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WOS:000737215300006; International audience; Ecological baselines for the structure and functioning of ecosystems in the absence of human activity can provide essential information on their health status. The Glorieuses islands are located in the Western Indian Ocean (WIO) and can be considered as "pristine" ecosystems that have not been subjected to anthropogenic pressure. Their nutrient context and the microbial assemblages were assessed by determining the abundance of heterotrophic prokaryotes (archaea and bacteria), picocyanobacteria, picoeukaryotes, microphytoplankton and protozooplankton communities in five stations, during two contrasted periods (November 2015 and May 2016). Chlorophyll-a concentrations were always under 1 mu g/L and associated to very low levels in orthophosphates, nitrate and dissolved organic carbon, revealing an ultra-oligotrophic status for the Glorieuses waters. Picocyanobacteria confirmed the ultra-oligotrophic status with a predominance of Synechococcus. Zeaxanthin associated with the presence of picocyanobacteria represented the major pigment in both surveys. Three indices of diversity (species richness, Shannon and Pielou indexes) from microscopy observations highlighted the difference of diversity in microphytoplankton between the surveys. A focus on a 16S metabarcoding approach showed a high dominance of picocyanobacteria, Alpha- and Gammaproteobacteria, regardless of station or period. Multivariate analyses (co-inertia analyses) revealed a strong variability of ecological conditions between the two periods, with (i) high nutrient concentrations and heterotrophic nanoflagellate abundance in November 2015, and (ii) high heterotrophic prokaryote and picoeukaryote abundance in May 2016. The impact of a category 5 tropical cyclone (Fantala) on the regional zone in April 2016 is also advanced to explain these contrasted situations. Relative importance of top-down factors between bacterial and heterotrophic nanoflagellates was observed in November 2015 with an active microbial food web. All the results indicate that three microbial indexes potentially can be considered to assess the ecological change in Glorieuses marine waters.

Lake Dziani Dzaha (Mayotte Island, Indian Ocean) is a tropical thalassohaline lake which geochemical and biological conditions make it a unique aquatic ecosystem considered as a modern analogue of Precambrian environments. In the present study, we focused on the diversity of phytoplanktonic communities, which produce very high and stable biomass (mean2014–2015 = 652 ± 179 μg chlorophyll a L−1). As predicted by classical community ecology paradigms, and as observed in similar environments, a single species is expected to dominate the phytoplanktonic communities. To test this hypothesis, we sampled water column in the deepest part of the lake (18 m) during rainy and dry seasons for two consecutive years. Phytoplanktonic communities were characterized using a combination of metagenomic, microscopy-based and flow cytometry approaches, and we used statistical modeling to identify the environmental factors determining the abundance of dominant organisms. As hypothesized, the overall diversity of the phytoplanktonic communities was very low (15 OTUs), but we observed a co-dominance of two, and not only one, OTUs, viz., Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta). We observed a decrease in the abundance of these co-dominant taxa along the depth profile and identified the adverse environmental factors driving this decline. The functional traits measured on isolated strains of these two taxa (i.e., size, pigment composition, and concentration) are then compared and discussed to explain their capacity to cope with the extreme environmental conditions encountered in the aphotic, anoxic, and sulfidic layers of the water column of Lake Dziani Dzaha. Electronic supplementary material The online version of this article (10.1007/s00248-019-01332-8) contains supplementary material, which is available to authorized users.

International audience; Thalassohaline ecosystems are hypersaline environments originating from seawater in which sodium chloride is the most abundant salt and the pH is alkaline. Studies focusing on microbial diversity in thalassohaline lakes are still scarce compared with those on athalassohaline lakes such as soda lakes that have no marine origin. In this work, we investigated multiple facets of bacterial, archaeal and eukaryotic diversity in the thalassohaline Lake Dziani Dzaha using a metabarcoding approach. We showed that bacterial and archaeal diversity were mainly affected by contrasting physicochemical conditions retrieved at different depths. While photosynthetic microorganisms were dominant in surface layers, chemotrophic phyla (Firmicutes or Bacteroidetes) and archaeal methanogens dominated deeper layers. In contrast, eukaryotic diversity was constant regardless of depth and was affected by seasonality. A detailed focus on eukaryotic communities showed that this constant diversity profile was the consequence of the high predominance of Picocystis salinarum, while nondominant eukaryotic groups displayed seasonal diversity turnover. Altogether, our results provided an extensive description of the diversity of the three domains of life in an unexplored extreme environment and showed clear differences in the responses of prokaryotic and eukaryotic communities to environmental conditions.

Discovery after discovery, host-associated microbiota reveal a growing list of positive effects on host homeostasis by contributing to host nutrition, improving hosts' immune systems and protecting hosts against pathogens. In that context, a collection of oyster associated bacteria producing antibacterial compounds have been established to evaluate their role in non-host-derived immunity. Here, we described alterins, potent anti-Gram negative compounds produced by Pseudoalteromonas hCg-6 and hCg-42 isolated from different healthy oyster hemolymph. The strains hCg-6 and hCg-42 produce a set of at least seven antibacterial compounds, ranging from 926 to 982 Da structurally characterized as cyclolipopeptides (CLPs). Alterins share the same cationic heptapeptidic cycle connected via an amido bond to different hydrophobic hydrocarbon tails. Their MICs disclosed a potent antibacterial activity directed against Gram-negative bacteria including oyster and human pathogens that may confer a beneficial defense mechanism to the host but also represents an untapped source of new antibiotics. The alterins&rsquo, mechanisms of action have been deciphered: after binding to lipopolysaccharides (LPS), alterins provoke a membrane depolarization and permeabilization leading to bacterial lysis. As hCg-6 and hCg-42 produced a set of natural derivatives, the structure/activity relationship linked to the carbon tail is clarified. We showed that the hydrocarbon tail determines the LPS-binding properties of alterins and consequently their antibacterial activities. Its length and saturation seem to play a major role in this interaction.

