Studies on arsenic transforming groundwater bacteria and their role in arsenic release from subsurface sediment

Ten different Gram-negative arsenic (As)-resistant and As-transforming bacteria isolated from As-rich groundwater of West Bengal were characterized to assess their role in As mobilization. 16S rRNA gene analysis confirmed the affiliation of these bacteria to genera Achromobacter, Brevundimonas, Rhizobium, Ochrobactrum, and Pseudoxanthomonas. Along with superior As-resistance and As-transformation abilities, the isolates showed broad metabolic capacity in terms of utilizing a variety of electron donors and acceptors (including As) under aerobic and anaerobic conditions, respectively. Arsenic transformation studies performed under various conditions indicated highly efficient As³⁺ oxidation or As⁵⁺ reduction kinetics. Genes encoding As³⁺ oxidase (aioA), cytosolic As⁵⁺ reductase (arsC), and As³⁺ efflux pump (arsB and acr3) were detected within the test isolates. Sequence analyses suggested that As homeostasis genes (particularly arsC, arsB, and acr3) were acquired by most of the bacteria through horizontal gene transfer. A strong correlation between As resistance phenotype and the presence of As³⁺ transporter genes was observed. Microcosm study showed that bacterial strain having cytosolic As⁵⁺ reductase property could play important role in mobilizing As (as As³⁺) from subsurface sediment.