohydrate temporal patterns were similar in winter (5.71 ± SD 3.95 and 4.67 ± SD 3.45 μg.g−1, respectively) butwere different in summer (14.9±SD 4.05 and 5.60±SD 4.50 μg.g−1, respectively). Carbohydrate colloidal EPS appeared to be related to light and salinity, while 1st-mm chl a concentration was negatively affected by strong salinities and predation pressure by P. ulvae. The fluctuations of colloidal carbohydrates were remarkably similar in the two seasons with peaks just after spring tideswhen the highest irradiance was received by microphytobenthic cells. Apparently, colloidal EPS carbohydrates can protect cells against the high salinity values ranging from 32.3 to 50.4 PSU. The presence of bound EPS carbohydrates may be linked to sediment colonization and resistance of biofilm activity. Proteins in EPS were absent in winter and represented a small proportion in summer (10%), but they appeared to be a good indicator of potential synergistic effects between MPB and bacteria in summer. Conversely, bound EPS carbohydrates reached high levels in winter, while the number of bacteria decreased simultaneously, suggesting a negative effect on bacterial growth in the absence of proteins in EPS. There was a lower proportion (31%) of low molecular weight EPS in summer than in winter (83%), possibly in relation to desiccation.

Our study compared the suprabenthic fauna collected in the Bay of Saint-Brieuc (western English Channel) in September 1995 and May 1996 at two sites colonized by Crepidula fornicata with that of one site where it was absent (site 23-0). Where Crepidula was present, no species dominated and abundance re- mained low. At the site without Crepidula , abundance was higher and the mysid Haplostylus lobatus domi- nated the suprabenthic community. Swimming activity of most suprabenthic species was increased by the presence of C. fornicata . The differential abundance and swimming activity of certain species are linked to the conversion of the formerly soft substrate to now hard, shelled substrate.

In French Atlantic coastal ponds of Charente, oysters can grow under conditions where phytoplankton production is limited by nutrients exhaustion. Such ponds typically show a high concentration of ciliates and flagellates during the growing season (1 x 104 to 3 x 105 cells l-1 in June 1997). In order to evaluate the importance of the "protozoan trophic link " for energy transfer from the " microbial food web" to large benthic suspension feeders, we offered a coastal pond community of ciliates and flagellates as potential prey to the oyster, Crassostrea gigas. Clearance rate, filtered particles and relative retention efficiency were evaluated. In the grazing experiment, 94 % of ciliates and 86 % of flagellates (size between 4 and 72 μm), were retained by the oyster. Whatever their size, protists were similarly retained by the oyster gills. In terms of carbon, oyster retain on average 126 μg carbon (C) h-1 g-1 dry weight, a value over 4 times higher than reported for phytoplankton. These results indicate that a field community of protists can contribute in coastal oyster rearing ponds to the energy requirements of the oyster Crassostrea gigas. We report here the first experimental evidence of a significant retention of a protist community by oysters, supporting the role of protists as a trophic link between picoplankton and benthic filter-feeding bivalves.

International audience; Numerous field-based investigations have highlighted that the production of extracellular polymeric substances (EPS) is physico-chemically and ecologically important for intertidal mudflats. EPS are largely secreted by marine benthic diatoms and their quantity and quality are environmental-dependant. This paper focused on the dynamic pathways, concentration rates and monosaccharides composition of colloidal, bound and residual carbohydrates extracted by using a cationic exchange resin from a diatom-dominated intertidal mudflat (Marennes-Oléron, France) during two different sampling periods: winter (February 2008) and summer (July 2008). A wide range of biotic and abiotic parameters were also studied to better understand the effect of environmental parameters, e.g., chlorophyll a, salinity, porewater amount, emersion time, luminosity, C:N ratio and tidal coefficient. Multiple colorimetric assays coupled to gas chromatographic analyseswere carried out to performthe biochemical characterizations. Firstly, the quantity of carbohydrates produced during winter (5.28 μg*μg chl a−1) was more important than during summer (2.04 μg*μg chl a−1). Yet, more proteins were found during summer for the colloidal and bound fractions (0.73 and 1.04 μg*μg chl a−1). Further investigations showed that the dynamic pathways were equivalent between winter and summer: bound carbohydrates (BC) quantities increased during the sediment emersion periods on the contrary to colloidal carbohydrates (CC) which tended to drop throughout the emersion time. The quality in monosaccharides was fraction-dependant, whatever the season. CC were always glucose-rich confirming their role of carbon source. BC were mainly composed of rhamnose whose the ratio increased during the emersion period, thus conferring adhesive properties to the extracellular matrix bounding diatoms cells. Residual carbohydrates (RC) were composed of various monosaccharides and a major increase of glucose content was found at the end of emersion, corresponding to intracellular C-storage in prevention to immersion times. Summer-RC were composed of fucose, a monosaccharide specific to these fractions and which was non-present during the winter campaign. Environmental parameters, such as salinity, pore water amount, and tidal coefficient could have a significant impact on the concentration rates and pathways of carbohydrates.

