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

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

CHEMOSPHERE

PERGAMON-ELSEVIER SCIENCE LTD

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 andbioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, beingthat the picophytoplankton fraction is a major entrance path of Hg2þ into pelagic food webs. This studyanalyzed the bioaccumulation of Hg2þ by the picocyanobacteria Synechococcus sp. using the radiotracer197Hg2þ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentrationand quality. Hg2þ bioaccumulation by Synechococcus was calculated as the uptake of Hg2þ perbiovolume unit (volume concentration factor VCF; pL mm3). Hg uptake showed a wide variation(13 < VCF< 300 pL mm3) in the natural DOC gradient tested (0.7e4mg L1; Hg2þ/DOC ratio: 1.8e14 ng mg1). The bioaccumulation of Hg2þ in Synechococcus decreased exponentially with DOC concentration.Differences in the quality of dissolved organic matter (DOM) among lake water influencedalso Hg2þ bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higherHg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromaticcompounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria arepivotal organisms in the Hg cycling, taking dissolved Hg2þ and transferring it to pelagic food webs, aswell as fueling the benthic Hg pathway through sedimentation.