The factors controlling estuarine phytoplankton diversity and production are relatively well known in temperate systems. Less however is known about the factors affecting phytoplankton community distribution in tropical estuaries. This is surprising given the economic and ecological importance of these large, deltaic ecosystems, such as are found in South East Asia. Here we present the results from an investigation into the factors controlling phytoplankton distribution and phytoplankton-bacterial coupling in the Bach Dang Estuary, a sub-estuary of the Red River system, in Northern Vietnam. Phytoplankton diversity and primary and bacterial production, nutrients and metallic contaminants (mercury and organotin) were measured during two seasons: wet (July 2008) and dry (March 2009). Phytoplankton community composition differed between the two seasons with only a 2% similarity between July and March. The large spatial extent and complexity of defining the freshwater sources meant that simple mixing diagrams could not be used in this system. We therefore employed multivariate analyses to determine the factors influencing phytoplankton community structure. Salinity and suspended particulate matter were important factors in determining phytoplankton distribution, particularly during the wet season. We also show that phytoplankton community structure is probably influenced by the concentrations of mercury species (inorganic mercury and methyl mercury in both the particulate and dissolved phases) and of tri-, di, and mono-butyl tin species found in this system. Freshwater phytoplankton community composition was associated with dissolved methyl mercury and particulate inorganic mercury concentrations during the wet season, whereas, during the dry season, dissolved methyl mercury and particulate butyl tin species were important factors for the discrimination of the phytoplankton community structure. Phytoplankton-bacterioplankton coupling was also investigated during both seasons. In the inshore, riverine stations the ratio between bacterial production and dissolved primary production was high supporting the hypothesis that bacterial carbon demand is supported by allochthonous riverine carbon sources. The inverse was true in the offshore stations, where BP:DPP values were less than 1, potentially reflecting differences in primary production due to shifting phytoplankton community diversity.

International audience; Since 2008, juvenile Crassostrea gigas oysters have suffered from massive mortalities in European fanning areas. This disease of complex etiology is still incompletely understood. Triggered by an elevated seawater temperature, it has been associated to infections by a herpes virus named OsHV-1 as well as pathogenic vibrios of the Splendidus Glade. Ruling out the complexity of the disease, most of our current knowledge has been acquired in controlled experiments. Among the many unsolved questions, it is still ignored what role immunity plays in the capacity oysters have to survive an infectious episode. Here we show that juvenile oysters susceptible to the disease mount an inefficient immune response associated with microbial permissiveness and death. We found that, in contrast to resistant adult oysters having survived an earlier episode of mortality, susceptible juvenile oysters never exposed to infectious episodes died by more than 90% in a field experiment. Susceptible oysters were heavily colonized by OsHV-1 herpes virus as well as bacteria including vibrios potentially pathogenic for oysters, which proliferated in oyster flesh and body fluids during the mortality event. Nonetheless, susceptible oysters were found to sense microbes as indicated by an overexpression of immune receptors and immune signaling pathways. However, they did not express important immune effectors involved in antimicrobial immunity and apoptosis and showed repressed expression of genes involved in ROS and metal homeostasis. This contrasted with resistant oysters, which expressed those important effectors, controlled bacterial and viral colonization and showed 100% survival to the mortality event. Altogether, our results demonstrate that the immune response mounted by susceptible oysters lacks some important immune functions and fails in controlling microbial proliferation. This study opens the way to more holistic studies on the "mass mortality syndrome", which are now required to decipher the sequence of events leading to oyster mortalities and determine the relative weight of pathogens, oyster genetics and oyster-associated microbiota in the disease.

The toxicity of polycyclic aromatic hydrocarbons (PAHs) mixtures was evaluated on natural phytoplankton communities sampled from lagoons of Bizerte (South-western Mediterranean Sea) and Thau (North-western Mediterranean Sea). PAHs induced short-term dose and ecosystem-dependant decreases in photosynthetic potential. Chlorophyll a was negatively affected by increasing PAHs concentrations, together with dramatic changes in phytoplankton community composition. Size classes were strongly affected in the Bizerte compare to the Thau lagoon, with a decrease in nano- and microphytoplankton densities compare to picophytoplankton. In both locations, the diatom Entomoneis paludosa appeared favoured under PAH exposure as evidenced by increase in cell density, whereas autotrophic flagellates and dinophytes were strongly reduced. Smaller cells were more tolerant to exposure to highest PAHs concentrations, with persistent picophytoplankton carbon biomass at the end of the incubations. Apparent recovery of photosynthetic potential, accompanied with a regrowth of chlorophyll a under the lowest PAH doses, coincided with a significantly altered community composition in both lagoons. Furthermore, sensitivity to PAHs was not related to the phytoplankton cell size, and toxicity-induced modification of top-down control by grazers during the experiment cannot be excluded. (C) 2018 Elsevier Ltd. All rights reserved.

International audience; The grazing activity by specific marine organisms represents a growing threat to the survival of many scleractinian species. For example, the recent proliferation of the corallivorous gastropod Drupella now constitutes a critical case in all South-East Asian waters. If the damaging effects caused by this marine snail on coral polyps are relatively well known, the indirect incidence of predation on coral microbial associates is still obscure and might also potentially impair coral health. In this study, we compared the main ecological traits of coral-associated bacterial and viral communities living in the mucus layer of Acropora formosa and Acropora millepora, of healthy and predated individuals (i.e., colonized by Drupella rugosa), in the Bay of Van Phong (Vietnam). Our results show a substantial impact of the gastropod on a variety of microbiological markers. Colonized corals harbored much more abundant and active epibiotic bacteria whose community composition shifted toward more pathogenic taxa (belonging to the Vibrionales, Clostridiales, Campylobacterales, and Alteromonadales orders), together with their specific phages. Viral epibionts were also greatly influenced by Drupella corallivory with spectacular modifications in their concentrations, life strategies, genotype richness, and diversity. Novel and abundant circular Rep-encoding ssDNA viruses (CRESS-DNA viruses) were detected and characterized in grazed corals and we propose that their occurrence may serve as indicator of the coral health status. Finally, our results reveal that corallivory can cause severe dysbiosis by altering virus-bacteria interactions in the mucus layer, and ultimately favoring the development of local opportunistic infections.

International audience; The impact of fish and oysters on components of the pelagic microbial food web (MFW) was studied in a 10 d mesocosm experiment using Mediterranean coastal waters. Two mesocosms contained natural water only (‘Controls’), 2 contained natural water with Crassostrea gigas (‘Oyster’), and 2 contained natural water with Atherina spp. (‘Fish’). Abundances and biomasses of microorganisms (viruses, bacteria, phytoplankton, heterotrophic flagellates, and ciliates) were measured to estimate their contribution to the total microbial carbon biomass. Two MFW indices, the microbial autotroph:heterotroph C biomass ratio (A:H) structural index and the gross primary production:respiration ratio (GPP:R) functional index, were defined. In the Fish mesocosms, selective predation on zooplankton led to a trophic cascade with 51% higher phyto- plankton C biomass and consequently higher A:H and GPP:R than in the Controls. By the end of the experiment, the Oyster mesocosms had a bacterial C biomass 87% higher and phytoplankton C biomass 93% lower than the Controls, giving significantly lower A:H and GPP:R (1), whereas oyster activities made the MFW more hetero- trophic (both indices

