Effects of Iron and Dissolved Organic Matter on Bioavailability of Arsenite under Anaerobic Conditions

Understanding the effects of water chemistry on the availability of arsenic (As) to biota is important for predicting the environmental fate of As. The "dissolved" fraction of As (<0.22 mu m) is often used as a proxy for bioavailable As. However, As speciation is also influenced by binding to dissolved organic matter (DOM) and colloidal iron (Fe) (oxy)-hydroxides, which can impact bioavailability. Here, we use a recently developed Escherichia coli anaerobic biosensor to elucidate the effects of DOM and Fe on arsenite (As-(III)) bioavailability under anaerobic conditions, where As can be highly mobile. Microbial As-(III) uptake decreased with greater DOM and Fe-(III) concentrations, while Fe-(II) had no effect. Higher organic sulfur content in DOM was associated with decreased biouptake at low As-(III)/C ratios, and X-ray absorption spectroscopy indicated that this was due to binding of As-(III) to sulfur ligands like thiols. The 0.1-0.5 kDa size fraction of As was most closely related to the bioavailable As fraction. Because the aquaporin channels mediating As-(III) uptake into both microbes and rice plants are structurally similar, our results may also have relevance for understanding of how biogeochemical conditions in rice paddies regulate the plant availability of arsenic.