Impact of temperature, CO₂ fixation and nitrate reduction on selenium reduction, by a paddy soil Clostridium strain

Aims: To elucidate the impact of CO₂ fixation, nitrate reduction and temperature on selenium reduction by a newly identified acetogenic bacterium, Clostridium sp. BXM. Methods and Results: A series of culture experiments were designed to evaluate the impact of temperature, CO₂ fixation and nitrate reduction on the rate and extent of selenium reduction by strain BXM. The products of selenium reduction, CO₂ fixation and nitrate reduction were determined. Molecular analysis was performed to identify the functional genes involved in the selenium reduction process. CO₂ may have enhanced the activity of hydrogenase I and/or the level of cytochrome b, thus increasing selenium reduction. Nitrate may inhibit selenium reduction due to its higher reduction potential and/or by decreasing selenite/selenate reductase activity. The suitable temperature was 37 and 30°C for selenite reduction under anaerobic and aerobic conditions, respectively. The optimum temperature was 30°C for selenate reduction under both anaerobic and aerobic conditions. CO₂ fixation and nitrate reduction by Clostridium sp. BXM stimulated each other. Conclusions: Clostridium sp. BXM was capable of reducing up to 36-94% of 1 mmol l^-1 selenate and selenite under anaerobic or aerobic conditions over 15 days. The strain might be used for the precipitation of Se from highly selenium-contaminated water or sediments. Significance and Impact of the Study: The findings contribute to the current understanding about the role that micro-organisms play in the detoxification of toxic selenium compounds in paddy soils. Micro-organisms in paddy soils can influence selenium accumulation in rice grain and hence human selenium intake.