Heterocyst and nitrogenase development in Anabaena cylindrica

The differentiation of filaments of a continuous culture of Anabaena cylindrica after removal of fixed nitrogen has been examined. Rapid development of proheterocysts (4-5 h) and mature heterocysts (14 h) in an ordered sequence was observed; the development of the latter was concomitant with the onset of nitrogenase activity. Commitment times of 2-3 h (proheterocyst) and 5-0 (heterocyst) were measured. Evidence is presented that shows that heterocyst development, rather than any product of heterocyst function in nitrogen fixation, is responsible for the imposition of the pattern of heterocysts in a filament of vegative cells. Both phycocyanin content and carbon dioxide fixation declined markedly throughout the filament during the early stages of heterocyst development, indicating that all the cells of the filament are involved in the initial stages of the differentiation process. Heterocysts do not evolve oxygen in the light, but do respire. Nitrogenase activity in isolated heterocysts and in intact filaments was stimulated by phosphoenolpyruvate, ATP and dichlorophenolindophenol-ascorbate.