The microbial metabolism of arsenic plays a prominent role in governing the biogeochemical cycle of arsenic. Although diverse microbes are known to be involved in the redox transformation of inorganic arsenic, the underlying mechanisms about the arsenic redox cycle mediated by a single microbial strain remain unclear yet. Herein, we discover that Shewanella putrefaciens CN32, a well-known arsenate-respiring and dissimilatory metal-reducing bacterium, could mediate the reversible arsenic redox transformation under aerobic conditions. Genetic analysis shows that S. putrefaciens CN32 contains both ars and arr operon but lacks an As(III) oxidase encoding gene. Arsenic(V) reduction tests demonstrate that the ars operon is advantageous but not essential for As(V) respiration in S. putrefaciens CN32. The Arr complex encoded by the arr operon not only plays a crucial role in arsenate respiration under anaerobic conditions but also participates in the sequential process of As(V) reduction and As(III) oxidation under aerobic conditions. The Arr enzyme also contributes to the microbial As(III) resistance. The expression and catalysis directionality of Arr in S. putrefaciens CN32 are regulated by the carbon source types. Our results highlight the complexity of arsenic redox biotransformation in environments and provide new insights into the important contribution of Arr to the As biogeochemical cycle in nature.
Keywords: Arr complex; arsenic; arsenic-respiring bacteria; biogeochemical cycling; reversible arsenic redox; single strain.