Microbial Mercury Methylation in Aquatic Environments: A Critical Review of Published Field and Laboratory Studies

Abstract

Methylmercury (MeHg) is an environmental contaminant of concern because it biomagnifies in aquatic food webs and poses a health hazard to aquatic biota, piscivorous wildlife and humans. The dominant source of MeHg to freshwater systems is the methylation of inorganic Hg (IHg) by anaerobic microorganisms; and it is widely agreed that in situ rates of Hg methylation depend on two general factors: the activity of Hg methylators and their uptake of IHg. A large body of research has focused on the biogeochemical processes that regulate these two factors in nature; and studies conducted within the past ten years have made substantial progress in identifying the genetic basis for intracellular methylation and defining the processes that govern the cellular uptake of IHg. Current evidence indicates that all Hg methylating anaerobes possess the gene pair hgcAB that encodes proteins essential for Hg methylation. These genes are found in a large variety of anaerobes, including iron reducers and methanogens; but sulfate reduction is the metabolic process most often reported to show strong links to MeHg production. The uptake of Hg substrate prior to methylation may occur by passive or active transport, or by a combination of both. Competitive inhibition of Hg uptake by Zn speaks in favor of active transport and suggests that essential metal transporters are involved. Shortly after its formation, MeHg is typically released from cells, but the efflux mechanisms are unknown. Although methylation facilitates Hg depuration from the cell, evidence suggests that the hgcAB genes are not induced or favored by Hg contamination. Instead, high MeHg production can be linked to high Hg bioavailability as a result of the formation of Hg(SH) 2 , HgS nanoparticles, and Hg−thiol complexes. It is also possible that sulfidic conditions require strong essential metal uptake systems that inadvertently bring Hg into the cytoplasm of Hg methylating microbes. In comparison with freshwaters, Hg methylation in open ocean waters appears less restricted to anoxic environments. It does seem to occur mainly in oxygen deficient zones (ODZs), and possibly within anaerobic microzones of settling organic matter, but MeHg (CH 3 Hg + ) and Me 2 Hg ((CH 3 ) 2 Hg) have been shown to form also in surface water samples from the euphotic zone. Future studies may disclose whether several different pathways lead to Hg methylation in marine waters and explain why Me 2 Hg is a significant Hg species in oceans but seemingly not in most freshwaters.

Widespread microbial mercury methylation genes in the global ocean

“…These results show that the taxonomy and the HgcA‐phylogeny are not always congruent. The phylogenetically irregular distribution of hgcA can be an indication of horizontal gene transfers (HGT) or gene deletions in response to stress, suggesting the environment's prevalent influence on Hg‐methylation ability (Regnell and Watras, ).…”

Widespread microbial mercury methylation genes in the global ocean

“…These two sequences are separated from other Chloroflexi HgcA sequences and more closely related to HgcA sequences from Syntrophobacterales, showing that the taxonomy and the HgcA-phylogeny are not always congruent. The phylogenetically irregular distribution of hgcA can be an indication of horizontal gene transfers (HGT) and/or gene deletions in response to stress, suggesting the prevalent influence of environment on Hg-methylation ability 23 .…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Recent reviews critically summarized Hg II uptake and MMHg efflux in methylating anaerobes, methods and equations to analyze Hg methylation rates (Regnell and Watras ) as well as chemotrophic and biotic Hg methylation and demethylation processes by anaerobes and phototrophs (Grégoire and Poulain ; Du et al ). In this review, we aim to summarize the main findings on mechanisms responsible for the formation of MMHg, focusing in detail on the new knowledge recently gained on microorganisms involved in MMHg formation.…”