Contamination of coastal environments is often due to a complex mixture of pollutants, sometimes in trace levels, that may have significant effects on diversity and function of organisms. The aim of this study was to evaluate the short-term dynamics of bacterioplankton exposed to natural and artificial mixtures of contaminants. Bacterial communities from a southwestern Mediterranean ecosystem, lagoon and the bay (offshore) of Bizerte were exposed to i) elutriate from resuspension of contaminated sediment, and ii) an artificial mixture of metals and herbicides mimicking the contamination observed during sediment resuspension. Elutriate incubation as well as artificial spiking induced strong enrichments in nutrients (up to 18 times), metals (up to six times) and herbicides (up to 20 times) relative to the in situ concentrations in the offshore station, whereas the increases in contaminants were less marked in the lagoon station. In the offshore waters, the artificial mixture of pollutants provoked a strong inhibition of bacterial abundance, production and respiration and significant modifications of the potential functional diversity of bacterioplankton with a strong decrease of the carbohydrate utilization. In contrast, incubation with elutriate resulted in a stimulation of bacterial activities and abundances, suggesting that the toxic effects of pollutants were modified by the increase in nutrient and DOM concentrations due to the sediment resuspension. The effects of elutriate and the artificial mixture of pollutants on bacterial dynamics and the functional diversity were less marked in the lagoon waters, than in offshore waters, suggesting a relative tolerance of lagoon bacteria against contaminants.

International audience; Coral reef environments are generally recognized as being the most threatened of marine ecosystems. However, it is extremely difficult to distinguish the effects of climate change from other forcing factors, mainly because it is difficult to study ecosystems that are isolated from human pressure. The five Iles Eparses (Scattered Islands) are located in the Western Indian Ocean (WIO) and can be considered to be "pristine" ecosystems not subject to anthropogenic pressure. This study characterized their plankton assemblages for the first time, by determining the abundances of microbial (virus, bacteria, heterotrophic protists and phytoplankton) and metazooplankton communities in various lagoon and ocean sites around each island. The Europa lagoon has extensive, productive mangrove forests, which have the highest nutrient concentrations (nitrogen forms, dissolved organic carbon) and whose microbial communities present a peculiar structure and functioning. By means of bioassay experiments, we observed that bacterial production and growth rates are higher in Europa than those reported for the other islands. Tromelin, which lies outside the Mozambique Channel, had the lowest biological productivity, nutrient concentrations, and bacterial growth rates. Multivariate analysis indicated that distinct microbial assemblages occur in association to varying nutrient concentrations. Molecular fingerprinting showed clear discrimination of the structure of the archaea, bacteria and eukaryotes community between the sites. Our results suggest that the geographical distance can influence the diversity of dominant microbial taxa in the WIO.

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; Size fractionation was performed using water from the Great Reef of Toliara (Madagascar) taken from two different habitats (ocean and lagoon) during the dry and wet seasons, to study the growth and mortality rates of bacterioplankton. Experiments were conducted with 1 and 100% of heterotrophic nanoflagellate (HNF) concentrations and virus-free water was obtained by tangential filtration (10 kDa). During the dry season, in both environments, bacterial abundance and production were significantly lower than values recorded during the wet season. Bacterial growth rates without grazers were 0.88 day–1 in the lagoon and 0.58 day–1 in the ocean. However, growth rates were statistically higher without grazers and viruses (1.58 day–1 and 1.27 day–1). An estimate of virus-induced bacterial mortality revealed the important role played by viruses in the lagoon (0.70 day–1) and the ocean (0.69 day–1). During the wet season, bacterial growth rates without grazers were significantly higher in both environments than were values obtained in the dry season. However, the bacterial growth rates were paradoxally lower in the absence of viruses than with viruses in both environments. Our results suggest that changes in nutrient concentrations can play an important role in the balance between viral lysis and HNF grazing in the bacterial mortality. However, virus-mediated bacterial mortality is likely to act simultaneously with nanoflagellates pressure in their effects on bacterial communities.

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L−1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l−1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 104 cells l−1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

Vibrio splendidus, strain LGP32, is an oyster pathogen associated with the summer mortalities affecting the production of Crassostrea gigas oysters worldwide. Vibrio splendidus LGP32 was shown to resist to up to 10 µM Cg-Def defensin and Cg-BPI bactericidal permeability increasing protein, two antimicrobial peptides/proteins (AMPs) involved in C. gigas immunity. The resistance to both oyster Cg-Def and Cg-BPI and standard AMPs (polymyxin B, protegrin, human BPI) was dependent on the ompU gene. Indeed, upon ompU inactivation, minimal bactericidal concentrations decreased by up to fourfold. AMP resistance was restored upon ectopic expression of ompU. The susceptibility of bacterial membranes to AMP-induced damages was independent of the ompU-mediated AMP resistance. Besides its role in AMP resistance, ompU proved to be essential for the adherence of V. splendidus LGP32 to fibronectin. Interestingly, in vivo, ompU was identified as a major determinant of V. splendidus pathogenicity in oyster experimental infections. Indeed, the V. splendidus-induced oyster mortalities dropped from 56% to 11% upon ompU mutation (Kaplan-Meier survival curves, P < 0.01). Moreover, in co-infection assays, the ompU mutant was out competed by the wild-type strain with competitive indexes in the range of 0.1-0.2. From this study, ompU is required for virulence of V. splendidus. Contributing to AMP resistance, conferring adhesive properties to V. splendidus, and being essential for in vivo fitness, the OmpU porin appears as an essential effector of the C. gigas/V. splendidus interaction.

Planktothrix agardhii dynamics, microcystin concentration and limnological variables were monitored every 2 weeks for 2 years (2004-2006) in a shallow hypereutrophic artificial lake (BNV, Viry-Chatillon, France). Time-series analysis identified two components in the P. agardhii biomass dynamics: (1) a significant decreasing trend in P. agardhii biomass (65% of the overall variance) and (2) a residual component without significant seasonal periodicity. A path-analysis model was built to determine the main factors controlling the P. agardhii dynamics over the period studied. The model explained 66% of P. agardhii biomass changes. The decreasing trend in P. agardhii biomass was significantly related to a decrease in the PO(4)(3-) concentration resulting from an improved treatment of the incoming watershed surface water. The residual component was related to zooplankton dynamics (cyclopoid abundances), supporting the hypothesis of a top-down control of P. agardhii, but only when the biomass was low. Forty-nine percent of the variability in the microcystin (MC) concentration (min:< 0.1 mu g equivalent MC-LR L(-1); max: 7.4 mu g equivalent MC-LR L(-1)) could be explained by changes in the P. agardhii biomass. The highest toxin content was observed when P. agardhii biomass was the lowest, which suggests changes in the proportion of microcystin-producing and -nonproducing subpopulations and/or the physiological status of cells.

