Ca requirement for aerobic nitrogen fixation by heterocystous blue-green algae

The requirement of Ca(2+) for growth and nitrogen fixation has been investigated in two strains of heterocystous blue-green algae (Anabaena sp. and Anabaena ATCC 33047). With combined nitrogen (nitrate or ammonium) or with N(2) under microaerobic conditions, Ca(2+) was not required for growth, at least in concentrations greater than traces. In contrast, Ca(2+) was required as a macronutrient for growth and nitrogen fixation with air as the nitrogen source. Addition of Ca(2+) to an aerobic culture without Ca(2+) promoted, after a lag of several hours, development of nitrogenase activity and cell growth. Provision of air to a microaerobic culture in the absence of Ca(2+) promoted a drastic drop in nitrogenase activity, which rapidly recovered its initial level upon restoration of microaerobic conditions. Development of nitrogenase activity in response to either Ca(2+) or low oxygen tension was dependent on de novo protein synthesis. The role of Ca(2+) seems to be related to protection of nitrogenase from inactivation, by conferring heterocysts resistance to oxygen.