AIMS:To elucidate the impact of CO(2) 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, CO(2) fixation and nitrate reduction on the rate and extent of selenium reduction by strain BXM. The products of selenium reduction, CO(2) fixation and nitrate reduction were determined. Molecular analysis was performed to identify the functional genes involved in the selenium reduction process. CO(2) 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. CO(2) 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.