We investigated the characteristics of the sulfur-oxidizing symbiont hosted in the gills of Codakia orbicularis, a bivalve living in shallow marine tropical environments. Special attention was paid to describing the heterogeneity of the population by using single-cell approaches including flow cytometry (FCM) and different microscopic techniques and by analyzing a cell size fractionation experiment. Up to seven different subpopulations were distinguished by FCM based on nucleic acid content and light side scattering of the cells. The cell size analysis of symbionts showed that the symbiotic population was very heterogeneous in size, i.e., ranging from 0.5 to 5 m in length, with variable amounts of intracellular sulfur. The side-scatter signal analyzed by FCM, which is often taken as a proxy of cell size, was greatly influenced by the sulfur content of the symbionts. FCM revealed an important heterogeneity in the relative nucleic acid content among the subclasses. The larger cells contained exceptionally high levels of nucleic acids, suggesting that these cells contained multiple copies of their genome, i.e., ranging from one copy for the smaller cells to more than four copies for the larger cells. The proportion of respiring symbionts (5-cyano-2,3-ditolyl-terazolium chloride positive) in the bacteriocytes of Codakia revealed that around 80% of the symbionts hosted by Codakia maintain respiratory activity throughout the year. These data allowed us to gain insight into the functioning of the symbionts within the host and to propose some hypotheses on how the growth of the symbionts is controlled by the host. Thioautotrophic symbioses in marine invertebrates have been studied for the last 25 years. Most of those studies were focused on hydrothermal vent environments. Nevertheless, similar types of symbiosis occur in sulfide-rich environments in the coastal zone with invertebrate hosts belonging to annelids, bivalves, gastropods, and protozoans (19, 42). These coastal organisms may represent attractive models to study thioautotrophic symbiosis, since these animals are much easier to sample and can often be kept alive in the laboratory with their symbionts for some extended period. This is an advantage for studies aiming to unravel the functioning of the symbionts within the host tissues and their interactions with the host.

International audience; The quantification of interaction stoechiometry and binding constant between bacteria (or other microorganism) and(macro)molecules remains a challenging issue for which only a few adapted methods are available. In this paper, a new methodologywas developed for the determination of the interaction stoichiometry and binding constant between bacteria and (macro)molecules. The originality of this work is to take advantage of the bacterial aggregation phenomenon to directly quantify the freeligand concentration in equilibrated bacteria-ligand mixtures using frontal analysis continuous capillary electrophoresis. The describedmethodology does not require any sample preparation such as filtration step or centrifugation. It was applied to the study ofinteractions between Erwinia carotovora and different generations of dendrigraft poly-L-lysines leading to quantitative informations(i.e. stoichiometry and binding site constant). High stoichiometries in the order of 106-107 were determined between nanometricdendrimer-like ligands and the rod-shaped micrometric bacteria. The effect of the dendrimer generation on the binding constantand the stoichiometry is discussed. Stoichiometries were compared with those obtained by replacing the bacteria by polystyrenemicrobeads to demonstrate the internalization of the ligands inside the bacteria and the increase of the specific surface via theformation of vesicles.

The trophic relationships between metazooplankton and natural particles were studied in May 2002 in the Senegal River Estuary (16 degrees N, 16 degrees W) in low water conditions (dry season). Environmental factors, micro-organism and metazooplankton were analyzed through sampling at a fixed station. Gut fluorescence measurements of and field experiments on zooplankton metabolism were also performed. Chlorophyll a concentrations ranged from 6.5 to 10.2 mu g 1(-1). The phytoplankton was dominated by picoplanktonic cells (83-94% of total numbers). The particulate organic carbon (1.2-2.7 mg l(-1)) originated for a large amount from organic detritus (20-70%). The zooplankton biomass was dominated by Cirripedia larvae and calanoid copepods (Acartia clausi, Temora stylifera and Paracalanus spp.). These taxa showed diel vertical migrations and maximal gut fluorescence at night, independently of tidal effects. Metabolic budgets show that their daily ingestion rates on phytoplankton (27-55% of body carbon weight) were insufficient to balance their respiration needs (40-51% of body carbon) and suggest that a selective feeding upon micro-heterotrophs (Heterotrophic NanoFlagellates, HNF) and/or detritus would be necessary to complete their energetic needs. The daily grazing pressure of metazooplankton represented only 5% of the in situ chlorophyll a and 14% of the primary production, but this pressure was mainly orientated towards nanophytoplankton. The daily recycling of nutrients by the metazooplankton excretion was rather high (83 and 46% of the in situ NH4-N and PO4-P concentrations, respectively). Therefore, the impact of metazooplankton on phytoplankton through top-down (grazing) and bottom-up (nutrient recycling) processes seemed substantial in this tropical estuary. (c) 2005 Elsevier Ltd. All rights reserved.

on microbial plankton isolated from the Senegal River Estuary. Experiments performed in microcosms were based on size-class fractionation (3, 12, 20 and 60 m size classes) of the microbial community in order to obtain different assemblages, including bacteria, heterotrophic nanoflagellates (HNF), ciliates, pico- and nanophytoplankton. Removal of bacterial predators >3 m increased the net growth rate of heterotrophic bacteria from 0.437 day � 1 in the presence of all predators (fraction 12 m caused an increased net growth rate of flagellates from 0.102 day � 1 (fraction 12 m by this copepod, allowing an increase in HNF growth rates through indirect effect. The highest ingestion rate over 60 h of experiment was recorded for the copepod Temora (61.1 m 3 ngC � 1 h � 1 ) and the lowest value for barnacle larvae (25.6 m 3 ngC � 1 h � 1 ). An intermediate value (47.5m 3 ngC � 1 h � 1 )was recorded for thecopepodAcartia clausi.Alltheseingestion ratesweremainlychanneled in the size classes between

We conducted experiments to determine whether bacterial growth, with or without predators, is limited by inorganic (N and P) and organic (C) substrates in Selingue, a mesotrophic reservoir located in Mali, West Africa. Significant increases (relative to controls) in bacterial cell vol- umes and thymidine incorporation rates were observed after 24 h incubation only for samples amended with the combination +PC and +CNP. The data revealed a colimitation of bacterioplankton growth by organic carbon during the dry season. Flow cytometry discriminated 3 groups of bacteria (Bact I, Bact II, Bact III) differing in increases in nucleic acid content and cell size. The Bact I group, comprising cells with low nucleic acid content and of small size, was the dominant population in all experiments. In the absence of bacterial predators, only the Bact II group showed significant differ- ences between the control and the +PC and +CNP treatments, indicating that this bacterial group was the most sensitive to nutrient additions. The Bact II group, corresponding to cells with high DNA content, are active members of the bacterioplankton community. The Bact III group did not increase in any treatment but the proportions of these cells always increased in the presence of bacterial predators. These cells may be, at least partly, grazing-resistant bacteria.

