Oxidative and reductive transformations of arsenic by photosynthetic microbial communities from hot springs on Pahoa Island, Mono Lake, California

The shoreline of Pahoa Island in hypersaline Mono Lake in California is characterized by numerous volcanogenic hot springs that display a wide range of temperatures between 30 and 85 degrees C. A variety of distinctive photosynthetic microbial mats are evident in these hot springs and their spatial distribution appears to be a function of water temperature. The suboxic hydrothermal waters of these seeps typically contain ~100 uM dissolved arsenic, which is rapidly oxidized from arsenite [As(III)] to arsenate [As(V)] as the springs flow over these microbial communities. We conducted experiments with anaerobic cultures of red or green photosynthetic bacteria from these hot springs, which we amended with radio-labeled 73As(III) or 73As(V) and incubated at 42 degrees C to measure arsenite oxidation and arsenate reduction activity. In order to assess the potential for As(III) to serve as an electron donor during anoxygenic photosynthesis, As(III) oxidation incubations were conducted under both light and dark conditions. Both light and dark incubations of these thermophiles rapidly oxidized amendments of 100 uM As(III) within 7 hours of incubation, however no significant difference was observed in the rate of As(III) oxidation for light compared to dark samples. Arsenate reduction was also observed in both light and dark anaerobic cultures after 48 hours incubation. In all cases, As oxidation or reduction activity was eliminated by autoclaving. These results suggest that biological As(III) oxidation by these bacteria is primarily a mechanism of detoxification or chemoautotrophy, however the potential significance of As(III) as a photosynthetic electron acceptor will be discussed.