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International audience; The effects of sediment elutriate, prepared from a sediment resuspension simulation process, on the growth of bacterioplankton and of different sized phytoplankton as well as on their grazing by protozooplankton were investigated in Mediterranean coastal ecosystems during spring phytoplankton bloom. In parallel, the response of bacterioplankton, phytoplankton and protozooplankton to sediment elutriate enrichment was assessed in microcosm experiments. Elutriate addition provoked a high enrichment in nutriments of the overlying water, particularly in ammonium, dissolved organic matter but also in trace metal elements and herbicides. Overall, elutriate addition significantly increased the biomass of bacterioplankton and of total phytoplankton, but the phytoplankton size fractions responded differently to elutriate enrichment. The picophytoplankton showed a significant strong enhancement in both growth and production rates, relatively to control, while these rates significantly decreased for nano- and microphytoplankton. Accordingly, the phytoplankton size structure shifted in elutriate microcosm toward a dominance of the picophytoplankton concomitant with a significant reduction in larger size fractions. The elutriate addition caused also a clear shift in the taxonomic composition of protozooplankton, associated with a significant modification of its functional diversity, with the dominance of pico-sized cell consumers (such as aloricate ciliates, heterotrophic nanoflagellates and mixotrophic dinoflagellates) in the detriment of grazers of nano- and microphytoplankton (such as heterotrophic dinoflagellate). The protozooplankton grazing on small prey (bacterioplankton and picophytoplankton) increased, while feeding on nano- and micro-sized phytoplankton decreased after elutriate addition. These results have implication for the carbon transfer pathway, which could change from an herbivorous food web to a microbial food web, influencing so the exportation of biogenic carbon through the other trophic compartments.

International audience; We assessed the spatial variability in the size structure of phytoplankton, community composition, primary production and carbon fluxes through the planktonic food web of the Gulf of Gabès (GG; Southeastern Mediterranean Sea) in the fall of 2017 during the MERMEX-MERITE cruise. High concentrations in nutrients, chlorophyll a (~ 2–6 µg L−1) and primary production (1816–3674 mg C m−2 d−1) revealed an eutrophic status of the studied stations in the GG. In accordance with hydrodynamic features, inorganic nutrients showed increases in concentrations from North to South and from coast to offshore, these nutrient gradients impacting the spatial distribution of phytoplankton community. Size-fractioned phytoplankton biomass and production were the lowest in the northernmost zone where they were mainly sustained by pico-sized fraction. Concomitantly, in this area, small aloricate ciliates were dominant leading to a high microbivory. Conversely, higher biomass and production were measured towards the South and offshore with prevalence of larger phytoplankton (nano- and/or micro-sized fractions) supported by diatoms. The herbivorous protozooplankton and metazooplankton were more abundant in these zones, resulting in an increase of the herbivory. The vertical particulate organic carbon flux followed also a north–south and coast-offshore increasing gradient, with a higher contribution of phytoplankton, and zooplankton fecal pellets to the sinking organic matter in the southernmost area. Our results suggest that even in nutrient-rich and highly productive waters, a continuum of trophic pathways, ranging from microbial to multivorous and herbivorous food webs, may exist, which implies different efficiencies in carbon export and carrying capacity within the ecosystem.

International audience; The toxicity of four herbicides in mixture (alachlor, diuron, des-isopropyl-atrazine and simazine) on the growth and the photosynthesis parameters of two marine diatoms Pseudo-niszchia mannii and Chaetoceros decipiens have been investigated for 9 days in monoculture and co-culture tests. The catalase (CAT) and guaiacol peroxidase (GPX) were also monitored to assess the oxidative stress response. In single-species assays, while both species displayed no affected instantaneous growth rate by herbicides, their physiological responses were different. Chl a content of P. mannii significantly decreased upon herbicide exposure, due probably to pigment destruction or inhibition of their synthesis. This decrease was associated with a reduction in the chlorophyll fluorescence parameters (ABS0/RC, TR0/RC, ET0/RC and DI0/RC). In contrast, C. decipiens maintained an effective photosynthetic performance under herbicide exposure, as Chl a per cell content and the specific energy fluxes per reaction center remained unchanged relative to control values. GPX activity was significantly higher in contaminated P. mannii and C. decipiens monocultures than in controls at early herbicide exposure (1 day), whereas a significant induction of CAT activity occurred later (from day 3 for C. decipiens and at day 9 for P. mannii) in response to herbicides. In control co-culture, P. mannii was eliminated by C. decipiens. As observed in the monoculture, the herbicides did not affect the photosynthetic performance of C. decipiens in co-culture, but significantly reduced its instantaneous growth rate. The oxidative stress response in co-culture has similar trends to that of C. decipiens in monoculture, but the interspecies competition likely resulted in higher CAT activity under herbicide exposure. Results of this study suggest that herbicide toxicity for marine diatoms might be amplified by interspecies interactions in natural communities, which might lead to different physiological and growth responses.

