The microbial mercury link in oligotrophic lakes: Bioaccumulation by picocyanobacteria in natural gradients of dissolved organic matter.

Andean Patagonian lakes are oligotrophic systems characterized by low dissolved organic carbon (DOC) levels and moderate to high Hg concentration that determine naturally high Hg/DOC ratios and bioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, being that the picophytoplankton fraction is a major entrance path of Hg into pelagic food webs. This study analyzed the bioaccumulation of Hg by the picocyanobacteria Synechococcus sp. using the radiotracer Hg and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg bioaccumulation by Synechococcus was calculated as the uptake of Hg per biovolume unit (volume concentration factor VCF; pL μm). Hg uptake showed a wide variation (13 < VCF< 300 pL μm) in the natural DOC gradient tested (0.7-4 mg L; Hg/DOC ratio: 1.8-14 ng mg). The bioaccumulation of Hg in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higher Hg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromatic compounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria are pivotal organisms in the Hg cycling, taking dissolved Hg and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.