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Arsenotrophy, growth coupled to autotrophic arsenite oxidation or arsenate respiratory reduction, occurs only in the prokaryotic domain of life. The enzymes responsible for arsenotrophy belong to distinct clades within the DMSO reductase family of molybdenum-containing oxidoreductases: specifically arsenate respiratory reductase, ArrA, and arsenite oxidase, AioA (formerly referred to as AroA and AoxB). A new arsenite oxidase clade, ArxA, represented by the haloalkaliphilic bacteriumAlkalilimnicola ehrlichiistrain MLHE-1 was also identified in the photosynthetic purple sulfur bacteriumEctothiorhodospirasp. strain PHS-1. A draft genome sequence of PHS-1 was completed and anarxoperon similar to MLHE-1 was identified. Gene expression studies showed thatarxAwas strongly induced with arsenite. Microbial ecology investigation led to the identification of additionalarxA-like sequences in Mono Lake and Hot Creek sediments, both arsenic-rich environments in California. Phylogenetic analyses placed these sequences as distinct members of the ArxA clade of arsenite oxidases. ArxA-like sequences were also identified in metagenome sequences of several alkaline microbial mat environments of Yellowstone National Park hot springs. These results suggest that ArxA-type arsenite oxidases appear to be widely distributed in the environment presenting an opportunity for further investigations of the contribution of Arx-dependent arsenotrophy to the arsenic biogeochemical cycle.