This study investigates the effects of polycyclic aromatic hydrocarbons (PAHs) on two potentially toxic Pseudo-nitzschia hasleana and P. mannii, isolated from a PAH contaminated marine environment. Both species, maintained in non-axenic cultures, have been exposed during 144 h to increasing concentrations of a 15 PAHs mixture. Analysis of the domoic acid, showed very low concentrations. Dose–response curves for growth and photosynthesis inhibition were determined. Both species have maintained their growth until the end of incubation even at the highest concentration tested (120 µg l-1), Nevertheless, P mannii showed faster growth and seemed to be more tolerant than P. hasleana. To reduce PAH toxicity, both species have enhanced their biovolume, with a higher increase for P. mannii relative to P hasleana. Both species were also capable of bio-concentrating PAHs and were able to degrade them probably in synergy with their associated bacteria. The highest biodegradation was observed for P. mannii, which could harbored more efficient hydrocarbon-degrading bacteria. This study provides the first evidence that PAHs can control the growth and physiology of potentially toxic diatoms. Future studies should investigate the bacterial community associated with Pseudo-nitzschia species, as responses to pollutants or to other environmental stressors could be strongly influence by associated bacteria. info:eu-repo/semantics/publishedVersion

The seasonal variations in Pseudo-nitzschia species and domoic acid (DA) concentration were investigated, at three shellfish farms in SW coastal Mediterranean. In parallel, the toxicity of mussels was tested. Two distinct groups of species were enumerated according to morphology and size (Pseudo-nizschia delicatissima and P. seriata groups). DA was detected over a nine-week period from July to October 2012 in the Lagoon, with a maximum concentration recorded in July (12.71 ng DA L−1). DA was positively correlated with the presence of P. seriata-group and P. delicatissima-group and was mostly occurred during P limitation period in seawater. No DA was found in mussels that were collected during the period of DA absence in seawater. Our results suggest that temperature, salinity, inorganic and organic nutrients were significant for the seasonal dynamics of P. seriata and P. delicatissima groups, but that the P limitation was the most driving factor for DA production in these areas. The relative influence of environmental factors should be further studied to better understand the recent surfacing of massive blooms of toxigenic Pseudo-nitzschia in SW Mediterranean coast. info:eu-repo/semantics/acceptedVersion

We investigated the efficiency of a benthic diatom-associated bacteria in removing benzo(a)pyrene (BaP) and fluoranthene (Flt). The diatom, isolated from a PAH-contaminated sediment of the Bizerte Lagoon (Tunisia), was exposed in axenic and non-axenic cultures to PAHs over 7 days. The diversity of the associated bacteria, both attached (AB) and free-living bacteria (FB), was analyzed by the 16S rRNA amplicon sequencing. The diatom, which maintained continuous growth under PAH treatments, was able to accumulate BaP and Flt, with different efficiencies between axenic and non-axenic cultures. Biodegradation, which constituted the main process for PAH elimination, was enhanced in the presence of bacteria, indicating the co-metabolic synergy of microalgae and associated bacteria in removing BaP and Flt. Diatom and bacteria showed different capacities in the degradation of BaP and Flt. Nitzschia sp. harbored bacterial communities with a distinct composition between attached and free-living bacteria. The AB fraction exhibited higher diversity and abundance relative to FB, while the FB fraction contained genera with the known ability of PAH degradation, such as Marivita, Erythrobacter, and Alcaligenes. Moreover, strains of Staphylococcus and Micrococcus, isolated from the FB community, showed the capacity to grow in the presence of crude oil. These results suggest that a “benthic Nitzschia sp.-associated hydrocarbon-degrading bacteria” consortium can be applied in the bioremediation of PAH-contaminated sites. This work was supported by IRD through the Laboratoire Mixte International (LMI) COSYS-Med (Contaminants et Ecosystèmes Sud Méditerranéens). English grammar and syntax of the manuscript were revised by Proof-Reading-Service.com. We thank anonymous reviewers for useful suggestions.

The effects of inorganic and organic nitrogen supply on the growth and domoic acid (DA) production of Pseudo-nitzschia cf. seriata and Pseudo-nitzschia calliantha from Bizerte Lagoon (SW Mediterranean Sea) were studied during field and laboratory experiments. Nitrogen enrichments (40 µM NO3 (-); 10 µM NH4 (+); 20 µM CH4N2O) and a control, with no added N, were carried out in separate carboys with seawater collected from Bizerte Lagoon. In the field experiments, all N-enrichments resulted in significant increases in chlorophyll a concentration, and maintained exponential growth until the end of the experiment. The initial diatom community was dominated by a bloom of P. cf. seriata (9.3 × 10(5) cells l(-1)). After 6 days of incubation, the abundance of P. cf. seriata was greatest in the urea addition (1.52 × 10(6) cells l(-1)), compared to the ammonium treatment (0.47 × 10(6) cells l(-1)), nitrate treatment (0.70 × 10(6) cells l(-1)) and control (0.36 × 10(6) cells l(-1)). The specific growth rates, calculated from increases in chlorophyll a and cell abundance, were statistically different across all treatments, with the highest in the urea and nitrate additions. Similar results were obtained from the laboratory experiments. These were carried out with P. calliantha isolated from Bizerte Lagoon and grown in f/2 medium enriched with 40 µM nitrate, 10 µM ammonium and 20 µM urea. The exponential growth rate was significantly faster for the cells cultured with urea (1.50 d(-1)) compared to the nitrate (0.90 d(-1)) and ammonium (0.80 d(-1)) treatments and the control (0.40 d(-1)). Analysis of DA, performed at the beginning and the end of the both experiments in all treatments, revealed very low concentrations (below the limit of quantification, 0.02- 1.310(-7) pg cell(-1), respectively).The field and laboratory experiments demonstrate that P.cf. seriata and P. calliantha are able to grow efficiently on the three forms of N, but with a preference for urea.