Methylmercury and selenium interactions: Mechanisms and implications for soil remediation

The bioaccumulation of methylmercury (MeHg), a highly toxic Hg species, and its associated ecological and health risks are of global concerns. Mercury–selenium (Se) antagonism is well-recognized as an important "natural" mechanism mitigating the risk of MeHg bioaccumulation and toxicity. However, the impact of Se on microbial MeHg production, the first step in MeHg bioaccumulation, is unclear. By evaluating the current state of knowledge on Hg–Se antagonisms observed in plants and deposit-feeders, we propose that Se-mediated decrease in microbial MeHg production in soils and sediments plays a key role. The underlying mechanisms responsible for Se-inhibited microbial MeHg production may include (i) HgSe nanoparticle formation, which reduces the availability of inorganic Hg to microbial methylators, as well as Se-impacted (ii) microbial methylators and (iii) MeHg degradation and evaporation. These mechanisms and the factors influencing the Se-induced decrease in microbial MeHg production are considered. In addition, the results of a recent field trial using Se to remediate Hg-contaminated soils are presented as an example of a remediation strategy for Hg-contaminated environments, a topic of interest to scientists and regulators. Finally, after highlighting Se-impacted microbial MeHg production, a process that has been largely underestimated, we suggest future avenues for exploring Hg–Se antagonism.