Worldwide Diversity and Occurrence of Arsenite Transporter acr3(2) Suggests An Important and Overlooked Pathway

Arsenic is a toxic element that occurs naturally in the environment.  Microorganisms have detoxification mechanisms that involve the expulsion of arsenite from the cytoplasm.  The genes, including acr3(2), encoding these processes have been well studied in laboratory.  However, comparatively less is known of detoxification genes in the environment.  Here the environmental diversity of acr3(2), an arsenite transporter gene, was examined in 12 samples: five water samples from the Amazon outflow, one from Mono Lake, CA sediment, one from Searle Lake, CA sediment, five groundwater samples from Bangladesh, one from Union Lake, NJ sediment and two treatments from acetate and arsenate amended microcosms.  Microcosms contained sediment and groundwater from the Vineland Chemical Co Superfund site, NJ.  These sites were chosen to represent the wide diversity of arsenic impacted environments and concentrations ranged from below 1 ppb to 422 ppm.  Of the 145 unique nucleotide sequences, 94 sequences were clustered among known Proteobacterial sequences and the rest in between clusters of Cyanobacteria and Euryarcheota and unique sequences from Acidobacteria, Planctomycetes, Bacteroides and Firmicutes.  Thus, the acr3(2) gene appears to be highly conserved worldwide and across the domains of Archaea and Bacteria.  Rarefraction analysis and CHAO1 estimation of species richness showed that even with 200 sequences derived from JGI and this study, the degree of diversity is not saturated.  These results suggests that the acr3(2) gene and detoxification may be more important than previously thought in environmental arsenic cycling and mobilization.