Illuminating the Black Box: Trace Element Biogeochemistry from a Microbial Perspective

Microbial processes are central to the transformation and fate of trace elements in the environment (e.g., mercury (Hg), arsenic (As)), but the complexities underlying microbial transformation rates and the influence of human impacts present considerable hurdles to developing conceptual and quantitative models of these processes. This perspective highlights processes that govern microbial activity in the environment as it pertains to trace elements, including redox, energy generation, resource limitation, and ecology and evolution. In this context, we compare and contrast the microbial-driven processes of Hg and As cycling, two elements with a genetic basis for microbial transformations (e.g., hgcAB, mer, arsH) that are used for microbial (i) metabolism (e.g., conversion of arsenate to arsenite), (ii) detoxification (e.g., reduction of divalent Hg(II) to volatile Hg(0)), (iii) warfare (e.g., conversion of arsenite to highly toxic trivalent methylated As(III)), or (iv) reasons not yet known (e.g., methylation of Hg(II) to toxic methylmercury). We argue for experimental approaches that quantify contaminant transformation(s) of interest in parallel with relevant metrics of microbial community activity. This microbe-centric framework may catalyze advancement that facilitates microbial integration into conceptual and quantitative models used to forecast environmental and human exposure to contaminants.