Biotic generation of arsenic(III) in metal(loid)-contaminated freshwater lake sediments

Sediments of Coeur d'Alene Lake, ID, are heavily contaminated with mine tailings that contain high levels of arsenic, iron, lead, and other trace elements. Maximal abundance of redox-active elements such as As and Fe is generally found close to the sediment-water interface, whereas peak abundance of less redox-active elements such as Pb is found at >25 cm. The suggestion that As is mobile within reduced sediments led us to characterize the sediment microbiota with regard to organisms whose activities favor As mobilization. Most probable number (MPN) estimates reveal that the densities of cultivable sulfate-, iron-, and arsenate-reducing bacteria approach 10⁶, 10⁵, and 10⁴ cells g^-1 wet weight sediment, respectively. Because As is considered more mobile in environments that produce As(III), we measured aqueous As(III) generation within As(V)-amended sediment microcosms. In organic acid-stimulated microcosms, >50% of a 10mM As(V) amendment is transformed to As(III), compared to 30% and 5% in unstimulated microcosms and abiotic controls, respectively. In microcosms amended with an inhibitor of SRB metabolism (molybdate), As(V) reduction was in some cases diminished, suggesting that SRB may contribute to As(V) reduction. The capacity for biotic As(V) reduction clearly exists in CDAL sediments, and the profile of As abundance may be partly attributed to metal(loid)-transforming bacteria.