PRECIPITATION OF ARSENIC TRISULFIDE BY DESULFOTOMACULUM-AURIPIGMENTUM

A newly discovered bacterium, Desulfotomaculum auripigmentum, precipit ates arsenic trisulfide (As2S3). Precipitation of As2S3 by this organi sm results front 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 st ability of As2S3 is highly sensitive to pH and [S(-II)]. Thus, the rel ative rates at which D. auripigmentum reduces As(V) and S(VI) are crit ical to its formation of As2S3. Other As(V)- or S(VI)-reducing bacteri a are unable to precipitate As2S3 either due to their inability to red uce both As(V) and S(VI) or because they reduce S(VI) too rapidly. Ele ctron microscopy of thin sections showed that the precipitate forms bo th intra- and extracellularly. Microbial As2S3 formation nucleates pre cipitation of the mineral in the bulk milieu, whereas heat-killed cell s alone do not serve as templates for its formation. Precipitation of As2S3 by D. auripigmentum suggests that As2S3 formation may be importa nt in the biogeochemical cycle of arsenic.