Regulatory Effect of pH and Incident Irradiance on the Levels of Nitrogenase Activity in the Cyanobacterium Nostoc UAM 205

Abstract

Summary Physiological response of Nostoc UAM 205 to the combined influence of pH and light intensity was examined. Increase in light intensity from 10 to 100μE · m−2 · s−1 had no stimulatory affect on growth rates of cultures grown at pHs 6 and 7, but enhanced growth at alkaline pH. Chlorophyll concentration of cells grown at alkaline pH was higher than that of cells grown at pHs 6 and 7. Chlorophyll concentration decreased with increasing light intensity. The light-saturated photosynthetic rate normalized to chlorophyll was slightly higher in cells grown at high light intensity than in cells grown at low light intensity. The photosynthetic rate did not change with pH. Major physiological changes in response to variations in external pH and irradiance were observed in nitrogenase activity. Optimal rates of nitrogenase activity were found at alkaline pH. Nitrogenase activity showed a negative correlation with the irradiance at which cultures were grown. Cells grown at low light intensity showed a higher maximal rate of dinitrogen fixation and a higher light utilization efficiency than cells grown at high light intensity. These results suggested a special role of light in regulating nitrogenase activity in cyanobacteria.