Synergistic effects of temperature and light affect the relationship between Taonia atomaria and its epibacterial community: a controlled conditions study.

In the context of global warming, this study aimed to assess the effect of temperature and irradiance on the macroalgal Taonia atomaria holobiont dynamics. We developed an experimental set-up using aquaria supplied by natural seawater with three temperatures combined with three irradiances. The holobiont response was monitored over 14 days using a multi-omics approach coupling algal surface metabolomics and metabarcoding. Both temperature and irradiance appeared to shape the microbiota and the surface metabolome, but with a distinct temporality. Epibacterial community first changed according to temperature, and later in relation to irradiance, while the opposite occurred for the surface metabolome. An increased temperature revealed a decreasing richness of the epiphytic community together with an increase of several bacterial taxa. Irradiance changes appeared to quickly impact surface metabolites production linked with the algal host photosynthesis (e.g. mannitol, fucoxanthin, dimethylsulfoniopropionate), which was hypothesized to explain modifications of the structure of the epiphytic community. Algal host may also directly adapt its surface metabolome to changing temperature with time (e.g. lipids content) and also in response to changing microbiota (e.g. chemical defences). Finally, this study brought new insights highlighting complex direct and indirect responses of seaweeds and their associated microbiota under changing environments.

Elutriate preparation affects embryo development test with Paracentrotus lividus: An in-depth study on the differences between two protocols and three different sediment/water mixing times.

Coastal areas are under continuous and increasing pressure from different human activities. A mixture of contaminants (e.g. hydrocarbons, pesticides, persistent organic pollutants (POPs), emerging contaminants, and others), originating mainly from populated, industrialised and agricultural areas, can reach the marine environment through different means such as wastewater discharge, soil runoffs, leaching from agriculture, and volatilisation/deposition. In this context, marine sediments have increasingly been considered repositories for a variety of pollutants that can accumulate and be stored for long periods, acting as a secondary source of contaminants during subsequent dredging operation or vessel manoeuvring. Chemical and ecotoxicological analyses of sediments are routinely conducted to evaluate the potential hazard/risk to the environment, either on bulk sediment or elutriate. In general, sediment elutriates are commonly prepared according to ASTM Guide even if alternative protocols are proposed by USACE for the various condition that they have to represent. The goal of the present study was to determine if the toxicological properties of ASTMprepared elutriates are comparable to those obtained from the USACE protocol. Sediment coming from 3 harbours (Olbia, Cagliari, and Toulon), as part of the "Se.D.Ri.Port" Interreg Project, were processed to obtain elutriates according to ASTM Guide and USACE Dredging Elutriate protocol and tested with the sea urchin Paracentrotus lividus embryo development test. Moreover, the significance of different stirring times of water/sediment mixture (1 h, 3 h, and 24 h) was tested with both the ASTM and USACE protocol. In addition to the biological analysis, for each sediment sample, heavy metals concentration, granulometry, and organic matter were determined. Even if for the ports of Toulon and Cagliari, the ASTM and USACE elutriates showed comparable results with P. lividus bioassay, for the port of Olbia the two protocols showed different criticalities. Preliminary results show that for the site Olbia elutriates prepared with the USACE protocol resulted in higher toxicity than elutriates obtained with ASTM (p < 0.001). In conclusion, differences in preparation protocols appear to be significant and can lead to different results in biological testing. To overcome this problem and to obtain more reliable evaluations of risk to the environment, standardisation and regulation must be the next goals in sediment management procedure.

Genetic diversity along the life cycle of the cyanobacterium Microcystis: highlight on the complexity of benthic and planktonic interactions.

Microcystis is a toxic freshwater cyanobacterium with an annual life cycle characterized by the alternation of a planktonic proliferation stage in summer and a benthic resting stage in winter. Given the importance of both stages for the development and the survival of the population, we investigated the genotypic composition of the planktonic and benthic Microcystis subpopulations from the Grangent reservoir (France) during two distinct proliferation periods. Our results showed a succession of different dominant genotypes in the sediment as well as in the water all along the study periods with some common genotypes to both compartments. Analysis of molecular variance and UniFrac analysis confirmed the similarity between some benthic and planktonic samples, thus evidencing exchanges of genotypes between water and sediment. Thanks to these data, recruitment and sedimentation were proven not to be restricted to spring and autumn, contrary to what was previously thought. Finally, genetic diversity was significantly higher in the sediment than in the water (P < 0.01; Student's t-test). Taken together, our results shed light on the hidden contribution of the benthic compartment in maintaining the genetic diversity of Microcystis populations throughout their annual cycle, which could explain their ecological success in aquatic ecosystems.