International audience; Benthic foraminifera are heterotrophic protists that utilize different trophic mechanisms and nutritional resources. They exhibit a wide range of trophic behaviours: selective (grazing) and indiscriminate herbivory, symbiosis, carnivory, parasitism, uptake of dissolved organic matter, passive suspension feeding and, most commonly, deposit feeding. The benthic foraminifera Ammonia tepida, previously known as an herbivore, fed as a carnivore in laboratory experiments where mobile metazoans were provided. We observed predation on the three types of metazoans provided: nematodes, copepods, and a larval gastropod. This foraminifera used its pseudopodial network to entrap the invertebrates, which were then stripped of their soft internal tissues within 24 hours. Our experiments are the first to demonstrate that Ammonia tepida, despite its limited motility, is able to utilize larger mobile animals as a food source. The great abundance of small metazoans in most marine environments suggests that they are a food source for foraminifera. Further study of foraminiferal feeding strategies will enhance our understanding of their role in marine communities.

The fate of benthic bacterial biomass in benthic food webs is a topic of major importance but poorly described. This paper describes an alternative method for evaluation of bacterial grazing rate by meiofauna and macrofauna using bacteria pre-enriched with stable isotopes. Natural bacteria from the sediment of an intertidal mudflat were cultured in a liquid medium enriched with 15NH4Cl. Cultured bacteria contained 2.9% of 15N and were enriched sufficiently to be used as tracers during grazing experiments. Cultured bacteria presented a biovolume (0.21 μm3) and a percentage of actively respiring bacteria (10%) similar to those found in natural communities. The number of Operational Taxon Units found in cultures fluctuated between 56 and 75% of that found in natural sediment. Despite this change in community composition, the bacterial consortium used for grazing experiments exhibited characteristics of size, activity and diversity more representative of the natural community than usually noticed in many other grazing studies. The bacterial ingestion rates of three different grazers were in the range of literature values resulting from other methods: 1149 ngC ind−1h−1 for the mud snail Hydrobia ulvae, 0.027 ngC ind−1 h−1 for the nematode community, and 0.067 ngC ind−1 h−1 for the foraminifera Ammonia tepida. The alternative method described in this paper overcomes some past limitations and it presents interesting advantages such as short time incubation and in situ potential utilisation.

The chemical and biological characteristics of the surface microlayer were determined during a transect across the South Pacific Ocean in October-December 2004. Concentrations of particulate organic carbon (1.3 to 7.6-fold) and nitrogen (1.4 to 7-fold), and POC:PON ratios were consistently higher in the surface microlayer as compared to surface waters (5 m). The large variability in particulate organic matter enrichment was negatively correlated to wind speed. No enhanced concentrations of dissolved organic carbon were detectable in the surface microlayer as compared to 5 m, but chromophoric dissolved organic matter was markedly enriched (by 2 to 4-fold) at all sites. Based on pigment analysis and cell counts, no consistent enrichment of any of the major components of the autotrophic and heterotrophic microbial community was detectable. CE-SSCP fingerprints and CARD FISH revealed that the bacterial communities present in the surface microlayer had close similarity (>76%) to those in surface waters. By contrast, bacterial heterotrophic production (3H-leucine incorporation) was consistently lower in the surface microlayer than in surface waters. By applying CARD-FISH and microautoradiography, we observed that Bacteroidetes and Gammaproteobacteria dominated leucine uptake in the surface microlayer, while in surface waters Bacteroidetes and Alphaproteobacteria were the major groups accounting for leucine incorporation. Our results demonstrate that the microbial community in the surface microlayer closely resembles that of the surface waters of the open ocean. Even a short residence in the surface microlayer influences leucine incorporation by different bacterial groups, probably as a response to the differences in the physical and chemical nature of the two layers.

