Résumé
Mercury (Hg), a persistent and toxic element, is found both naturally and as an anthropogenically-produced compound in the environment. Industrial use of Hg, and its subsequent release to the environment, has contributed to increasing Hg levels in soil, sediments and aquatic ecosystems worldwide. The Arctic is at risk for Hg toxicity. This is particularly the case for coastal sites, which appear to be involved in Hg cycling. The role of micro-organisms in the biogeochemical Hg cycle has recently become the focus of a large number of studies. The inorganic form of Hg, HgII, can be reduced to the relatively inert gaseous elemental mercury (GEM) form by bacteria possessing the mer operon. Bacteria, such as sulphate or iron reducers, are also reported to mediate the transformation of HgII to the more toxic and bioaccumulative organic form, methyl mercury (MeHg). However, the extent to which bacterial populations interact with mercury remains to be elucidated, especially in polar ecosystems. In order to understand how microbial populations respond to mercury, we used a metagenomic approach to describe both community structure and gene function variations in the Arctic snow pack over a two-month period during the spring of 2008 in Ny-Alesund, Svalbard, Norway. Using taxonomic DNA microarrays, a high throughput molecular biology technique, we monitored the evolution of bacterial communities within the snowpack. Community structure was then correlated to environmental factors and mercury concentrations. Gene function was analyzed by pyrosequencing as well as Q-PCR. The potential interdependence of the microbial community controlled mercury transformations and the community structure could provide insights into mercury cycling in the Arctic. In addition, the role of mercury resistant bacteria in the survival of the entire microbial community can be induced by the correlation between community members and mercury functional genes.