Carbon Monoxide Inhibition of Azotobacter in Microrespiration Experiments

Abstract

In microrespiration expts. it was demonstrated that CO affects the assimilation of different forms of N by Azotobacter vinelandii in the following ways:[long dash]Fixation of N2 is inhibited[long dash]first detected at a conc. of 0.1% it is 75-85% complete at 0.6% CO. In the same range of CO conc. the assimilation of ammonium- and urea-N (and probably reduced organic forms such as asparagine, aspartate and glutamate) is scarcely affected. At times, a small non-specific effect on growth is noted with these sources (10-15%), but it is not consistent. Assimilation of nitrate (or nitrite) is slightly but definitely inhibited by 0.6% CO. The inhibition is much smaller than that observed with N2 but it is consistently observed. It is concluded that CO specifically inhibits N2 fixation by this organism in the sense that the enzymes responsible for fixation of free N2 are more sensitive to CO than those concerned with assimilation of combined N (or with other aspects of growth). The inhibition of N2 fixation may be only a special case of the inhibition of the reduction of oxidized N compounds by the organism. A. vinelandii grown in N-free medium uses N2, NIL and urea readily; NO3 (or NO2) is much less suitable. If, however, the organism is adapted to the oxidized form by previous transfer in its presence, it will use nitrate (nitrite) as readily as N2. When kept on N-free medium, it does not assimilate glutamate, aspartate or asparagin very rapidly. Adaptation increases the rate of assimilation of asparagin; with the others attempts at adaptation gave indefinite results.