Effect of External cAMP on Growth, Cell Differentiation and Nitrogenase Activity of Nostoc linckia

Summary Three strains of Nostoc linckia (Roth Born et Flah., i.e., the wild type, a mutant producing heterocysts but lacking nitrogenase (Het + Nif − ) and a mutant displaying reduced heterocyst frequency and nitrogenase activity (Het rf Nif ra ), were used in this study. In the parent strain, external cAMP suppresses heterocyst formation and nitrogenase activity, and supports less growth than that measured in the presence of nitrate or in the absence of any source of combined nitrogen. In both mutant strains the supply of cAMP stimulates growth and concurrently inhibits heterocyst differentiation and nitrogenase synthesis. The results do not indicate a regulatory function of external cAMP in nitrogen fixation by N. linckia but suggest that exogenous cAMP is utilized by this alga as an alternative source of combined nitrogen.