International audience; Deposit feeders are able to process a considerable volume of sediment, containing large 9 quantities of associated bacteria. However, conclusions concerning the trophic role played by 10 benthic bacteria in marine sediments are still not fully elucidated. This study deals with 11 bacterivory by the gastropod Hydrobia ulvae, one of the most abundant deposit-feeding 12 species in intertidal mudflats in Western Europe. Ingestion rates of bacteria were determined 13 during grazing experiments using 15 N pre-enriched bacteria. Grazing experiments were 14 performed in order to measure effects of abiotic (temperature, salinity and luminosity) and 15 biotic (bacterial and algal abundances) factors on ingestion rates of bacteria by H. ulvae of an 16 intertidal mudflat (Brouage, Marennes-Oléron, France). The mean ingestion rate of bacteria 17 by H. ulvae was 1149 ngC ind-1 h-1. The general trend showed a temperature effect with an 18 optimum around 30°C, and the assimilation rate was significantly lower at 5°C. Bacterial 19 assimilation did not significantly differ between salinity 18 and salinity 31. Ingestion was the 20 same in light and in dark conditions. Results were compared with those of other grazing 21 experiments conducted simultaneously in similar conditions with two other grazers with 22 different size and feeding modes: the foraminifera Ammonia tepida and a nematode 23 community from the superficial sediment of the Brouage mudflat. Hydrobia ulvae and 24 nematodes presented a feeding behavior less influenced by environmental changes than A. tepida. Hydrobia ulvae ingested bacteria at a higher rate than smaller meiofaunal grazers and seemed to have a lower ability to selectively ingest diatoms than meiofaunal grazers.

International audience; The structure and summertime production of planktonic communities and the role of nondiatom planktonic cells were studied in coastal ponds, which are areas traditionally used for fattening and greening table-sized oysters. The abundance and biomass of nano–microplanktonic protists were determined at weekly intervals between February 1998 and February 1999 in a coastal pond without oysters in the French Atlantic coast near La Rochelle. The production of these microbiotas was determined in the summer period. The structure of plankton communities revealed the following observations: (1) microphytoplanktonic cells were mostly diatoms and dinoflagellates, (2) microzooplank-tonic cells were mainly ciliates, and (3) nanoplanktonic cells were represented by pigmented (80–90% of the nanoplankton biomass) and colorless nanoflagellates. Diatoms were dominated by Naviculiineae. Dinoflagellates were dominated by Peridiniales. Oligotrichida were predominant in the ciliate community. Protist biomass levels were nine times higher from April to August (summer period 1033 mg C L _ 1) than from September to March (winter period 114 mg C L _ 1). Whatever the season, nanoflagellates were dominant in the water column (66 and 53% of the entire protist biomass in the summer and winter periods, respectively). Nanoflagellates represented the highest production of nano–microplanktonic communities (76% of carbon protist production) in the coastal pond in summer and showed the shortest generation time (7.1 h). Dinoflagellates came after nanoflagellates in production (19.5% of carbon protist production). Diatoms represented only a supplementary carbon resource available for higher trophic levels, whereas, until now, they were considered as the principal food of oysters in coastal ponds. Ciliates were a small source of carbon, but their growth rate was high. We suggest, first, that nanoflagellates represented the primary resource available in the pond and could constitute an important food resource for higher trophic levels, such as oysters, farmed in this type of pond. Overall, the system appeared to be more autotrophic than hetero-trophic. Because inorganic nutrients are quickly exhausted in a semiclosed pond, pigmented flagellates dominated the carbon biomass, production and biomass of bacteria were high (thus, the microbial food web appeared to be active in this pond), and mixotrophy seemed to be an important trophic mode there.

