Missing pieces in the puzzle of ecology of microbial arsenate reduction

Arsenic pollution and its associated health risks have raised widespread concern. Under anaerobic conditions, arsenic mobility and toxicity increase when arsenate [As(V)] is reduced to arsenite [As(III)] by microbes through the cytoplasmic and dissimilatory pathways. However, the relative importance of these two pathways in the environment remains unclear, restricting our ability to effectively predict and regulate the environmental behavior of arsenic. Here, we review reports that declared a major role of the cytoplasmic or dissimilatory pathway in the environment and point out their limitations. We then summarize the key environmental factors influencing microbial As(V) reduction. Based on studies examining the expression of genes involved in the two As(V) reduction pathways, we hypothesize that the cytoplasmic pathway predominates at relatively high environmental As(III) concentrations, while the dissimilatory pathway is more significant at low concentrations. Future research is needed to test this hypothesis, and the expression of As(V)-reducing genes as a function of As(III) concentration can be investigated with various environmental samples and gradients.