H2 metabolism in the photosynthetic bacterium Rhodopseudomonas capsulata: H2 production by growing cultures

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Abstract
Purple photosynthetic bacteria produce H2 from organic compounds by an anaerobic light-dependent electron transfer process in which nitrogenase functions as the terminal catalyst. The H2-evolving function of nitrogenase is inhibited by N2 and ammonium salts and is maximally expressed in cells growing photoheterotrophically with certain amino acids as N sources. The nutritional factors affecting the rate and magnitude of H2 photoproduction in R. capsulata cultures growing with amino acid N sources were examined. The highest H2 yields and rates of formation were observed with organic acids (lactate, pyruvate, malate and succinate) in media containing glutamate as the N source; under optimal conditions with excess lactate, H2 was produced at rates of .apprx. 130 ml/h per g (dry wt) of cells. H2 production is significantly influenced by the N/C ratio in the growth substrates; when this ratio exceeds a critical value, free NH3 appears in the medium and H2 is not evolved. In the standard lactate + glutamate system, H2 production and growth are saturated at a light intensity of .apprx. 600 ft-c (6500 lx). Evolution of H2 occurs during growth at light intensities as low as 50-100 ft-c (540-080 lx), i.e., under conditions of energy limitation. In circumstances where energy conversion rate and supplies of reducing power exceed the capacity of the biosynthetic machinery, energy-dependent H2 production presumably represents a regulatory device that facilitates energy-idling. Even when light intensity (energy) is limiting, a significant fraction of the available reducing power and ATP is probably diverted to nitrogenase, resulting in H2 formation and a bioenergetic burden to the cell.