The Reduction of Nitrate to Ammonium by a Clostridium sp. Isolated from Soil

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Abstract
Cultures of Clostridium KDHS2 reduced 15NO3 − to 15NH4 + with a concurrent increase in molar growth yield of 15·7% compared with fermentatively grown bacteria. The bacteria exhibited a K 8 (NO3 −) of 0·5 mM and reduced NO3- maximally at a rate of 0·1 μmol h-1 (mg dry wt)-1. A partially purified nitrate reductase was obtained which had a K m (NO3-) of 0·15 mM. The reduction of 13NO3 − to 13NH4 + by resting bacteria was not inhibited by NH4 +, glutamate, glutamine, methionine sulphoximine or azaserine. Glutamine synthetase affected neither the synthesis nor the activity of the NO3 −-reducing enzymes. The results are consistent with the hypothesis that NO3 − reduction to NH4 + in this Clostridium sp. is dissimilative. SO3 2-, but not SO4 2-, inhibited the reaction, apparently at the level of NO2 − reduction.