Nitrogen Fixation by Rhodospirillum rubrum Grown in Nitrogen-limited Continuous Culture

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Abstract
Cell-free extracts of the photosynthetic bacterium Rhodospirillum rubrum were inconsistent in reducing N2. An internally illuminated fermentor, designed for the continuous culture of this organism on N2 under nitrogen-limited conditions, produced cells which yielded cell extracts with consistent activity for cell-free N2 fixation. A nitrogen-limited continuous culture, supplied ammonia rather than N2, gave cell-free extracts with even more active N2 fixation. Extracts of cells grown in the fermentor with glutamate nitrogen as the limiting nutrient in continuous culture did not reduce N2, but whole cells fixed 15N-enriched N2. The discovery that cells from ammonia and glutamate nitrogen-limited continuous cultures are capable of N2 reduction suggests that R. rubrum cells produce the N2-reducing enzymes in response to conditions of nitrogen deficiency rather than in response to the presence of N2. Examination of the effect of the pN2 on N2 reduction by cell-free preparations of R. rubrum indicated that the K(N2) is approximately 0.071 atm. Cell-free extracts from R. rubrum were tested for their ability to reduce substrates other than N2.

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