International audience; In an attempt to compare the microbial community structure between the sea surface microlayer (SML) and subsurface waters (SSW), we determined the enrichment factors (EF: the ratio of abundance or activity in the SML to abundance or activity in SSW) of 13 biological parameters. Samples were taken at 2 contrasting coastal sites in the Mediterranean Sea, corresponding to a high (Barcelona, Spain) and low (Banyuls-sur-Mer, France) urbanized area. Principal component analysis showed that temporal variability was much higher at Barcelona than at Banyuls, and that the characteristics of the SML and SSW samples were more closely related at Barcelona. At both sites, the SML was weakly enriched in heterotrophic bacteria (on average 1.1-fold), Synechococcus spp. (on average 1.1-fold), and photosynthetic picoeukaryotes (on average 1.5-fold) relative to SSW. In contrast, the SML was considerably enriched in chlorophyll a (chl a) (on average 1.9-fold), phaeophytin a (on average 7.4-fold), autotrophic (on average 6.1-fold) and heterotrophic nanoflagellates(on average 5.1-fold) relative to SSW. Enrichments in bacterial production and culturable bacteria were highly variable. EF were significantly different between the 2 sites only for concentrations of chl a, b, and c and the abundance of autotrophic nanoflagellates, with higher EF at the Barcelona site.Except for autotrophic flagellates, the abundance or activity of the parameters determined in the SML was highly correlated with that determined in SSW, suggesting that enrichment of the SML results mainly from upward transport of microorganisms attached to buoyant particles or bubble scavenging.In contrast, the high contribution of autotrophic nanoflagellates to overall phytoneuston biomass (mean = 26%) is likely due to their rapid colonization of the SML. The high abundances of auto- and heterotrophic nanoflagellates in the SML indicated that these organisms play a key role in the functioning of the microbial food webs at the air–sea interface.

International audience; During a 4-week experiment on oyster grazing in coastal ponds (claires) near La Rochelle (France), a red coloration was observed in the digestive gland of oysters. This pigmentation was attributed to an intensive grazing on cryptophyte nanoflagellates. Three different techniques led to this conclusion, mentioning for the first time free-living cryptophytes as responsible for such a phenomenon. Spectrofluorometry was used to demonstrate that the red coloration was due to the presence of a phycoerythrin, characteristic of cryptophytes. Microscopic observations and accessory pigments analyses in water ponds confirmed that the source of this pigment was of cryptomonadal origin. Oyster grazing on these algae is evidenced by large differences in alloxanthin concentrations and flagellate abundances between the two experimental ponds. The winter occurrence of this phenomenon is of importance for oyster commercialization and a short-term depuration is suggested to remove the undesirable pigmentation. D

International audience; To assess the in situ impact of oysters, Crassostrea gigas, on planktonic protist and bacteria communities and the potential contribution of protozoa to their food resource intake, the abundance and the diversity of protists and bacteria were followed in 2 Atlantic coastal ponds, with and without oysters. The protist biomass in such ponds was high, with a maximum in spring of 982 μg C l-1 and a minimum in winter of 179 μg C l-1. Whatever the season, the presence of oysters (20 m-2 corresponding to on average 23 mg dry weight m-2) induced a significant decrease in > 5 μm protist abundance. On the contrary, planktonic organisms < 5 μm, such as Chlorophyta flagellates and bacteria, developed similarly in both ponds. It can be assumed that such a depletion in micro-sized protists was especially related to the grazing activity of C. gigas, which efficiently retains > 5 μm particles. In spring, oyster grazing triggered dramatic changes in the protist community by lowering the taxonomic diversity. In autumn and winter, the presence of oysters deeply influenced the taxonomic structure of the protist communities: > 5 μm protists could only develop in the control pond, whereas they were removed by filtration into the oyster pond; on the contrary, < 5 μm protists that were not retained, were favoured in the oyster pond. The set of results showed that hetero/mixotrophic protists represent an important potential resource in coastal ponds: flagellates > 5 μm were the main protist resource for C. gigas; ciliates represented the second resource with a substantial contribution in autumn; diatoms and dinoflagellates, though efficiently removed represented a weak carbon resource. Our study supports the hypothesis that oysters may access to the strong bacterioplanktonic production through hetero/mixotrophic protists, which would thus allow the transfer of carbon from the microbial loop towards C. gigas.

Pearl oysters are farmed in oligotrophic tropical atoll lagoons where planktonic communities are dominated by production from cyanobacteria smaller than 2 pm. Paradoxically, the pearl oyster Pinctada margaritifera only retains particles larger than 2 Pm. In this study, we assess the relative contribution of hetero/mixotrophic microbiota to the available planktonic resource. In Takapoto Atoll, picocyanobacteria are the dominant biomass (20 mug C l(-1)). The carbon biomass of ciliates and dinoflagellates ranges from 1 to 24 and 0.5 to 5 mug C l(-1) respectively, with a mean of 6 mug C l(-1) for ciliates and 2 mug C l(-1) for dinoflagellates. The possible retention by P. margaritifera on a natural protist suspension was investigated. Due to its high clearance rates (ca 20 l h(-1) g(-1)) the pearl oyster retained 85 mug C h(-1) g(-1) from ciliates and 55 mug C h(-1) g(-1) from dinoflagellates. Conversely, cyanobacteria were not efficiently retained by the bivalve and did not efficiently contribute to its diet. From our experiments, we concluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonic resource towards filter-feeders, particularly pearl oysters.

