Anaerobic As(III) Oxidation Coupled with Nitrate Reduction and Attenuation of Dissolved Arsenic by Noviherbaspirillum Species

Arsenic pollution is a worldwide problem. In flooded arsenic-contaminated paddy soils, arsenic bioavailability to rice plants is elevated due to the reductive dissolution of arsenate (As(V)) containing Fe(III) (oxyhydr)oxides. However, microbial-mediated arsenite (As(III)) and Fe(II) oxidation has great potential for attenuating arsenic mobility in an anoxic environment. In this study, a bacterium strain HC18 belonging to the genus Noviherbaspirillum was isolated from arsenic-contaminated paddy soil. This microbe is able to oxidize both As(III) and Fe(II) under anoxic nitrate-reducing conditions. However, strain HC18 was not able to oxidize As(III) to As(V) under oxic conditions, suggesting that the process is nitrate-dependent. Genome mining analysis revealed that the As(III) oxidase aio and arx gene clusters and denitrification gene clusters are present in Noviherbaspirillum denitrificans HC18. Oxidation of aqueous Fe(II) resulted in precipitation of Fe(III)-containing minerals, and As(V) produced by As(III) oxidation was bound to Fe(III) (oxyhydr)oxides such as goethite. The effectiveness of N. denitrificans HC18 for As(III) and Fe(II) oxidation under anoxic nitrate-reducing conditions reveals a potential for its use in bioremediation of arsenic-contaminated environments.