Microbial ecology of selenium-respiring bacteria

Irrigated agriculture, steam-power generation, mining, and other human activities result in water that is co-contaminated by selenium (Se), sulfur (S), and nitrogen (N) that typically exist as selenate (SeO42−) and/or selenite (HSeO3−), sulfate (SO42−), and nitrate (NO3−), respectively. Usually, their concentrations are very different, whether in irrigated agriculture run-off or in wastewater. The S-to-Se mass ratio (S:Se) is typically in the order of 1000:1 (g S:g Se) and the N-to-Se mass ratio (N:Se) is typically in the order of 50:1 (g N:g Se). The target contaminant concentrations in treated effluent also show great disparity. For example, the United States Environmental Protection Agency (EPA, 2020) requires existing steam-power-generation facilities to discharge water having less than 3 g N/m and 0.029 g Se/m (average daily concentrations over a consecutive 30-day period); this is a N:Se ratio ∼100:1 (g N:g Se). Selenium is among the first micro-pollutants that, according to regulation (EPA, 2020), require biological wastewater treatment and have regulated surface-water discharge standards. When the contaminated water has a pH of 6 to 8 and a temperature of 15 to 30°C, bacteria can anaerobically reduce these oxyanions at a rate that makes bioreactors an economically viable treatment alternative (Boltz &