Understanding fundamental constraints of microbial mercury methylation through the evolutionary perspective

Methylmercury (MeHg), primarily produced by anaerobic microorganisms, is a major concern in both natural and engineered aquatic systems. Assessing the risk of MeHg pollution is essential for protecting human health and supporting effective pollution management. However, due to the complex interactions between environmental variables, the controlling factors of microbial mercury (Hg) methylation exhibit strong environmental specificity, which restricts our ability to predict MeHg pollution risk across diverse environments. We therefore adopt an evolutionary perspective to examine the emergence, mechanistic establishment, and functional significance of microbial Hg methylation, in order to identify fundamental controls that may be obscured by the complexity of modern systems. Our analysis suggests that all prerequisites required for microbial Hg methylation were probably present in the last universal common ancestor (LUCA). This ability may originate from the abiotic version of geochemical Hg methylation reaction in hydrothermal environments. Microbial Hg methylation may represent an important way in which Hg methylators interact with surrounding environments. These analyses suggest that strain-specific microbial physiology, methyl-donor and Hg availability, and ecological and environmental contexts may be fundamental factors controlling microbial MeHg production, whereas affiliation with a particular microbial lineage may not constitute a strict constraint. These inferences are consistent with recent experimental observations. After these analyses, we propose experimental approaches to investigate the emergence, establishment, and function of microbial Hg methylation. This article aims to provide a new path toward understanding the controlling factors of microbial Hg methylation in aquatic environments and propose feasible guidelines for future studies.