Blanchet, Marine ... et. al.-- Special issue Microbial Ecology of the Continental and Coastal Environments.-- 16 pages, 8 figures, 2 tables, electronic supplementary material http://dx.doi.org/10.1007/s11356-014-3848-x, Spatial increases and temporal shifts in outbreaks of gelatinous plankton have been observed over the past several decades in many estuarine and coastal ecosystems. The effects of these blooms on marine ecosystem functioning and particularly on the dynamics of the heterotrophic bacteria are still unclear. The response of the bacterial community from a Mediterranean coastal lagoon to the addition of dissolved organic matter (DOM) from the jellyfish Aurelia aurita, corresponding to an enrichment of dissolved organic carbon (DOC) by 1.4, was assessed for 22 days in microcosms (8 l). The high bioavailability of this material led to (i) a rapid mineralization of the DOC and dissolved organic nitrogen from the jellyfish and (ii) the accumulation of high concentrations of ammonium and orthophosphate in the water column. DOM from jellyfish greatly stimulated heterotrophic prokaryotic production and respiration rates during the first 2 days; then, these activities showed a continuous decay until reaching those measured in the control microcosms (lagoon water only) at the end of the experiment. Bacterial growth efficiency remained below 20 %, indicating that most of the DOM was respired and a minor part was channeled to biomass production. Changes in bacterial diversity were assessed by tag pyrosequencing of partial bacterial 16S rRNA genes, DNA fingerprints, and a cultivation approach. While bacterial diversity in control microcosms showed little changes during the experiment, the addition of DOM from the jellyfish induced a rapid growth of Pseudoalteromonas and Vibrio species that were isolated. After 9 days, the bacterial community was dominated by Bacteroidetes, which appeared more adapted to metabolize high-molecular-weight DOM. At the end of the experiment, the bacterial community shifted toward a higher proportion of Alphaproteobacteria. Resilience of the bacterial community after the addition of DOM from the jellyfish was higher for metabolic functions than diversity, suggesting that jellyfish blooms can induce durable changes in the bacterial community structure in coastal lagoons. © 2014, Springer-Verlag Berlin Heidelberg, This study was supported by the CNRS-EC2CO program through the GELPROC project (2010–2012): “Interactions between gelatinous plankton and dynamics of prokaryotic communities.”

cited By 4; International audience; Sediments from Bizerta lagoon were used in an experimental microcosm setup involving three scenarios for the bioremediation of anthracene-polluted sediments, namely bioaugmentation, biostimulation, and a combination of both bioaugmentation and biostimulation. In order to investigate the effect of the biotreatments on the benthic biosphere, 16S rRNA gene-based T-RFLP bacterial community structure and the abundance and diversity of the meiofauna were determined throughout the experiment period. Addition of fresh anthracene drastically reduced the benthic bacterial and meiofaunal abundances. The treatment combining biostimulation and bioaugmentation was most efficient in eliminating anthracene, resulting in a less toxic sedimentary environment, which restored meiofaunal abundance and diversity. Furthermore, canonical correspondence analysis showed that the biostimulation treatment promoted a bacterial community favorable to the development of nematodes while the treatment combining biostimulation and bioaugmentation resulted in a bacterial community that advantaged the development of the other meiofauna taxa (copepods, oligochaetes, polychaetes, and other) restoring thus the meiofaunal structure. The results highlight the importance to take into account the bacteria/meiofauna interactions during the implementation of bioremediation treatment.

International audience; While considered as sustainable and low-cost agricultural amendments, the impacts of organic fertilizers on downstream aquatic microbial communities remain poorly documented. We investigated the quantity and quality of the dissolved organic matter leaching from agricultural soil amended with compost, vermicompost or biochar and assessed their effects on lake microbial communities, in terms of viral and bacterial abundances, community structure and metabolic potential. The addition of compost and vermicompost significantly increased the amount of dissolved organic carbon in the leachate compared to soil alone. Leachates from these additions, either with or without biochar were highly bioavailable to aquatic microbial communities, though reducing the metabolic potential of the community and harboring more specific communities. Although not affecting bacterial richness or taxonomic distributions, the specific addition of biochar affected the original lake bacterial communities resulting in a strongly different community. This could be partly explained by viral burst and converging bacterial abundances throughout the samples. These results underline the necessity to include off-site impacts of agricultural amendments when considering their cascading effect on downstream aquatic ecosystems. This article is protected by copyright. All rights reserved.

International audience; The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of 13C (H13CO3) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from 13C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the 13C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

The effects of starvation and salinity on the physiology of Salmonella typhimurium were investigated in a microcosm study. The physiological changes were monitored by using fluorochromes dyes such as DAPI (4',6-diamidino-2-phenylindole) for evaluation of the genomic content, CTC (5-cyano-2,3-ditolyl tetrazolium chloride) for respiratory activity and syto 9 and propidium iodide for cytoplasmic membrane damages. The metabolic activity of the cellular population was assessed with the method of Kogure (direct viable count), to enumerate the substrate-responsive cells. These different staining procedures were objectively analysed by an image analysis system. This paper describes the progressive alteration of Salmonella typhimurium physiology under salinity and starvation conditions.

