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

Abstract

Mercury (Hg) is a natural and widespread trace metal, but is considered a priority pollutant, particularly its organic form methylmercury (MMHg), because of human's exposure to MMHg through fish consumption. Pioneering studies showed the methylation of divalent Hg (Hg^II) to MMHg to occur under oxygen-limited conditions and to depend on the activity of anaerobic microorganisms. Recent studies identified the hgcAB gene cluster in microorganisms with the capacity to methylate Hg^II and unveiled a much wider range of species and environmental conditions producing MMHg than previously expected. Here, we review the recent knowledge and approaches used to understand Hg^II-methylation, microbial biodiversity and activity involved in these processes, and we highlight the current limits for predicting MMHg concentrations in the environment. The available data unveil the fact that Hg^II methylation is a bio-physico-chemical conundrum in which the efficiency of biological Hg^II methylation appears to depend chiefly on Hg^II and nutrients availability, the abundance of electron acceptors such as sulfate or iron, the abundance and composition of organic matter as well as the activity and structure of the microbial community. An increased knowledge of the relationship between microbial community composition, physico-chemical conditions, MMHg production, and demethylation is necessary to predict variability in MMHg concentrations across environments.

Figure 1

Conceptual summary of the biological and chemical interplays affecting Hg^II methylation in the environment. Orange boxes and arrows refer to geochemical variables directly affecting microbial activity and Hg speciation. Purple boxes refer to Hg chemical forms. The red arrow indicates the transformation of Hg^II to CH₃Hg⁺. Blue refers to a compendium of metabolic processes occurring in the environment, among these processes Hg^II methylation carried out by the hgcAB gene cluster.

Figure 2

Schematic representation of the proposed conceptual iterative strategy for studying Hg^II methylation.

Figure 3

Proposed model to quantify MMHg in freshwater systems. The model considers formation (k _m, Hg^II methylation) and degradation (k _d, MMHg demethylation) and the inputs and outputs of MMHg. Processes are represented with orange arrows, known functional genes are shown in brackets. Transport of MMHg to the system or out the system is represented by blue arrows.