EFFECT OF OXYGEN AND TEMPERATURE ON THE INDUCTION OF A CIRCADIAN NITROGENASE ACTIVITY RHYTHM IN SYNECHOCOCCUS RF-1

Evidence is presented that the periodic cessation of photosynthetic CO2 assimilation rather than O2 evolution appears to induce an endogenous nitrogenase activity rhythm in Synechococcus RF-1. Apart from the inductive effect of L/D cycles, a 4 h cold pulse (0-degrees-C) on four consecutive days in continuous light also induces the rhythm. At 0-degrees-C the organism ceases to photosynthesize. The rhythms induced by means of UD cycles at temperatures below 23-degrees-C (15-degrees-C-0-degrees-C) exhibit similar phase shifts relative to the rhythms induced at 23-degrees-C and higher temperatures. Synechococcus RF-1 does not exhibit nitrogenase activity in nitrogen-free medium at 5-degrees-C or lower temperatures and appears to be unable to survive exposure to -5-degrees-C for a few hours.