The release of enteric pathogenic bacteria in aquatic environments poses the problem of the fate of these bacteria under the effects of environmental factors (solar radiation, salt concentration, temperature, nutrient level, pH, competition). Frequently, these bacterial cells, potentially pathogenic, enter into a non-culturable state on routine bacteriological plating media. However, the use of direct detection methods (DAPI stained cells) allows the visualization of these Viable but Non Culturable cells (VNC). But, beyond the characterization of the viability of the cells (electron transport activity, metabolic activity, membrane integrity, structure and/or quantity of DNA), what happens with the virulence of these cells? This problem was experimentally investigated according to the bacterial model Salmonella Typhimurium. The virulence of this strain, which is the agent of the murine typhoid, was evaluated on a mouse model. Experimentally, the effects of some environmental factors on the survival and on the maintenance of virulence of Salmonella Typhimurium were measured in microcosms exposed to UV radiation (four germicidal lamps 8 mW s(-1) cm(-2), wave length: 254 nm), salt concentration (Sea Salt Sigma, 37), nutrient starvation. The microcosms were simultaneously submitted to these three factors, with variable exposure times. For each of those times, the viability of the nonculturable cells (which became nonculturable because of the exposure to the three factors) was measured through different physiological states notable in the cells, after using different fluorescent dyes. The stained cells were observed by epifluorescence microscopy and analysed by image cytometry. So, the cellular populations are characterised by enumeration of respiring bacteria (CTC, [39]), metabolising bacteria (YEK, [22] modified), bacteria owning an undamaged cytoplasmic membrane (LD, Live/Dead BacLight Viability Kit. Molecular Probes Inc.); we also determined the quantity and/or structure of DNA of the cells (fluorescence level of DAPI stained cells). After exposure to the three factors for one hour (13.56 J cm(-2)), while the plate count cell density rapidly decreased from #10(7) cells mL(-1) to < 0.1 cell mL(-1), physiological states of these viable but non-culturable cells are similar to those of nonexposed cells. On the other hand, after exposure for three hours, only 10 % of the cells deposit a CTC formazan-crystal and 20 % are substrate responsive cells (enlarged cells in presence of Yeast Extract and Cephalexin: YEC). Half of the cellular population presents an undamaged cytoplasmic membrane and the level of fluorecence of DAPI stained cells is close to 85 %, which shows that the DNA of these cells is weakly damaged. After exposure to the three experimental factors for 24 hours (315 J cm(-2)), weak replies to the physiological tests used in this study to characterize the viability of the non-culturable cellular population are observed (CTC: 4 %; YEC: 2 %; LD: 11.8 %) while the fluorescence level of DAPI stained cells remains firm at 80 %. At the same time, the virulence expression of VNC cells of Salmonella Typhimurium, evaluated by intraperitoneal injection to the mouse (route which excludes uncontrolled parameters, unlike the per os route) does not seem to be correlated with the cellular viability such as it has been evaluated in this study. A 30 min exposure (6.73 J cm(-2)) to the three environmental factors, leading to the non-culturability of almost the entire exposed cell population (0.08 culturable cell mL(-1)) whereas the level of viability of those culturable cells is closed to the one of non-exposed cells. The injection of 1000 of those cells (, Le rejet de bactéries entériques pathogènes dans l'environnement aquatique pose le problème du devenir de ces microorganismes sous les effets des facteurs environnementaux (rayonnement solaire, salinité, température, oligotrophie, pH, compétition). L'évolution fréquente de ces cellules bactériennes potentiellement pathogènes vers un état non cultivable implique l'absence de détection de celles-ci par des méthodes traditionnelles. Cependant, l'utilisation des méthodes de détection directes permet de visualiser ces cellules non cultivables. Au stade suivant, il est nécessaire de caractériser le niveau de viabilité de ces cellules non cultivables. Mais au-delà de la caractérisation de l'état viable de la cellule (activité des chaînes respiratoires, activité métabolique, intégrilé membranaire, structure et/ou quantité d'ADN) se pose parallèlement le problème du maintien du pouvoir pathogène de ces bactéries. Cette problématique a été abordée expérimentalement sur le modèle bactérien Salmonella Typhimurium. Pour évaluer le pouvoir pathogène de cette souche, il est fait appel au modèle souris, Salmonella Typhimurium étant l'agent de la typhoïde murine. Pour évaluer les effets de certains facteurs environnementaux sur cette souche pathogène, il a été fait appel sur le plan expérimental en microcosmes au rayonnement UV (quatre lampes germicides 8 mW s−1 cm −2à 254 nm), à la salinité (SeaSalt Sigma, 37) et à l'oligotrophie. Ces trois facteurs sont appliqués simultanément avec des temps d'exposition variables. Pour chaque temps d'exposition, le niveau de viabilité des cellules qui ont perdu le pouvoir de cultiver sous les effets de ces facteurs est mesuré à travers différents états physiologiques, au moyen de marqueurs fluorochromiques. La réaction marqueur-cellule est observée par microscopic en épifluorescence et analysée par cytométrie en images. Ainsi, les populations cellulaires sont caractérisées sur le plan de la respiration (CTC, [39]), de la capacité à métaboliser un nutriment (YEC, [22] modifié), de l'intégrité membranaire (LD, Kit Live/Dead Baclight-Molecular Probes Inc) et de la structure et/ou quantité d'ADN [36]. Alors que la disparition du pouvoir de cultiver de toutes les cellules exposées (zéro cultivabilité) se réalise en 1 h (13,56 J cm−2), les états physiologiques que présentent ces cellules non cultivables sont proches de ceux présentés par les cellules d'une culture non exposée. Par contre, après 3 h d'exposition, 10 % seulement des cellules présentent une activité de leur chaîne respiratoire (CTC) et 20 % présentent une activité métabolique (extrait de levure en présence de céphalexine). La moitié des cellules présente une atteinte de la membrane cytoplasmique et le niveau de fluorescence de la population cellulaire est voisin de 85 % montrant par là que ces cellules ne présentent à ce stade qu'une faible atteinte de l'ADN. Après 24 h d'exposition (315 J cm−2) à ces trois facteurs stressants expérimentaux, les tests physiologiques appliqués dans cette étude sur les cellules non cultivables sont quasiment négatifs alors que le niveau de fluorescence de leur ADN marqué se maintient à 80 %. En parallèle, le suivi du pouvoir pathogène de ces cellules de Salmonella Typhimurium, évalué par injection intrapéritonéale à la souris (voie permettant de s'affranchir de paramètres non contrôlés comme la voie per os), ne paraît pas évoluer en fonction de la disparition de la viabilité cellulaire mesurée par les tests utilisés dans cette étude. Une exposition de 30 min (6,73 J cm−2) aux trois facteurs stressants expérimentaux fait disparaître le pouvoir de cultiver de la quasi-totalité des cellules exposées (0,08 cellules cultivables mL−1) alors que ces cellules présentent un état de viabilité proche de celui d'une souche non exposée. L'injection de 1000 de ces cellules ( 0,001 cellule cultivable dans 100 μL injecté) à la souris (un lot de dix souris) n'entraîne aucun état pathologique ni mortalité sur quatre semaines d'observations alors que l'injection du même nombre de cellules non exposées aux trois facteurs stressants entraîne la mort du lot de souris en une semaine. D'après ces quelques expérimentations préliminaires sur Salmonella Typhimurium, la perte du pouvoir de cultiver paraît s'accompagner de la perte de la virulence testée chez la souris par voie intrapéritonéale après exposition aux effets associés de l'irradiation UV, de la salinité 37 et de l'oligotrophie.

cited By 14; International audience; Biological interactions between metazoans and the microbial community play a major role in structuring food webs in aquatic sediments. Pollutants can also strongly affect the structure of meiofauna and microbial communities. This study aims investigating, in a non-contaminated sediment, the impact of meiofauna on bacteria facing contamination by a mixture of three PAHs (fluoranthene, phenanthrene and pyrene). Sediment microcosms were incubated in the presence or absence of meiofauna during 30. days. Bioremediation treatments, nutrient amendment and addition of a hydrocarbon-degrading bacterium, were also tested to enhance PAH biodegradation. Results clearly show the important role of meiofauna as structuring factor for bacterial communities with significant changes observed in the molecular fingerprints. However, these structural changes were not concomitant with changes in biomass or function. PAH contamination had a severe impact on total meiofaunal abundance with a strong decrease of nematodes and the complete disappearance of polychaetes and copepods. In contrast, correspondence analysis, based on T-RFLP fingerprints, showed that contamination by PAH resulted in small shifts in microbial composition, with or without meiofauna, suggesting a relative tolerance of bacteria to the PAH cocktail. The PAH bioremediation treatments were highly efficient with more than 95% biodegradation. No significant difference was observed in presence or absence of meiofauna. Nutrient addition strongly enhanced bacterial and meiofaunal abundances as compared to control and contaminated microcosms, as well as inducing important changes in the bacterial community structure. Nutrients thus were the main structural factor in shaping bacterial community composition, while the role of meiofauna was less evident.

