Effects of light and CO2 on nitrite liberation by the heterocystous cyanobacterium Anabaena variabilis

Air-grown cultures of the heterocystous cyanobacterium Anabaena variabilis ATCC 29413, when incubated under continuous illumination in the presence of nitrate, liberated nitrite at a low rates during growth. Ammonium liberation was also observed but at a very low level. Cultures that were shifted to a higher light intensity showed increased rates of nitrite liberation. Increasing the CO2 supply (from 0.035% to 5%, v/v) induced rapid nitrite uptake, irrespective of whether cultures were incubated in the light or in the dark, and stimulated photosynthetic O-2 evolution, glutamine synthetase activity, and nitrogenase activity. Results indicate that CO2 availability is an important factor in the control of nitrite assimilation and are consistent with regulatory interactions involving products of CO2 and nitrogen assimilation. Although nitrite accumlated up to 0.25 mM in A. variabilis cultures tested, nitrite toxicity was detected at concentrations higher than 0.6 mM. These results may be relevant to environments where cyanobacteria are subjected to diurnal, seasonal, and geographical variations of CO2 levels and illumination rates.