Microbial controls of mercury geochemistry

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My research program investigates how microbes control the entry of contaminants into food webs. I study how microbes sense and transform metals and how such processes evolved. Although I am broadly interested in threats to ecosystem health that are associated with toxic metal(oid)s, this proposal focuses on mercury (Hg) biogeochemistry. Globally, Hg is a priority contaminant due to biomagnification of the potent neurotoxin methylmercury (MeHg) in food webs. Microbes are involved in most reactions controlling the relative abundance of Hg species in the environment. Some microbes catalyze the interconversion between Hg(II) and its elemental volatile form, Hg(0), while others catalyze the production and degradation of MeHg. The relative importance of such reactions varies throughout geochemical gradients, from oxic to low redox potential methanogenic or sulfidic environments. My long-term objective is to identify the microbial controls of Hg geochemistry in past and current environments. The continued importance of Hg as a chemical of concern to human health was underscored by the recent ratification of the UNEP Minamata convention (2017) that requires governments to regulate Hg emissions and their environmental impacts, and to ban the export of Hg waste to other countries. With this new international regulatory context, industries must develop solutions to deal with their domestic Hg waste. Over the next 5 years, my program will train 31 HQPs while addressing knowledge gaps identified in the UNEP Global Hg assessment and supporting implementation of the Minamata convention, namely the link between Hg deposition, methylation and uptake by organisms, the parameters that determine the rates of exchange between air, water, air and soil and a better characterization of the lag in aquatic ecosystem response to atmospheric mercury deposition. My first objective is to evaluate whether there are universal controls of Hg bioavailability across microbial taxa. My second objective is to determine anaerobic microbial reduction processes. My third objective aims to develop genomic paleo-proxies for historical Hg deposition and determine to which extent bacterial metal resistance mechanisms have evolved in response to recent changes in metal delivery to ecosystems over broad spatial scales. Indeed, the development of effective risk reduction strategies depends on a comprehensive understanding of the present and past effects of toxic pollutants on ecosystems.