International audience; Mangroves are threatened ecosystems that provide numerous ecosystem services, especially through their wide biodiversity, and their bioremediation capacity is a challenging question in tropical areas. In a mangrove in Mayotte, we studied the potential role of microbial biofilm communities in removing nutrient loads from pre-treated wastewater. Microbial community samples were collected from tree roots, sediments, water, and from a colonization device, and their structure and dynamics were compared in two areas: one exposed to sewage and the other not. The samples from the colonization devices accurately reflected the natural communities in terms of diversity. Communities in the zone exposed to sewage were characterized by more green algae and diatoms, higher bacteria densities, as well as different compositions. In the area exposed to sewage, the higher cell densities associated with specific diversity patterns highlighted adapted communities that may play a significant role in the fate of nutrients.

International audience; Purpose: The main goals of this study were to investigate (1) the behavior of microbial communities in response to low-dose bioavailable anthracene addition in lightly contaminated sediment from Bizerte Lagoon and (2) the effects of bioremediation treatments on microbial biomass, activity, and community structure. Methods: Sediment microcosms amended with 1 ppm anthracene were incubated in triplicate during 30 days. Biostimulation (addition of nitrogen and phosphorus fertilizer) and bioaugmentation (inoculation of a hydrocarbonoclastic bacterium) were used as bioremediation treatments. Bacterial biomass was estimated using flow cytometry. Sediment oxygen consumption was measured with oxygen microelectrodes. Bacterial community structure was assessed by molecular fingerprints (terminal restriction fragment length polymorphism; T-RFLP) analysis. Results: Anthracene contamination resulted in a significant reduction of bacterial abundance with an impact on cell integrity. Concomitantly, sediment oxygen consumption was strongly inhibited. Correspondence analysis on T-RFLP data indicated that bacterial community structures from anthracene-contaminated microcosms were different from that of the control. Interestingly, the changes observed in microbial biomass, structure, and activities as a result of anthracene contamination were not alleviated even with the use of biostimulation and combination of biostimulation and bioaugmentation strategy for anthracene bioremediation. Nevertheless, both treatment methods resulted in different community structures relative to the contaminated and control microcosms with the appearance of distinct populations. Conclusion: Anthracene spiking severely affected microbial communities, suggesting dominance of nontolerant populations in this lightly-contaminated sediment. Although biostimulation and/or bioaugmentation treatments did not alleviate the anthracene toxic effects, the changes observed in microbial population and structure suggest that the proposed treatments might be promising to promote bacterial growth. Further works are still required to propose a more efficient strategy to stimulate biodegradation that takes into account the complex interactions between species for resource access. © 2012 Springer-Verlag.

International audience; Few studies on the consequences of future global warming and increased ultraviolet-B radiation (UVBR, 280-320nm) have been carried out at the food web scale and very few have considered the effect of the combination of these two climatic stressors. The single and combined effect of 3°C warming and 20% increased UVBR on planktonic carbon and nitrogen dynamics was investigated during a 10-day mesocosm experiment in coastal Mediterranean waters. Planktonic organisms from Thau Lagoon were sampled and incubated into eight in situ moored 2000L mesocosms and exposed to 4 different temperature and light conditions in duplicates; 1) natural light and temperature conditions; 2) +3°C above in situ water temperature; 3) +20% above incident UVBR, and 4) simultaneous +3°C and +20% above natural conditions. Compared to the natural in situ temperature and light conditions, increasing UVBR alone reduced bacterial carbon production and biomass by an average of 20%, with some reduction in ammonium net production. Warming alone significantly reduced bacterial carbon production by about 40% as a consequence of a trophic cascade effect on bacterial biomass. Warming also increased phytoplankton carbon production and associated nitrate uptake rates by an average of 90% and 200%, respectively. Thus, the results showed a direct positive effect of temperature increase on phytoplankton metabolism and greater nutrient availability owing to the reduction in potential bacterial competition. The combination of temperature and increased UVBR significantly increased the autotrophic production but without accumulation of freshly produced particulate organic matter. This suggests an accumulation of dissolved organic matter under these climatic conditions.

In intermittent rivers, characterised by a specific hydrological behaviour, sediment–water column interactions may influence water quality during low flows. The main objective of this work was to assess the extent of anthropogenic pollution (organotins, faecal indicator bacteria and nutrients) in the river sediment of an intermittent river and its impact on the downstream water bodies: The Vene River, main tributary of the Thau lagoon. We first investigated anthropogenic pollution from water and sediment samples collected in situ along the river course (1.5 km); then, during laboratory experiments, we assessed the survival of faecal bacteria and quantified the degradation rates of organotins on the same sediments. The results indicate the presence of anthropogenic pollution all along the study reach. The waste water treatment plant effluent is a direct pollution input source for anthropogenic pollution. The sediment data points to an urban drainage ditch as a secondary point pollution source while the organotins data highlights the presence of other diffuse sources, specific to this substance. The results of the laboratory experiments show that both faecal bacteria and organotins may persist in the river sediments for up to two months in summer and even longer in winter. This indicates that sediments may, under favourable conditions, become important pollutant stores which may later be released and transported to the Thau lagoon during floods.

OmpU porins are increasingly recognized as key determinants of pathogenic host Vibrio interactions. Although mechanisms remain incompletely understood, various species, including the human pathogen Vibrio cholera , require OmpU for host colonization and virulence. We have shown previously that OmpU is essential for virulence in the oyster pathogen Vibrio splendidus LGP32. Here, we showed that V. splendidus LGP32 invades the oyster immune cells, the hemocytes, through subversion of host-cell actin cytoskeleton. In this process, OmpU serves as an adhesin/invasin required for β-integrin recognition and host cell invasion. Furthermore, the major protein of oyster plasma, the extracellular superoxide dismutase Cg -EcSOD, is used as an opsonin mediating the OmpU-promoted phagocytosis through its RGD sequence. Finally, the endocytosed bacteria were found to survive intracellularly, evading the host defense by preventing acidic vacuole formation and limiting reactive oxygen species production. We conclude that ( i ) V. splendidus is a facultative intracellular pathogen that manipulates host defense mechanisms to enter and survive in host immune cells, and ( ii ) that OmpU is a major determinant of host cell invasion in Vibrio species, used by V. splendidus LGP32 to attach and invade oyster hemocytes through opsonisation by the oyster plasma Cg -EcSOD.