International audience; To explore the variability of bacterial biomass conversion factors in aquatic environments, 5 manne and 5 non-marine bacterial strains were exposed to nutrient deprivation in artificial seawater and physiological (NaC1, 9 g I-') water, respectively. Carbon content per cell (C,) and per volume (C,) were estimated from total cell number, particulate organic carbon and cell volume (V) measurements after 0. 7, 15 and 28 d of starvation. While Cc appeared to be rather constant (26.02 * 1.08 fg C cell-'; mean k standard error. n = 50) during starvation, C, showed a significant increase. In general , C , increased more rapidly in marine strains than in non-marine strains. Cc and V were not correlated, but a highly significant relationship between C , and V was found. The non-linear relation was well fitted by a power function model (r2 = 0.847, p < 0.0001). Parameters of the power function adjusted to marine or non-marine strains were not significantly different. We conclude that C, and, to a lesser extent, Cc were dependent on species and physiological status. However , it seems that the differences in C, between marine and non-marine bacteria observed in this study and by others are not the consequence of different relationships between C, and volume, but, more Ilkely, of the low cell volume of marine bacteria and their ability to reduce their water content faster than can non-marine bactena.

Although microorganisms are the primary drivers of biogeochemical cycles, the structure and functioning of microbial food webs are poorly studied. This is the case in Sphagnum peatlands, where microbial communities play a key role in the global carbon cycle. Here, we explored the structure of the microbial food web from a Sphagnum peatland by analysing (1) the density and biomass of different microbial functional groups, (2) the natural stable isotope (δ13C and δ15N) signatures of key microbial consumers (testate amoebae), and (3) the digestive vacuole contents of Hyalosphenia papilio, the dominant testate amoeba species in our system. Our results showed that the feeding type of testate amoeba species (bacterivory, algivory, or both) translates into their trophic position as assessed by isotopic signatures. Our study further demonstrates, for H. papilio, the energetic benefits of mixotrophy when the density of its preferential prey is low. Overall, our results show that testate amoebae occupy different trophic levels within the microbial food web, depending on their feeding behavior, the density of their food resources, and their metabolism (i.e., mixotrophy vs. heterotrophy). Combined analyses of predation, community structure, and stable isotopes now allow the structure of microbial food webs to be more completely described, which should lead to improved models of microbial community function.

International audience; The Mediterranean Thau Lagoon is an important European oyster farming area. Oyster growth levels are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant in the Thau Lagoon. Moreover, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in water columns of the Thau Lagoon and to understand the particular structure of the Thau microbial food web, which may explain such a paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter with a maximum Summer abundance. On 17 August 1998, the picophytoplankton and nanophytoplankton abundances were not as high as expected and we observed the development of large diatoms. At this time, available carbon resources arose from microphytoplankton (84.5 %) and picoplanktonic cells represented only 1.27 % in terms of carbon. The heterotrophic cells were few in abundance and constitued only < 14 % of carbon resources. In order to evaluate the importance of the "protozoan trophic link" for energy transfer from "microbial food web" to large benthic suspension feeders, the oyster Crassostrea gigas was offered a planktonic community as potential prey. In the grazing experiment, all > 5 μm flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only < 5 μm flagellates and picoeukaryotic cells, Ostreococcus tauri, were not very well retained (45 % and 2 %). The high clearance rates of Crassostrea gigas found in this experiment can be explained by a low suspended particulate matter (0.65 mg l-1). Oysters adapted their retention mechanism when they lived in oligotrophic waters. These results indicate that, under the given experimental conditions, picophytoplankton did not represent a valuable trophic resource for farmed oysters because (1) Crassostrea can not retain picoparticles and (2) the picoplankton represented a poor available carbon resource to be transferred via a weak heterotrophic protist community. In the oyster pends of the Thau Lagoon during this study, which followed a rainfall event, microphytoplanktonic primary producers, in particular diatoms, could be considered as the main food sources for bivalve suspension feeders.