Summary: The growth of three hydrogenase minus (Hup-) mutants of was compared with that of the parent Hup strain in batch or continuous cultures. All three mutants gave similar yields to the parent under N-fixing conditions at an optimum dilution rate of 0.1 h in sucrose-limited N-fixing cultures. However, at higher values the steady-state yields of sucrose-limited mutants were lower than those of the parent and washout occurred at lower values. These observations were confirmed in carbon-limited mixed cultures where the parent strain outgrew the mutant at high values. Such marked differences were not obtained in SO -or O-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-N-fixing (NH -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.


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