Soil surface colonization by phototrophic indigenous organisms, in two contrasted soils treated by formulated maize herbicide mixtures.

Soil phototrophic microorganisms, contributors to soil health and food webs, share their particular metabolism with plants. Current agricultural practices employ mixtures of pesticides to ensure the crops yields and can potentially impair these non-target organisms. However despite this environmental reality, studies dealing the susceptibility of phototrophic microorganisms to pesticide mixtures are scarce. We designed a 3 months microcosm study to assess the ecotoxicity of realistic herbicide mixtures of formulated S-metolachlor (Dual Gold Safeneur(®)), mesotrione (Callisto(®)) and nicosulfuron (Milagro(®)) on phototrophic communities of two soils (Limagne vertisol and Versailles luvisol). The soils presented different colonizing communities, with diatoms and chlorophyceae dominating communities in Limagne soil and cyanobacteria and bryophyta communities in Versailles soil. The results highlighted the strong impairment of Dual Gold Safeneur(®) treated microcosms on the biomass and the composition of both soil phototrophic communities, with no resilience after a delay of 3 months. This study also excluded any significant mixture effect on these organisms for Callisto(®) and Milagro(®) herbicides. We strongly recommend carrying on extensive soil studies on S-metolachlor and its commercial formulations, in order to reconsider its use from an ecotoxicological point of view.

Assessing the diversity of plankton-associated prokaryotes along a size-fraction gradient: A methodological evaluation.

Marine free-living (FL) and plankton-associated prokaryotes (plankton-microbiota) are at the basis of trophic webs and play crucial roles in the transfer and cycling of nutrients, organic matter, and contaminants. Different ecological niches exist along the plankton size fraction gradient. Despite its relevant ecological role, the plankton-microbiota has rarely been investigated with a sufficient level of size-fraction resolution, and it can be challenging to study because of overwhelming eukaryotic DNA. Here we compared the prokaryotic diversity obtained by 16S rRNA gene sequencing from six plankton size fractions (from FL to mesoplankton), through three DNA recovery methods: direct extraction, desorption pretreatment, enrichment post-treatment. The plankton microbiota differed strongly according to the plankton size-fraction and methodological approach. Prokaryotic taxa specific to each size fraction, and methodology used, were identified. Vibrionaceae were over-represented by cell desorption pretreatment, while prokaryotic DNA enrichment had taxon-specific effects, indicating that direct DNA extraction was the most appropriate method.

Contamination of planktonic food webs in the Mediterranean Sea: Setting the frame for the MERITE-HIPPOCAMPE oceanographic cruise (spring 2019).

This paper looks at experiential feedback and the technical and scientific challenges tied to the MERITE-HIPPOCAMPE cruise that took place in the Mediterranean Sea in spring 2019. This cruise proposes an innovative approach to investigate the accumulation and transfer of inorganic and organic contaminants within the planktonic food webs. We present detailed information on how the cruise worked, including 1) the cruise track and sampling stations, 2) the overall strategy, based mainly on the collection of plankton, suspended particles and water at the deep chlorophyll maximum, and the separation of these particles and planktonic organisms into various size fractions, as well as the collection of atmospheric deposition, 3) the operations performed and material used at each station, and 4) the sequence of operations and main parameters analysed. The paper also provides the main environmental conditions that were prevailing during the campaign. Lastly, we present the types of articles produced based on work completed by the cruise that are part of this special issue.

Cd transfers during marine sediment resuspension over short and long-term period: Associated risk for coastal water quality.

