Precipitation of arsenic trisulfide by Desulfotomaculum auripigmentum

A newly discovered bacterium, Desulfotomaculum auripigmentum, precipitates arsenic trisulfide (As₂S₃). Precipitation of As₂S₃ by this organism results from its reduction of As(V) to As(III) and S(VI) to S(-II). At the As(III) concentration range of interest (0.1 to 1 mM), the stability of As₂S₃ is highly sensitive to pH and [S(-II)]. Thus, the relative rates at which D. auripigmentum reduces As(V) and S(VI) are critical to its formation of As₂S₃. Other As(V)- or S(VI)-reducing bacteria are unable to precipitate As₂S₃ either due to their inability to reduce both As(V) and S(VI) or because they reduce S(VI) too rapidly. Electron microscopy of thin sections showed that the precipitate forms both intra- and extracellularly. Microbial As₂S₃ formation nucleates precipitation of the mineral in the bulk milieu, whereas heat-killed cells alone do not serve as templates for its formation. Precipitation of As₂S₃ by D. auripigmentum suggests that As₂S₃ formation may be important in the biogeochemical cycle of arsenic.