SUMMARY: The cyanobacterium CCAP 1403/13a (UTEX 1444, ATCC 29413) was grown diazotrophically in the dark in the presence of fructose with a doubling time of 25 h at 35 °C. This was 40% of the maximum rate observed in the light. Growth rates in the dark were similar with nitrate or ammonium as nitrogen sources or under diazotrophic conditions. Dark-grown cyanobacteria reduced acetylene in the dark at 25% of the rate observed for photoautotrophic cultures in the light. Heterotrophic growth yields for dark growth with different nitrogen sources were estimated from the extent of fructose-dependent growth in batch cultures. The ratio between cell carbon and fructose carbon was 0.38 in diazotrophically grown cells, 0.38 with nitrate and 0.52 with ammonia. Thus the energy requirement for growth on molecular nitrogen is similar to the energy requirement for growth on nitrate, but greater than that for growth on ammonia. Since the diazotrophic culture always had a dissolved O concentration of at least 80% of air saturation, this demonstrates that there are no detectable extra energy requirements for aerobic nitrogen fixation compared to nitrate reduction, such as would result from a need for respiratory protection of nitrogenase.


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