Precipitation of alacranite ( As 8 S 9 ) by a novel As ( V )‐respiring anaerobe strain MPA ‐ C 3

Summary Strain MPA ‐ C3 was isolated by incubating arsenic‐bearing sediments under anaerobic, mesophilic conditions in minimal media with acetate as the sole source of energy and carbon, and A s( V ) as the sole electron acceptor. Following growth and the respiratory reduction of A s( V ) to A s( III ), a yellow precipitate formed in active cultures, while no precipitate was observed in autoclaved controls, or in uninoculated media supplemented with A s( III ). The precipitate was identified by X ‐ray diffraction as alacranite, As 8 S 9 , a mineral previously only identified in hydrothermal environments. Sequencing of the 16S rRNA gene indicated that strain MPA ‐ C3 is a member of the D eferribacteres family, with relatively low (90%) identity to D enitrovibrio acetiphilus   DSM 12809. The arsenate respiratory reductase gene, arrA , was sequenced, showing high homology to the arrA gene of D esulfitobacterium halfniense . In addition to A s( V ), strain MPA ‐ C3 utilizes NO 3 − , S e( VI ), S e( IV ), fumarate and F e( III ) as electron acceptors, and acetate, pyruvate, fructose and benzoate as sources of carbon and energy. Analysis of a draft genome sequence revealed multiple pathways for respiration and carbon utilization. The results of this work demonstrate that alacranite, a mineral previously thought to be formed only chemically under hydrothermal conditions, is precipitated under mesophilic conditions by the metabolically versatile strain MPA ‐ C3 .