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

SOTO CÁRDENAS, CAROLINA; QUEIMALIÑOS, CLAUDIA; RIBEIRO-GUEVARA, SERGIO; GEREA, MARINA; DIÉGUEZ, M.C.

CHEMOSPHERE

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 230 p. 360 - 368

0045-6535

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 Hg2+ into pelagic food webs. This study analyzed the bioaccumulation of Hg2+ by the picocyanobacteria Synechococcus sp. using the radiotracer 197Hg2+ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg2+ bioaccumulation by Synechococcus was calculated as the uptake of Hg2+ per biovolume unit (volume concentration factor VCF; pL mm-3). Hg uptake showed a wide variation (13 < VCF< 300 pL mm-3) in the natural DOC gradient tested (0.7-4 mgL-1; Hg2+/DOC ratio: 1.8 -14 ng mg-1). The bioaccumulation of Hg2+ in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg2+ 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 Hg2+ and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.