Heterocyst formation and nitrogenase synthesis in Anabaena sp.

Summary

When filaments from a culture of Anabaena sp. growing photoautotrophically with nitrate as a nitrogen source are placed in a nitrate-free mineral medium and incubated anaerobically in the light, the formation of heterocysts and the synthesis of nitrogenase both begin after a lag of about 24 hours. During the lag period, about 70% of the phycocyanin is destroyed. Under an atmosphere of N₂-CO₂, the nitrogenase activity rises to a peak value, and then falls markedly as growth at the expense of N₂ begins. Phycocyanin synthesis resumes concomitantly with growth. Under an atmosphere of Ar-CO₂, the formation of heterocysts and the synthesis of nitrogenase proceed to higher levels than those observed under N₂-CO₂, and the nitrogenase level is thereafter maintained. Under these conditions, neither growth nor resynthesis of phycocyanin occurs, and phycocyanin eventually falls to about 10% of its initial level in the filaments; however, growth can be promptly initiated if N₂ is admitted to the system. The implications of these findings are discussed.