The Beneficial Effect of Hydrogenase in Azotobacter chroococcum Under Nitrogen-fixing, Carbon-limiting Conditions in Continuous and Batch Cultures

Abstract

Summary: The growth of three hydrogenase minus (Hup-) mutants of Azotobacter chroococcum was compared with that of the parent Hup+ strain in batch or continuous cultures. All three mutants gave similar yields to the parent under N2-fixing conditions at an optimum dilution rate (D) of 0.1 h−1 in sucrose-limited N2-fixing cultures. However, at higher D values the steady-state yields of sucrose-limited mutants were lower than those of the parent and washout occurred at lower D values. These observations were confirmed in carbon-limited mixed cultures where the parent strain outgrew the mutant at high D values. Such marked differences were not obtained in SO2- 4-or O2-limited continuous cultures. In batch culture at low sucrose levels the mutants displayed a long division lag compared with the parent, particularly with dilute inocula. Non-N2-fixing (NH+ 4-grown) conditions removed these differences. We suggest that one beneficial effect of hydrogenase is on the initiation of diazotrophic growth, particularly with restricted carbon/energy supply.