REDOX TRANSFORMATIONS OF ARSENIC(V) AND ARSENIC(III) ASSOCIATED WITH PERIPHYTON COMMUNITIES

Arsenic in drinking water is a prominent environmental cause of toxicity and cancer mortality. The mobility of arsenic in the environment and its availability for biological uptake in As-contaminated ecosystems are largely controlled by the oxidation state in which it occurs. In nature arsenic oxyanions occur primarily in two oxidation states: As(V) (arsenate) and As(III) (arsenite). While both of these forms are toxic to a wide range of organisms, arsenate is much more particle-reactive than arsenite and tends to associate with sediment solids in aquatic environments. Microorganisms mediate the majority of oxidative and reductive transformations of arsenic between these geochemical species. Previous studies have demonstrated dissimilatory arsenate reduction that is coupled to bacterial respiration during organic matter oxidation in arsenic-enriched environments such as hypersaline and alkaline Mono Lake. Arsenite oxidation coupled to chemolithoautotrophic growth has also been observed in bacteria isolated from Mono Lake water, as well as from geothermal and mine waters containing elevated levels of arsenic.

Here we report microbial redox transformations of arsenic in arsenate- and arsenite-amended live samples of fresh water and periphyton collected from San Francisquito Creek in Palo Alto, CA. Live, oxic samples of creek water oxidized micro- and milli-molar concentrations of arsenite to arsenate, while anoxic samples of creek water inoculated with algae reduced micro- and milli-molar concentrations of arsenate to arsenite. No activity was observed in autoclaved controls or in anoxic samples where arsenate was added without algae. These data suggest the presence of a bacterial population capable of chemoautotrophic arsenite oxidation and respiratory arsenate reduction in a freshwater stream characterized by low (< 1 ppb) ambient arsenic concentrations. These preliminary findings suggest that bacterial communities capable of biochemical redox transformations of arsenic are ubiquitous in the modern terrestrial environment, and may represent a significant pathway for the natural release of sedimentary arsenic to the aqueous environment or its removal from arsenic-contaminated drinking water.