Two marine, unicellular aerobic nitrogen-fixing cyanobacteria, Cyanothece strain BH63 and Cyanothece strain BH68, were isolated from the intertidal sands of the Texas Gulf coast in enrichment conditions designed to favor rapid growth. By cell morphology, ultrastructure, a GC content of 40%c, and aerobic nitrogen fitation ability, these strains were assigned to the genus Cyanothece. These strains can use molecular nitrogen as the sole nitrogen source and are capable of photoheterotrophic growth in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and glycerol. The strains demonstrated a doubling time of 10 to 14 h in the presence of nitrate and 16 to 20 h under nitrogen-fixing conditions. Rapid growth of nitrogen-fixing cultures can be obtained in continuous light even when the cultures are continuously shaken or bubbled with air. Under 12-h alternating light and dark cycles, the aerobic nitrogenase activity was confined to the dark phase. The typical rates of aerobic nitrogenase activity in Cyanothece strains BH63 and BH68 were 1,140 and 1,097 nmol of C2H2 reduced per mg (dry weight) per h, respectively, and nitrogenase activity was stimulated twofold by light. Ultrastructural observations revealed that numerous inclusion granules formed between the photosynthetic membranes in cells grown under nitrogen-fixing conditions. These Cyanothece strains possess many characteristics that make them particularly attractive for a detailed analysis of the interaction of nitrogen fixation and photosynthesis in an aerobic diazotroph.In all nitrogen-fixing organisms, the nitrogen fixation process is carried out by nitrogenase, an extremely oxygensensitive enzyme. Microorganisms have developed various strategies to protect their nitrogenase from oxygen inhibition (2, 5, 6). Among the nitrogen-fixing microorganisms, the cyanobacteria occupy a unique position because these are the only oxygenic photosynthetic organisms capable of nitrogen fixation under aerobic conditions (13). Nitrogen fixation has been reported for all three major morphological groups of cyanobacteria: heterocystous filamentous, nonheterocystous filamentous, and unicellular forms (5, 13). The oxygen protection mechanisms employed by these organisms vary considerably. In the heterocystous filamentous strains, about 5 to 10% of the cells undergo morphological differentiation into specialized cells called heterocysts under nitrogen-fixing conditions (13,49). In this arrangement, nitrogen fixation and photosynthetic oxygen evolution are spatially separated, so that oxygenic photosynthesis takes place in vegetative cells and nitrogen fixation occurs in heterocysts. The fixed nitrogen from these cells is exported to the neighboring vegetative cells, and the reductant for nitrogen fixation is imported from the vegetative cells (5, 49).There are many genera of nonheterocystous cyanobacteria, both filamentous and unicellular, that are capable of nitrogen fixation. The nonheterocystous filamentous forms have been placed in the genera Trichodesmium, Oscillatoria, an...

Aerobic photoproduction of H2 was demonstrated in Anabaena spp. strains CA and 1F when cells were growing under nitrogen-fixing conditions. The rates of production, measured either by the hydrogen electrode or in a flow system by gas chromatography, were 10 to 15% of the rate of photosynthetic 02 evolution or 50 to 80%6 of the rates of acetylene reduction. Strains CA, and 1F differed in 's9veral respects. In strain CA, H2 production was immediately paitially sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, whereas strain 1F was not immediately affected. Strain CA also showed a consistently higher rate of H2 production than did strain 1F. H2 production in strain CA was also markedly influenced by the light intensity used for growth., although the growth rates indicated that the light intensities used were essentially saturating.