The existence of Salmonella enterica serovar Typhimurium viable-but-nonculturable (VBNC) cells is a public health concern since they could constitute unrecognized sources of infection if they retain their pathogenicity. To date, many studies have addressed the ability of S . Typhimurium VBNC cells to remain infectious, but their conclusions are conflicting. An assumption could explain these conflicting results. It has been proposed that infectivity could be retained only temporarily after entry into the VBNC state and that most VBNC cells generated under intense stress could exceed the stage where they are still infectious. Using a Radioselectan density gradient centrifugation technique makes it possible to increase the VBNC-cell/culturable-cell ratio without increasing the exposure to stress and, consequently, to work with a larger proportion of newly VBNC cells. Here, we observed that (i) in the stationary phase, the S . Typhimurium population comprised three distinct subpopulations at 10, 24, or 48 h of culture; (ii) the VBNC cells were detected at 24 and 48 h; (iii) measurement of invasion gene ( hilA , invF , and orgA ) expression demonstrated that cells are highly heterogeneous within a culturable population; and (iv) invasion assays of HeLa cells showed that culturable cells from the different subpopulations do not display the same invasiveness. The results also suggest that newly formed VBNC cells are either weakly able or not able to successfully initiate epithelial cell invasion. Finally, we propose that at entry into the stationary phase, invasiveness may be one way for populations of S . Typhimurium to escape stochastic alteration leading to cell death.

The bivalve Codakia orbicularis , hosting sulfur-oxidizing gill endosymbionts, was starved (in artificial seawater filtered through a 0.22-μm-pore-size membrane) for a long-term experiment (4 months). The effects of starvation were observed using transmission electron microscopy, fluorescence in situ hybridization and catalyzed reporter deposition (CARD-FISH), and flow cytometry to monitor the anatomical and physiological modifications in the gill organization of the host and in the symbiotic population housed in bacteriocytes. The abundance of the symbiotic population decreased through starvation, with a loss of one-third of the bacterial population each month, as shown by CARD-FISH. At the same time, flow cytometry revealed significant changes in the physiology of symbiotic cells, with a decrease in cell size and modifications to the nucleic acid content, while most of the symbionts maintained a high respiratory activity (measured using the 5-cyano-2,3-ditolyl tetrazolium chloride method). Progressively, the number of symbiont subpopulations was reduced, and the subsequent multigenomic state, characteristic of this symbiont in freshly collected clams, turned into one and five equivalent genome copies for the two remaining subpopulations after 3 months. Concomitant structural modifications appeared in the gill organization. Lysosymes became visible in the bacteriocytes, while large symbionts disappeared, and bacteriocytes were gradually replaced by granule cells throughout the entire lateral zone. Those data suggested that host survival under these starvation conditions was linked to symbiont digestion as the main nutritional source.

This study focused on the effects of two pesticides, paraquat (herbicide) and deltamethrin (insecticide), which are two common molecules used intensively in Burkina Faso. Natural bacterial populations, phytoplankton cultures (one cyanobacterium, Cylindrospermopsis raciborskii, and one chlorophycea, Monoraphidium sp.), and two species of zooplankton (Diaphanosoma excisum and Moina micrura) were isolated from aquatic communities and were used as biological targets in the experimental protocols. Paraquat was moderately toxic to bacteria and phytoplankton, whereas deltamethrin was significantly toxic only to the zooplankton species. Paradoxically, the chlorophycea Monoraphidium sp. exhibited a significant increase of in vitro fluorescence after 48 h at the highest doses. Preliminary tests were also performed from natural water extract of the main drinking water supply of the country (Loumbila Reservoir) by using solid-phase extraction. Obviously, the natural extract proved to be toxic to the same biological targets. Despite the absence of any determination of pesticides in the natural extract, the question of contamination and toxicity of these waters affects concerns about the safety of water supply and the effect of human pressure on the dynamics of planktonic communities of freshwater reservoirs in arid regions of western Africa.

Temporal variability in bacterial communities was studied on a daily scale in the estuar- ine part of the largest river on the West African coast, the Senegal River. Duplicate mesocosms (3 m 3 volume) were placed in the upper part of the estuary at the end of the dry season (May 2002) and treated with low and high inorganic nutrient (N and P) enrichment. High nutrient additions were followed by a 7-fold increase in phytoplankton biomass and a 6-fold increase in bacterial abundance after 4 and 9 d, respectively. Heterotrophic nanoflagellates (HNF) showed their maximal abundance (1 × 10 6 ml -1 ) 2 d after the bacterial peak. The low bacteria to flagellate ratios recorded on Day 10 may suggest enhanced bacterivory from HNF. Simultaneous measurements of growth and grazing rates on bacteria during the bacterial growth phase were performed with the dilution method and seemed to indicate that the HNF community was capable of quickly controlling bacterial development. How- ever, estimates of the carbon demand of HNF during their growth phase (915 µg C l -1 d -1 ) appeared to be more elevated than the bacterial carbon production (63% of the HNF carbon demand). To cover HNF carbon requirements, an alternative/complementary prey might be the picophytoplanktonic cells, which were very numerous during the study (i.e. 85% of the total phytoplankton count). During the period of high grazing pressure, bacterial populations were characterized by higher specific activity (from tritiated thymidine incorporation) and culturability (from plate counts). The presence of very large bacteria, as detected by epifluorescence microscopy and flow cytometry measure- ments, may be an escape response from flagellate grazing. 16S rDNA sequences from bacterial isolates showed the presence of 2 types of taxonomic units (Vibrio natriegens- and Flexibacter maritimus-like bacteria), which can be considered by their forms and growth rates to be strains that have developed strategies to protect against grazing. Thus, as demonstrated in various temperate systems, predation by HNF in estuarine tropical ecosystems may also be of importance in shaping the structure and functions of the bacterial community.

Maintenance of pathogenicity of viable but nonculturable Salmonella typhimurium cells experimentally stressed with UV-C and seawater, was investigated relative to the viability level of the cellular population. Pathogenicity, tested in a mouse model, was lost concomitantly with culturability, whereas cell viability remained undamaged, as determined by respiratory activity and cytoplasmic membrane and genomic integrities.