FIG. 5.

Schematic representation of microbial arsenic cycling that occurs in contemporary ecosystems like the Paoha Island red biofilms. Under reducing conditions, chemoautotrophic dissimilatory As(V) reduction is fueled by inorganic electron donors like hydrogen or sulfide, which results in the fixation of inorganic carbon into cell material. Chemoheterotrophic metabolism can also fuel As(V) reduction (although not evident in these biofilms), resulting in the oxidation of the organic matter to CO₂. On the oxidative side, As(III) generated by the above reactions, or from waters originally emanating from the hot springs themselves, can be biologically oxidized to As(V) in the presence of strong oxidants like O₂ (or NO₃⁻). Alternatively, in the presence of light photosynthetic bacteria (PSBs) use As(III) as an electron donor to drive anoxygenic photosynthesis, resulting in the formation of As(V) and the fixation of inorganic carbon into cell material.