Detoxification of arsenic mediated by microbial sulphate reduction in                 Mediterranean marine sediments
Micro-organisms which dwell in sediments have a crucial role in                 biogeochemical cycles and are thus expected to have a strong influence on the cycle                 of arsenic, a metalloid responsible for severe water pollution and presenting a                 major health risk for the populations. Nowadays, environmental genomics techniques                 allow to study the adaptive and cooperative strategies of microbial communities                 thriving in polluted environments. We sequenced the sediment metagenomes from                 l'Estaque marina (Marseille, France), close to a former metallurgic industrial site                 highly polluted with arsenic, and from St Mandrier marina, near Toulon (France). The                 functional and taxonomic profiles of both metagenomes, along with those from four                 publicly available metagenomes used as control data sets, were obtained using the                 RAMMCAP workflow. The biodiversity was higher in both harbour sediment communities                 in comparison to the controls. The order of Desulfobacterales represented 54.7% of                 the observed orders in l'Estaque and 31.7% in St Mandrier. Other orders were evenly                 distributed though the diversity was lower in the highly polluted site of l'Estaque                 than in St Mandrier. The Gene Ontology (GO) entries describing the functional                 profiles were compared with the controls in order to highlight the over-represented                 categories in the metagenomes of interest. We observed that categories related to                 arsenic resistance and oxidative stress were over-represented in l'Estaque. More                 importantly, the integration of metagenomic and physico-chemical data showed that in                 l'Estaque, dissimilatory sulphate reduction in the upper layer of sediment could                 increase arsenic diffusion to the water column and deeper layers, resulting in local                 detoxification of the sediment.