Repression of microbial arsenite uptake and methylation by dissolved organic carbon.

Arsenic methylation is the microbe-mediated transformation of inorganic As into methylated species, an important component of the biogeochemical arsenic cycle in rice paddies. Prior to methylation, arsenite is taken up into bacterial cells through GlpF, an aquaglyceroporin channel for uptake of glycerol and other low-molecular-weight organics. The uptake and subsequent biotransformation of arsenite are therefore linked to the bacterial utilization of organics. We hypothesized that increasing concentrations of carbon substrates will repress the uptake and methylation of arsenite through a carbon catabolite repression (CCR) mechanism. An arsenic biosensor assay demonstrated that arsenite uptake was repressed in the presence of glucose and environmental dissolved organic matter (DOM) isolates. RT-qPCR analysis of glpF expression linked the decrease in arsenite uptake at higher carbon concentrations to the repression of glycerol-transporting GlpF channels. Methylation of arsenite by Arsenicibacter rosenii, a rice paddy isolate, was repressed by the upper glycolytic substrates glucose, xylose, and mannose, but was not affected by pyruvate and succinate. This result is consistent with current CCR theories. Our findings provide a new perspective on the impacts of organic carbon on microbial arsenic transformations, and suggest that arsenic biotransformation can be repressed in systems that are rich in upper glycolytic carbon substrates.