SUMMARY: Nitrogen fixation (acetylene reduction) by was displayed weakly and capriciously in lactate/sulphate media and sequential or continuous diazotrophic cultures were not successfully established. Washed cells from lactate-grown N-limited chemostat populations at 28 °C showed reproducible anaerobic derepression of acetylene reduction, accompanied by limited growth, in low-N buffer, if sodium pyruvate + sodium sulphate were provided. Hydrogenase activity was not affected by the concentrations of acetylene used. Optimum concentrations of cells, pyruvate + sulphate, casein hydrolysate, NaMoO and FeSO were established: peak activities of ~10 nmol CH reduced min (mg bacterial protein) occurred with 10% (v/v) CH after about 48 h; Ni, Mn or derepression under Ar did not influence activity. NH or air prevented derepression. An oxidant, usually sulphate, was essential. Thiosulphate was a poor substitute for sulphate; sulphite was apparently ineffective. Lactate, fumarate or H did not replace pyruvate as derepression substrate.

Pyruvate-derepressed populations showed reversible inactivation when exposed briefly to air. Activity was substantially inhibited by 10 or 100μm-NH , reversibly at low NH concentrations.


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