Role of reduced exogenous organic compounds in the physiology of the blue-green bacteria (algae): photoheterotrophic growth of a "heterotrophic" blue-green bacterium.

Nostoc sp. (strain Mac) was shown to be capable of using glucose, fructose, or sucrose as a sole source of carbon and energy in the dark. In the light in the absence of exogenously supplied CO(2), this strain exhibited a more versatile metabolism. In addition to the three sugars above, glycerol and acetate served as sole sources of carbon. This photoheterotrophic growth in the absence of exogenously supplied CO(2) appears to involve O(2)-evolving photosynthesis. The action spectrum for photoheterotrophic growth on acetate closely resembles the action spectrum for photosynthesis. The physiology of photoheterotrophic growth was further investigated through determinations of stable carbon isotope ratios and measurements of gas exchanges. These investigations suggest that respired CO(2) from substrate oxidation is assimilated by the photosynthetic machinery.