The sources and factors controlling concentrations of monomethylmercury (MMHg) in aquatic ecosystems need to be better understood. Here, we investigated Hg transformations in sediments, periphyton associated with green algae's or aquatic plants, and benthic biofilms from the Lake Titicaca hydrosystem and compared them to the occurrence of active methylating microorganisms and extracellular Hg ligands. Intense Hg methylation was found in benthic biofilms and green algae's periphyton, while it remained low in sediments and aquatic plants' periphyton. Demethylation varied between compartments but remained overall in the same range. Hg methylation was mainly carried out by sulfate reducers, although methanogens also played a role. Its variability between compartments was first explained by the presence or absence of the hgcAB genes. Next, both benthic biofilm and green algae's periphyton exhibited a great diversity of extracellular low-molecular-weight (LMW) thiols (13 or 14 compounds) present at a range of a few nmol L-1 or μmol L-1 but clearly dominated by cysteine and 3-mercaptopropionic acid. Hg methylation was overall positively correlated to the total thiol concentrations, albeit to different extents according to the compartment and conditions. This work is the first examining the interplay between active methylating bacterial communities and extracellular ligands in heterotrophic biofilms and supports the involvement of LMW thiols in Hg methylation in real aquatic systems.
Bibliographical noteFunding Information:
This work is a contribution to the LA PACHAMAMA project (ANR CESA program, no. ANR-13-CESA-0015-01). The Swedish International Development Cooperation Agency in Bolivia is also thanked for making part of this work possible through the "Aquatic Pollution and Remediation" project at Universidad Mayor de San Andrés. Laurent Chauvaud, Erwan Amice, Julien Thébault, and Thierry Lebec (LEMAR, UMR 6539, France) are greatly acknowledged for sample collection as well as Thomas Creusot, Emilie Loustau, Hugues Preudhomme, and Véronique Vacchina for their technical help during analyses.
© 2018 American Chemical Society.