ABSTRACT
100The effect of the lack of trace metals, namely tungsten and selenium, on the production of acids and alcohols by CO-enriched anaerobic sludge was investigated. The CO-enriched sludge was first supplied a tungsten-deficient medium (containing selenium) and, in a next assay, a selenium-deficient medium (containing tungsten) was fed to a continuous gas-fed bioreactor operating for 36 days at a CO flow rate of 10ml/min. An initial pH of 6.2 followed by a pH decrease to 4.9 yielded 7.34 g/L of acetic acid as major acid at the end of the pH 6.2 stage. Subsequently, bioconversion of the acids at a lower pH of 4.9 yielded 1.85 g/L ethanol and 1.2 g/L butanol in the absence of tungsten (tungstate). A similar follow up assay in the same bioreactor with two consecutive periods of pH maintained at 6.2 first and then decreased to 4.9, with selenium deficient medium yielded 6.6 g/L of acetic acid at pH 6.2 and 4 g/L of ethanol as well as 1.88 g/L butanol at the lower pH 4.9. The only known CO fixing organism able to produce alcohols, as revealed by the microbial community analysis, was Clostridium autoethanogenum which was found to be present in the bioreactor both in the tungsten and selenium deprived medium.