Cadmium (Cd) is a highly toxic metal, regularly monitored uniformly for water quality across Europe, but scarcely for sediments. This study was designed to compare the kinetics of Cd remobilization and the amplitude of its transfers with different marine sediments. The results showed a highly reproducible transfer kinetics. Dissolved Cd was strongly and quickly removed from the dissolved phase (from 5 min up to 7 h). Then, the dissolved Cd concentration increased progressively to reach a maximal value after two weeks of mixing. The influence of the resuspension intensity representing light wind-induced resuspension up to dredging operations was observed after 2 weeks. The intensity of the sediment resuspension clearly impacted the amplitude of Cd remobilization, dissolved Cd ranging from a few ngL-1 to few hundreds of ngL-1, exceeding the maximal dissolved Cd concentration accepted by the European Union Water Framework Directive (WFD-2008/105 32/EC).

Limited influence of marine sediment lyophilization on prokaryotic community structure assessed via amplicon sequencing: an example from environmentally contrasted sediment layers in Toulon harbor (France).

Sediment lyophilization is a common process that allows for long-term conservation and sharing of marine sediments for multiple downstream analyses. Although it is often used for geochemical studies, the effects of lyophilization on prokaryotic taxonomic diversity assessment remained to be assessed. Here, we tested the effect of lyophilization on microbial diversity assessment using three sediment layers corresponding to various sediment ages and chemical contamination levels sampled from a marine Mediterranean harbor. Duplicate DNA samples were extracted from wet frozen or lyophilized sediments, and 16S rRNA gene amplicon sequence variants were analyzed. We detected changes in community structure over depth linked to both dominant and less abundant taxa whether sediments were lyophilized or not. Data from both wet frozen and lyophilized sediments led us to conclude that historical chemical contamination of the sediment of Toulon Bay did not appear to be the main environmental variable shaping prokaryotic community structure on the vertical dimension, but that sediment diagenesis was. We conclude that sediment lyophilization is compatible with marine biogeochemical and ecotoxicological studies but that caution should be used when discussing small variations among samples.

Metal resistance genes enrichment in marine biofilm communities selected by biocide-containing surfaces in temperate and tropical coastal environments.

Microorganisms able to form biofilms in marine ecosystems are selected depending on immersed surfaces and environmental conditions. Cell attachment directly on toxic surfaces like antifouling coatings suggests a selection of tolerant (or resistant) organisms with characteristics conferring adaptive advantages. We investigated if environment would drive metal resistance gene abundance in biofilms on artificial surfaces. Biofilms were sampled from three surfaces (a PVC reference and two antifouling coatings) deployed in three coastal waters with dissimilar characteristics: The Mediterranean Sea (Toulon) and Atlantic (Lorient) and Indian (Reunion) Oceans. The two coatings differed in metals composition, either Cu thiocyanate and Zn pyrithione (A3) or Cu2O (Hy). Metal resistance genes (MRG) specific to copper (cusA, copA, cueO) or other metals (czcA and pbrT) were monitored with qPCR in parallel to the microbial community using 16S rRNA gene metabarcoding. A lower α-diversity on A3 or Hy than on PVC was observed independent on the site. Weighted Unifrac suggested segregation of communities primarily by surface, with lower site effect. Metacoder log2 fold change ratio and LeFSe discrimination suggested Marinobacter to be specific of Hy and Altererythrobacter, Erythrobacter and Sphingorhabdus of A3. Likewise, the relative abundance of MRG (MRG/bacterial 16S rRNA) varied between surfaces and sites. A3 presented the greatest relative abundances for cusA, cueO and czcA. The latter could only be amplified from A3 communities, except at Toulon. Hy surface presented the highest relative abundance for copA, specifically at Lorient. These relative abundances were correlated with LeFSe discriminant taxa. Dasania correlated positively with all MRG except cueO. Marinobacter found in greater abundance in Hy biofilm communities correlated with the highest abundances of copA and Roseovarius with czcA. These results prove the selection of specific communities with abilities to tolerate metallic biocides forming biofilms over antifouling surfaces, and the secondary but significant influence of local environmental factors.

Integration of spatio-temporal variations of surface metabolomes and epibacterial communities highlights the importance of copper stress as a major factor shaping host-microbiota interactions within a Mediterranean seaweed holobiont.

