INDUCTION OF NITROGEN-FIXING CIRCADIAN-RHYTHM SYNECHOCOCCUS RF-1 BY LIGHT SIGNALS

A circadian rhythm in nitrogen fixation could be induced by treating Synechococcus RF-1 cultures preadapted to continuous white light or red light at 680 nm (10 nm half-bandwidth) with a single 12-h dark pulse. The induction of rhythm in white light failed when the dark treatment was replaced by 680 nm red light at 100 muE m-2s-1. The rhythm induced in 680 nm red light was not affected when a 30-min far red light pulse at 730 nm was applied at various intervals during the dark treatment. The rhythm in a culture that had been maintained in continuous white light could also be induced by exposing the cells to 12/12 h bright/dim cycles, with the dim part of the cycle being at one fourth the intensity of the bright. Our results suggest that lowering the intensity of photosynthetic light is sufficient to induce a circadian rhythm in Synechococcus RF-1.