Dependency of growth yield, maintenance and K s-values on the dissolved oxygen concentration in continuous cultures of Azotobacter vinelandii

Abstract

Azotobacter vinelandii was grown diazotrophically in sucrose-limited chemostat cultures at either 12, 48, 108, 144 or 192 μM dissolved oxygen. Steady state protein levels and growth yield coefficients (Y) on sucrose increased with increasing dilution rate (D). Specific rate of sucrose consumption (q) increased in direct proportion to D. Maintenance coefficients (m) extrapolated from plots of q versus D, as well as from plots of 1/Y versus 1/D exhibited a nonlinear relationship to the dissolved oxygen concentration. Constant maximal theoretical growth yield coefficients (Y G) of 77.7 g cells per mol of sucrose consumed were extrapolated irrespective of differences in ambient oxygen concentration. For comparison, glucose-, as well as acetate-limited cultures were grown at 108 μM oxygen. Fairly identical m- and Y G-values, when based on mol of substrate-carbon with glucose and sucrose grown cells, indicated that both substrates were used with the same efficiency. However, acetate-limited cultures showed significantly lower m- and, at comparable, D, higher Y-values than cultures limited by either sucrose or glucose. Substrate concentrations (K s) required for half-maximal growth rates on sucrose were not constant, they increased when the ambient oxygen concentration was raised and, at a given oxygen concentration, when D was decreased. Since biomass levels varied in linear proportion to K s these results are interpreted in terms of variable substrate uptake activity of the culture.