Acetylene (0.1 atm) prevented rapid increase of CO2 evolution in glucose-amended anaerobically incubated sandy loam soil. This effect was explainable in terms of an effect of C2H2 on nitrogen-fixing Clostridia.In a N2ase-repressed Clostridium pasteurianum culture growing on a medium supplemented with 339 μg NH4-N/ml, C2H2 caused 60 to 100% inhibition of cell proliferation and of cell-nitrogen accumulation, and prevented the increase in rate of CO2 evolution normally associated with growth. In N2ase-containing cultures the competitive inhibition of nitrogen fixation by C2H2 was relieved when C2H2 reduction brought the pC2H2 to near 0.025 atm. The C2H2 inhibition of the NH4-grown culture, however, was not reversed by removal of the C2H2 after 11 h of exposure. Ethylene showed no inhibitory effect.Variation of NH4-N concentration and addition, to the medium, of casein hydrolysate and of pyruvate separately and in combination had no marked effect on the inhibition pattern.The effect of C2H2 reported here suggests that, until more information is available, the results of long-term C2H2 assays of low-activity materials such as soils should be interpreted with some caution.
The Enzymology of Nitrogen Fixation in Cell-free Extracts of Clostridium Pasteurianum
