Metagenomic insights into microbial metabolism affecting arsenic dispersion in Mediterranean marine sediments.

Microorganisms dwelling in sediments have a crucial role in biogeochemical cycles and are expected to have a strong influence on the cycle of arsenic, a metalloid responsible for severe water pollution and presenting major health risks for human populations. We present here a metagenomic study of the sediment from two harbours on the Mediterranean French coast, l'Estaque and St Mandrier. The first site is highly polluted with arsenic and heavy metals, while the arsenic concentration in the second site is below toxicity levels. The goal of this study was to elucidate the potential impact of the microbial community on the chemical parameters observed in complementary geochemical studies performed on the same sites. The metagenomic sequences, along with those from four publicly available metagenomes used as control data sets, were analysed with the RAMMCAP workflow. The resulting functional profiles were compared to determine the over-represented Gene Ontology categories in the metagenomes of interest. Categories related to arsenic resistance and dissimilatory sulphate reduction were over-represented in l'Estaque. More importantly, despite very similar profiles, the identification of specific sequence markers for sulphate-reducing bacteria and sulphur-oxidizing bacteria showed that sulphate reduction was significantly more associated with l'Estaque than with St Mandrier. We propose that biotic sulphate reduction, arsenate reduction and fermentation may together explain the higher mobility of arsenic observed in l'Estaque in previous physico-chemical studies of this site. This study also demonstrates that it is possible to draw sound conclusions from comparing complex and similar unassembled metagenomes at the functional level, even with very low sequence coverage.