BACKGROUND: Although considered as holobionts, macroalgae and their surface microbiota share intimate interactions that are still poorly understood. Little is known on the effect of environmental parameters on the close relationships between the host and its surface-associated microbiota, and even more in a context of coastal pollutions. Therefore, the main objective of this study was to decipher the impact of local environmental parameters, especially trace metal concentrations, on an algal holobiont dynamics using the Phaeophyta Taonia atomaria as a model. Through a multidisciplinary multi-omics approach combining metabarcoding and untargeted LC-MS-based metabolomics, the epibacterial communities and the surface metabolome of T. atomaria were monitored along a spatio-temporal gradient in the bay of Toulon (Northwestern Mediterranean coast) and its surrounding. Indeed, this geographical area displays a well-described trace metal gradient particularly relevant to investigate the effect of such pollutants on marine organisms. RESULTS: Epibacterial communities of T. atomaria exhibited a high specificity whatever the five environmentally contrasted collecting sites investigated on the NW Mediterranean coast. By integrating metabarcoding and metabolomics analyses, the holobiont dynamics varied as a whole. During the occurrence period of T. atomaria, epibacterial densities and α-diversity increased while the relative proportion of core communities decreased. Pioneer bacterial colonizers constituted a large part of the specific and core taxa, and their decrease might be linked to biofilm maturation through time. Then, the temporal increase of the Roseobacter was proposed to result from the higher temperature conditions, but also the increased production of dimethylsulfoniopropionate (DMSP) at the algal surface which could constitute of the source of carbon and sulfur for the catabolism pathways of these taxa. Finally, as a major result of this study, copper concentration constituted a key factor shaping the holobiont system. Thus, the higher expression of carotenoids suggested an oxidative stress which might result from an adaptation of the algal surface metabolome to high copper levels. In turn, this change in the surface metabolome composition could result in the selection of particular epibacterial taxa. CONCLUSION: We showed that associated epibacterial communities were highly specific to the algal host and that the holobiont dynamics varied as a whole. While temperature increase was confirmed to be one of the main parameters associated to Taonia dynamics, the originality of this study was highlighting copper-stress as a major driver of seaweed-epibacterial interactions. In a context of global change, this study brought new insights on the dynamics of a Mediterranean algal holobiont submitted to heavy anthropic pressures. Video abstract.

Trace Metal Contamination Impacts Predicted Functions More Than Structure of Marine Prokaryotic Biofilm Communities in an Anthropized Coastal Area.

Trace metal (TM) contamination in marine coastal areas is a worldwide threat for aquatic communities. However, little is known about the influence of a multi-chemical contamination on both marine biofilm communities' structure and functioning. To determine how TM contamination potentially impacted microbial biofilms' structure and their functions, polycarbonate (PC) plates were immerged in both surface and bottom of the seawater column, at five sites, along strong TM contamination gradients, in Toulon Bay. The PC plates were incubated during 4 weeks to enable colonization by biofilm-forming microorganisms on artificial surfaces. Biofilms from the PC plates, as well as surrounding seawaters, were collected and analyzed by 16S rRNA amplicon gene sequencing to describe prokaryotic community diversity, structure and functions, and to determine the relationships between bacterioplankton and biofilm communities. Our results showed that prokaryotic biofilm structure was not significantly affected by the measured environmental variables, while the functional profiles of biofilms were significantly impacted by Cu, Mn, Zn, and salinity. Biofilms from the contaminated sites were dominated by tolerant taxa to contaminants and specialized hydrocarbon-degrading microorganisms. Functions related to major xenobiotics biodegradation and metabolism, such as methane metabolism, degradation of aromatic compounds, and benzoate degradation, as well as functions involved in quorum sensing signaling, extracellular polymeric substances (EPS) matrix, and biofilm formation were significantly over-represented in the contaminated site relative to the uncontaminated one. Taken together, our results suggest that biofilms may be able to survive to strong multi-chemical contamination because of the presence of tolerant taxa in biofilms, as well as the functional responses of biofilm communities. Moreover, biofilm communities exhibited significant variations of structure and functional profiles along the seawater column, potentially explained by the contribution of taxa from surrounding sediments. Finally, we found that both structure and functions were significantly distinct between the biofilm and bacterioplankton, highlighting major differences between the both lifestyles, and the divergence of their responses facing to a multi-chemical contamination.

