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Metabolism and Enzymology

Ca²⁺ Requirement for Aerobic Nitrogen Fixation by Heterocystous Blue-Green Algae ¹

Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-Consejo Superior de Investigaciones Científicas, Apartado 1113, 41080 Sevilla, Spain, Facultad de Biología, Apartado 1113, 41080 Sevilla, Spain

The requirement of Ca²⁺ 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 N₂ under microaerobic conditions, Ca²⁺ was not required for growth, at least in concentrations greater than traces. In contrast, Ca²⁺ was required as a macronutrient for growth and nitrogen fixation with air as the nitrogen source. Addition of Ca²⁺ to an aerobic culture without Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ or low oxygen tension was dependent on de novo protein synthesis. The role of Ca²⁺ seems to be related to protection of nitrogenase from inactivation, by conferring heterocysts resistance to oxygen.