GROWTH, PHYSIOLOGY, AND ULTRASTRUCTURE OF A DIAZOTROPHIC CYANOBACTERIUM, CYANOTHECE SP STRAIN ATCC-51142, IN MIXOTROPHIC AND CHEMOHETEROTROPHIC CULTURES

The growth, physiology, and ultrastructure of the marine, unicellular, diazotrophic cyanobacterium, Cyanothece sp. strain ATCC 51142 was exa mined under mixotrophic and chemoheterotrophic conditions. Several org anic substrates were tested for the capacity to support heterotrophic growth. Glycerol was the only substrate capable of enhancing mixotroph ic growth in the light and supporting chemoheterotrophic growth in the dark. Dextrose enhanced mixotrophic growth bat could not support chem oheterotrophic growth. Chemoheterotrophic cultures in continuous darkn ess grew faster and to higher densities than photoautotrophic cultures , thus demonstrating the great respiratory capacity of this cyanobacte rial strain. Only small differences in the pigment content and ultrast ructure of the heterotrophic strains were observed in comparison to ph otoautotrophic control strains. The chemoheterotrophic strain grown in continuous darkness and the mixotrophic strain grown in light/dark cy cles exhibited daily metabolic oscillations in N-2 fixation and glycog en accumulation similar to those manifested in photoautotrophic cultur es grown in light/dark cycles or continuous light. This ''temporal sep aration'' helps protect O-2-sensitive N-2 fixation from photosynthetic O-2 evolution. The rationale for cyclic glycogen accumulation in cult ures with an ample source of organic carbon substrate is unclear, but the observation of similar daily rhythmicities in cultures grown in li ght/dark cycles, continuous light, and continuous dark suggests an und erlying circadian mechanism.