Have decades of abiotic studies in sediments been misinterpreted?

Sterilization techniques are largely employed to distinguish biotic and abiotic processes in biogeochemical studies as they inhibit microbial activity. Since one century, chemical sterilizers, supposed to preserve original environmental samples, have taken precedence over physical sterilization techniques considered too destructive. Sodium azide (NaN3) is nowadays the most commonly used inorganic chemical sterilizer. It is sufficiently purified to study trace metals, as well. Nevertheless, its (in)activity in physico-chemical processes was never ascertained. Through the investigation of sediment resuspension in seawater, the present work unequivocally demonstrated that NaN3 can impact carbon and trace metals' transfers by altering the redox balance and pH. Unlike decades of blind practice, NaN3 should be used with great care to track abiotic processes from organic matter rich and reductive matrices.

Dissolved organic matter controls of arsenic bioavailability to bacteria.

The presence of arsenic in irrigation and drinking waters is a threat to worldwide human health. Dissolved organic matter (DOM) is a ubiquitous and photoreactive sorbent of arsenic, capable of both suppressing and enhancing its mobility. Microbes can control the mobilization of mineral-bound arsenic, through redox processes thought to occur intracellularly. The role that DOM plays on the bioavailability of arsenic to microbes is often invoked but remains untested experimentally. Here, using a whole-cell biosensor, we tested the role of DOM on As(III) and As(V) bioavailability. Using cation amendments, we explored the nature of As-DOM interactions. We found As bioavailability to be dependent on [As]/[DOM] ratio and on the strength of As binding to DOM which varied as a function of time. We further tested the role of DOM on As(III) photooxidation and showed that As(III) photooxidation rate is limited by the strength of its interactions with DOM and sensitive to ionic competitive desorption. Our study demonstrates the dynamic control that photoreactive DOM poses on the bioavailability and reactivity of As in the environment and highlights the kinetic controls that DOM can possibly exert on As toxicity at various levels in foodwebs.

Impacts of copper and lead exposure on prokaryotic communities from contaminated contrasted coastal seawaters: the influence of previous metal exposure.

Our understanding of environmental factors controlling prokaryotic community is largely hampered by the large environmental variability across spatial scales (e.g. trace metal contamination, nutrient enrichment and physicochemical variations) and the broad diversity of bacterial pre-exposure to environmental factors. In this article, we investigated the specific influence of copper (Cu) and lead (Pb) on prokaryotic communities from the uncontaminated site, using mesocosm experiments. In addition, we studied how pre-exposure (i.e. life history) affects communities, with reference to previous metal exposure on the response of three prokaryotic communities to similar Cu exposition. This study showed a stronger influence of Cu contamination than Pb contamination on prokaryotic diversity and structure. We identified 12 and 34 bacterial families and genera, respectively, contributing to the significant differences observed in community structure between control and spiked conditions. Taken altogether, our results point toward a combination of direct negative responses to Cu contamination and indirect responses mediated by interaction with phytoplankton. These identified responses were largely conditioned by the previous exposure of community to contaminants.

A Multi-Omics Analysis Suggests Links Between the Differentiated Surface Metabolome and Epiphytic Microbiota Along the Thallus of a Mediterranean Seaweed Holobiont.

