Mechanisms Regulating Mercury Bioavailability for Methylating Microorganisms in the Aquatic Environment: A Critical Review

Abstract

Mercury is a potent neurotoxin for humans, particularly if the metal is in the form of methylmercury. Mercury is widely distributed in aquatic ecosystems as a result of anthropogenic activities and natural earth processes. A first step toward bioaccumulation of methylmercury in aquatic food webs is the methylation of inorganic forms of the metal, a process that is primarily mediated by anaerobic bacteria. In this Review, we evaluate the current state of knowledge regarding the mechanisms regulating microbial mercury methylation, including the speciation of mercury in environments where methylation occurs and the processes that control mercury bioavailability to these organisms. Methylmercury production rates are generally related to the presence and productivity of methylating bacteria and also the uptake of inorganic mercury to these microorganisms. Our understanding of the mechanisms behind methylation is limited due to fundamental questions related to the geochemical forms of mercury that persist in anoxic settings, the mode of uptake by methylating bacteria, and the biochemical pathway by which these microorganisms produce and degrade methylmercury. In anoxic sediments and water, the geochemical forms of mercury (and subsequent bioavailability) are largely governed by reactions between Hg(II), inorganic sulfides, and natural organic matter. These interactions result in a mixture of dissolved, nanoparticulate, and larger crystalline particles that cannot be adequately represented by conventional chemical equilibrium models for Hg bioavailability. We discuss recent advances in nanogeochemistry and environmental microbiology that can provide new tools and unique perspectives to help us solve the question of how microorganisms methylate mercury. An understanding of the factors that cause the production and degradation of methylmercury in the environment is ultimately needed to inform policy makers and develop long-term strategies for controlling mercury contamination.

Influence of rice straw amendment on mercury methylation and nitrification in paddy soils

“…For example, the microbial community in the high Hg paddy soil may a play more important role in microbial Hg methylation and decomposition of the rice straw than those in low Hg soil. Generally, Hg methylation in environments is mainly regulated by Hg availability to microbes and activity of Hg methylators (Barkay et al, 1997;Chiasson-Gould et al, 2014;Hsu-Kim et al, 2013). In the present study, the positive correlation between DOC and MeHg content suggests the great importance of DOC on enhancing Hg methylation.…”

Benefits of mercury controls for the United States

“…Many of the processes affecting the conversion of inorganic mercury to MeHg and subsequent uptake in biota are poorly understood, particularly in marine ecosystems (54,55). In addition, there is variability among ecosystem types, both freshwater (37) and marine and estuarine (32), in how quickly these systems and biota within them respond to changes in deposition.…”

Persistent Hg contamination and occurrence of Hg-methylating transcript (hgcA) downstream of a chlor-alkali plant in the Olt River (Romania)

“…and the merA gene (involved in Hg reduction) indicating the coexistence of these two processes in sediments of the Olt River and emphasizes the complexity of microbial communities involved in Hg biogeochemical cycling. Overall, these observations highlight the difficulty of linking MeHg concentration or proportion to bacterial activities in the field, pointing the limits of the current understanding of the Hg methylation mechanisms and the existing research gaps such as Hg bioavailability to bacteria in anoxic sediments, and biochemical pathway of MeHg production and degradation, particularly in the field settings (Hsu-Kim et al 2013). …”