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

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Abstract
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 N,-fixing conditions at an optimum dilution rate (D) of 0.1 h-l in sucrose-limited N2-fixing cultures. However, at higher D values the steady-state yields of sucroscllimited 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 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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