Marine macroalgae constitute an important living resource in marine ecosystems and complex ecological interactions occur at their surfaces with microbial communities. In this context, the present study aimed to investigate how the surface metabolome of the algal holobiont Taonia atomaria could drive epiphytic microbiota variations at the thallus scale. First, a clear discrimination was observed between algal surface, planktonic and rocky prokaryotic communities. These data strengthened the hypothesis of an active role of the algal host in the selection of epiphytic communities. Moreover, significant higher epibacterial density and α-diversity were found at the basal algal parts compared to the apical ones, suggesting a maturation gradient of the community along the thallus. In parallel, a multiplatform mass spectrometry-based metabolomics study, using molecular networking to annotate relevant metabolites, highlighted a clear chemical differentiation at the algal surface along the thallus with similar clustering as for microbial communities. In that respect, higher amounts of sesquiterpenes, phosphatidylcholines (PCs), and diacylglycerylhydroxymethyl-N,N,N-trimethyl-ß-alanines (DGTAs) were observed at the apical regions while dimethylsulfoniopropionate (DMSP) and carotenoids were predominantly found at the basal parts of the thalli. A weighted UniFrac distance-based redundancy analysis linking the metabolomics and metabarcoding datasets indicated that these surface compounds, presumably of algal origin, may drive the zonal variability of the epibacterial communities. As only few studies were focused on microbiota and metabolome variation along a single algal thallus, these results improved our understanding about seaweed holobionts. Through this multi-omics approach at the thallus scale, we suggested a plausible scenario where the chemical production at the surface of T. atomaria, mainly induced by the algal physiology, could explain the specificity and the variations of the surface microbiota along the thallus.

Long-term monitoring emphasizes impacts of the dredging on dissolved Cu and Pb contamination along with ultraplankton distribution and structure in Toulon Bay (NW Mediterranean Sea, France).

A long-term monitoring during dredging and non-dredging periods was performed. Total and dissolved Cu and Pb concentrations, DGT-labile Pb, ultraphytoplankton abundance and structure were monitored at four sites: dredging site, dumping site (inside/outside of a geotextile bag) and reference site. During the reference period (non-dredging), an increasing contamination in Pb, Cu and a progressive shift from Synechococcus to photosynthetic picoeukaryotes dominance was observed from reference to dumping site. Pb concentrations were significantly higher during dredging period, pointing out sediment resuspension as Pb major source of contamination. Unlike Pb, Cu concentrations were not statistically different during the two periods. Dredging period did not impact on ultraphytoplankton abundance and structure but influence heterotrophic prokaryotes abundance. Sediment resuspension is therefore a major driver of chemical and biological qualities in Toulon Bay. Furthermore, although the geotextile bag reduces particulate transport of the dredged sediment, the transport in the dissolved phase remains a major problem.

Effects of catchment area and nutrient deposition regime on phytoplankton functionality in alpine lakes.

High mountain lakes are a network of sentinels, sensitive to any events occurring within their waterbodies, their surrounding catchment and their airshed. In this paper, we investigate how catchments impact the taxonomic and functional composition of phytoplankton communities in high mountain lakes, and how this impact varies according to the atmospheric nutrient deposition regime. For two years, we sampled the post snow-melt and the late summer phytoplankton, with a set of biotic and abiotic parameters, in six French alpine lakes with differing catchments (size and vegetation cover) and contrasting nitrogen (N) and phosphorus (P) deposition regimes. Whatever the nutrient deposition regime, we found that the lakes with the smallest rocky catchments showed the lowest functional richness of phytoplankton communities. The lakes with larger vegetated catchments were characterized by the coexistence of phytoplankton taxa with more diverse strategies in the acquisition and utilization of nutrient resources. The nutrient deposition regime appeared to interact with catchment characteristics in determining which functional groups ultimately developed in lakes. Photoautotroph taxa dominated the phytoplankton assemblages under high NP deposition regime while mixotroph taxa were even more favored in lakes with large vegetated catchments under low NP deposition regime. Phytoplankton functional changes were likely related to the leaching of terrestrial organic matter from catchments evidenced by analyses of carbon (δ13C) and nitrogen (δ15N) stable isotope ratios in seston and zooplankton. Plankton δ15N values indicated greater water-soil interaction in lakes with larger vegetated catchments, while δ13C values indicated the effective mineralization of the organic matter in lakes. The role played by catchments should be considered when seeking to determine the vulnerability of high altitude lakes to future changes, as catchments' own properties will vary under changes related to climate and airborne contaminants.

Metabolomic and proteomic changes induced by growth inhibitory concentrations of copper in the biofilm-forming marine bacterium Pseudoalteromonas lipolytica.

