Microbial oxidation of arsenite and occurrence of arsenite‐oxidizing bacteria in acid mine water from a sulfur‐pyrite mine

The acid mine waters (pH 2.0–2.4) discharged from the Matsuo sul‐fur‐pyrite mine contained high concentrations of dissolved inorganic arsenic (2–13 ppm). Arsenic in the superficial acid mine waters was predominantly in the (V) state (arsenate); however, the element in the water from a deep mine drift was almost in the (III) state (arsenite). Microbial arsenite oxidation occurred in the acid mine waters and along the stream of the river, which was contaminated with a large volume of the mine drift water. Arsenite (500 ppm As)‐resistant bacteria (0–27 colonies/ml) were detected in the water samples and 208 slant cultures were obtained. Arsenite‐oxidizing activities of all the cultures were determined and six strains with strong arsenite‐oxidizing activity were isolated. These bacteria were acidophilic (optimum growth pH, 3—4), gram‐negative, aerobic, and rod‐shaped. They could not oxidize ferrous iron and elemental sulfur as a sole energy source and not derive the energy for chemoautotrophic growth from arsenite oxidation.