Quantifying Microbially Mediated Kinetics of Ferrihydrite Transformation and Arsenic Reduction: Role of the Arsenate-Reducing Gene Expression Pattern.

The behavior of arsenic (As) is usually coupled with iron (Fe) oxide transformation and mediated by both abiotic reactions and microbial processes in the environment. However, quantitative models for the coupled kinetic processes, which specifically consider the arsenate-reducing gene expression correspondent to different reaction conditions, are lacking. In this study, based on the pure cultured Shewanella putrefaciens incubation experiments, extended X-ray absorption fine structure spectroscopy, high resolution transmission electron microscopy, and a suite of microbial analyses, we developed a coupled kinetics model for microbially mediated As reduction and Fe oxide transformation and specifically quantified the As(V) reduction rate coefficients based on the expression patterns of arrA genes. The model reasonably described the temporal changes of As speciation and distribution. The microbial reduction rates of As(V) varied dramatically during the reactions, which were well represented by the varying transcript abundances of arrA genes at different As concentrations. The contributions of biotic and abiotic reactions to the overall reaction rates were assessed. The results improved our quantitative understanding on the key role of As(V)-reducing genes in regulating the speciation and distribution of As. The kinetic modeling approaches based on microbial gene expression patterns are promising for developing comprehensive biogeochemical models of As involving multiple coupled reactions.