Oxidation of arsenite to arsenate in growth medium and groundwater using a novel arsenite-oxidizing diazotrophic bacterium isolated from soil

• An As(III)-oxidizing diazotroph Azospirillum sp. was isolated for the first time.
• The strain was effective in oxidizing As(III) to As(V) in groundwater.
• This strain possessed arsenite oxidase gene.
• This strain has potential for remediation of arsenic contaminated water.

An arsenic hyper-tolerant diazotrophic bacterium was isolated from a heavy metal contaminated soil. The pure isolate MM-17 was identified as Azospirillum sp. based on phylogenetic analysis of 16S rRNA. The strain oxidized 100 μM As(III) to As(V) in both culture media (minimal salts) and real groundwater within 8 and 10 h, respectively. The oxidation of As(III) by this strain was observed within the pH range 5–10 with the best performance at pH 7–8. As(III) oxidation was found to be independent of cell growth which implies the oxidation enzymes are constitutively expressed. The whole cell kinetic study highlighted a lower value of kinetic constant, Ks as 32.9 μM As(III), which indicates that the strain MM-17 has greater affinity for As(III). The gene sequence of the large subunit of arsenite oxidase of MM-17 showed 99 and 72% similarity to the large subunit of arsenite oxidase of Stenotrophomonas sp. MM-7 and Sinorhizobium sp. M14, respectively. Sphaeroplasts experiments suggest that arsenite oxidase is a membrane associated protein in MM-17.