Copper is an essential element for living cells but this metal is present in some marine environments at such high concentrations that it can be toxic for numerous organisms. In polluted areas, marine organisms may develop specific adaptive responses to prevent cell damage. To investigate the influence of copper on the metabolism of a single organism, a dual approach combining metabolomics and proteomics was undertaken on the biofilm-forming bacterial strain Pseudoalteromonas lipolytica TC8. In order to highlight differential adaptation according to the phenotype, the response of P. lipolytica TC8 to copper stress was studied in planktonic and biofilm culture modes under growth inhibitory copper concentrations. As expected, copper exposure led to the induction of defense and detoxification mechanisms. Specific metabolite and protein profiles were thus observed in each condition (planktonic vs. biofilm and control vs. copper-treated cultures). Copper exposure seems to induce drastic changes in the lipid composition of the bacterial cell membrane and to modulate the abundance of proteins functionally known to be involved in copper cell homeostasis in both planktonic and biofilm culture modes. Much more proteins differentially expressed after copper treatment were observed in biofilms than in planktonic cells, which could indicate a more heterogeneous response of biofilm cells to this metallic stress.

Changes in Bacterioplankton Communities Resulting From Direct and Indirect Interactions With Trace Metal Gradients in an Urbanized Marine Coastal Area.

Unraveling the relative importance of both environmental conditions and ecological processes regulating bacterioplankton communities is a central goal in microbial ecology. Marine coastal environments are among the most urbanized areas and as a consequence experience environmental pressures. The highly anthropized Toulon Bay (France) was considered as a model system to investigate shifts in bacterioplankton communities along natural and anthropogenic physicochemical gradients during a 1-month survey. In depth geochemical characterization mainly revealed strong and progressive Cd, Zn, Cu, and Pb contamination gradients between the entrance of the Bay and the north-western anthropized area. On the other hand, low-amplitude natural gradients were observed for other environmental variables. Using 16S rRNA gene sequencing, we observed strong spatial patterns in bacterioplankton taxonomic and predicted function structure along the chemical contamination gradient. Variation partitioning analysis demonstrated that multiple metallic contamination explained the largest part of the spatial biological variations observed, but DOC and salinity were also significant contributors. Network analysis revealed that biotic interactions were far more numerous than direct interactions between microbial groups and environmental variables. This suggests indirect effects of the environment, and especially trace metals, on the community through a few taxonomic groups. These spatial patterns were also partially found for predicted bacterioplankton functions, thus indicating a limited functional redundancy. All these results highlight both potential direct influences of trace metals contamination on coastal bacterioplankton and indirect forcing through biotic interactions and cascading.

Shear Stress as a Major Driver of Marine Biofilm Communities in the NW Mediterranean Sea.

While marine biofilms depend on environmental conditions and substrate, little is known about the influence of hydrodynamic forces. We tested different immersion modes (dynamic, cyclic and static) in Toulon Bay (north-western Mediterranean Sea; NWMS). The static mode was also compared between Toulon and Banyuls Bays. In addition, different artificial surfaces designed to hamper cell attachment (self-polishing coating: SPC; and fouling-release coating: FRC) were compared to inert plastic. Prokaryotic community composition was affected by immersion mode, surface characteristics and site. Rhodobacteriaceae and Flavobacteriaceae dominated the biofilm community structure, with distinct genera according to surface type or immersion mode. Cell density increased with time, greatly limited by hydrodynamic forces, and supposed to delay biofilm maturation. After 1 year, a significant impact of shear stress on the taxonomic structure of the prokaryotic community developed on each surface type was observed. When surfaces contained no biocides, roughness and wettability shaped prokaryotic community structure, which was not enhanced by shear stress. Conversely, the biocidal effect of SPC surfaces, already major in static immersion mode, was amplified by the 15 knots speed. The biofilm community on SPC was 60% dissimilar to the biofilm on the other surfaces and was distinctly colonized by Sphingomonadaceae ((Alter)Erythrobacter). At Banyuls, prokaryotic community structures were more similar between the four surfaces tested than at Toulon, due possibly to a masking effect of environmental constraints, especially hydrodynamic, which was greater than in Toulon. Finally, predicted functions such as cell adhesion confirmed some of the hypotheses drawn regarding biofilm formation over the artificial surfaces tested here.