Uptake of Hg²⁺ by picocyanobacteria in natural water from four Andean lakes

¹ Laboratorio de Fotobiología, Instituto de Biodiversidad y Medioambiente, CONICET-Universidad Nacional del Comahue, Quintral 1250, 8400, San Carlos de Bariloche, ARGENTINA
² Laboratorio de Análisis por Activación Neutrónica, Centro Atómico Bariloche, CNEA, Bustillo km 9.5, 8400, San Carlos de Bariloche, ARGENTINA
³ U.S. Geological Survey, 345 Middlefield Rd. MS 480, Menlo Park, CA 94025, USA

Abstract

In lake food webs, planktonic bacteria and algae represent the greatest bioconcentration step for Hg₂₊ and monomethyl-Hg (MeHg). As they are the most abundant organisms in planktonic trophic webs and also the main food resource for herbivorous plankton, they can mobilize large amounts of Hg to higher trophic levels. In Andean Patagonian lakes (Argentina), dissolved organic matter (DOM) concentration and character, coupled with photo-reactions, play a central role in the complexation of Hg₂₊ in the water column and can even regulate the uptake of Hg₂₊ by planktonic algae. In this investigation we evaluated the DOM character of natural waters (NW) from four Andean lakes and studied its influence on the uptake of ₁₉₇Hg₂₊ in a strain of the picocyanobacteria Synechococcus by using Hg₂₊ labeled with ₁₉₇Hg₂₊. The uptake of radiolabeled Hg₂₊ by Synechococcus showed different magnitude in NW of lakes Moreno, El Trébol, Morenito and Escondido. Increasing lake DOM concentration reduced the bioavailability of Hg₂₊ as indicated by the lower uptakes rates found in NW with higher complexity and concentration of the DOM pool. Uptakes of Hg₂₊ by this picocyanobacteria contrasted among NW from pelagic (surface and bottom) and littoral compartments of Lake Escondido which suggest that the entry of this metal may be highly variable even in the same environment. The study of the uptake of radiolabeled Hg₂₊ in a set of dilutions of NW from Lake Escondido demonstrated that the bioavailability of Hg₂₊ decrease with increasing DOM concentration.