Respiratory differences associated with culture aeration in Azotobacter vinelandii

Respiratory oxygen uptake of nitrogen-fixing Azotobacter vinelandii cells was altered by culturing at different levels of culture agitation (aeration). Cells were grown at low agitation or moderate agitation and with three different carbon substrates. The low-agitation cultures had much lower dissolved-oxygen concentrations than moderate-agitation cultures at the stage of growth at which they were studied. The respiration systems of cells from the moderate-agitation cultures had lower apparent affinities (higher K s (O 2 ) values) for oxygen than cells grown at low agitation. The higher K s (O 2) ) values were dependent on the presence of Ca 2+ and (or) Mg 2+ in the medium. In low-agitation cultures, the oxygen concentrations were below the K s (O 2 values and the respiration rates in the cultures were therefore well below the maximal respiration (V max ) rates. The oxygen concentrations in moderate-agitation cultures were above the K s (O 2 ) values and the culture respiration rates were much higher. The culture oxygen concentration relative to the K s (O 2 ) had a much greater effect on culture respiration rate than did the V max . It is proposed that changes in the respiration system resulting from culture agitation (aeration) reflect an "oxygen-sensing mechanism" that regulates respiration. This would provide at least a partial explanation for the increased respiration rates with increased culture oxygen concentration in A. vinelandii.