Carbon dioxide regulation of autotrophy and diazotrophy in the nitrogen-fixing cyanobacterium Nostoc muscorum.

To understand how carbon and nitrogen metabolism are regulated in diazotrophically and non-diazotrophically grown cultures of the cyanobacterium Nostoc muscorum, we investigated the role of bicarbonate (HCO₃⁻) in regulating diazotrophy and autotrophy. Results showed that HCO₃⁻ concentration up to 12 mol m⁻³ enhanced growth, specific growth rate, photosynthetic pigments, photosynthetic O₂ evolution and nitrogenase activity under diazotrophic growth conditions. The co-existence of different nitrogen sources in the growth medium further accelerate the examined parameters in the order of NO₃⁻<NO₂⁻<NH₄⁺<proline. Further, we examined the Ca⁺⁺-dependent ATPase activity and cytochrome c oxidase activity in the presence of graded concentration of HCO₃⁻ under diazotrophically grown and non-diazotrophically grown cultures. The activity of these enzymes was higher in the cells grown under elevated HCO₃⁻ concentration. The Ca⁺⁺-dependent ATPase activity was higher than that of cytochrome c oxidase, when NAPH was supplied as the electron donor. This finding suggested that photosynthetic generation of ATP utilized NADPH as an electron donor and cytochrome c oxidase activity is independent of NADPH.