CYCLIC VARIATIONS OF PHOTOSYNTHETIC ACTIVITY UNDER NITROGEN-FIXING CONDITIONS IN SYNECHOCOCCUS RF-1

When nitrogen fixing cell cultures of Synechococcus RF-1 were subjecte d to an alternating light-dark regime (12 h:12 h), a cyclic decrease i n the photosynthetic oxygen evolution potential was observed during th e dark periods. This rhythm of net photosynthesis rate was maintained for at least two days after transition to continuous light. The decrea se in net photosynthesis was accompanied by a stimulation of dark resp iration. However, the magnitude of oxygen uptake was considerably smal ler than the observed decrease in oxygen evolution. The photosynthetic activity of cells taken from the dark period was characterized by (i) a significantly lower quantum yield and (ii) a strong reduction in th e Light-saturated rate of photosynthesis. Growing the cultures on nitr ate or under continuous Light completely suppressed this rhythm. Prote in synthesis was not necessary for the recovery of the light-saturated rate of photosynthesis during the light period. The cellular content of chlorophyll a and of phycobiliproteins did not vary between Light a nd dark period, indicating that quantitative changes in the compositio n of the photosynthetic apparatus are not the basis for the observed o scillations. Regulatory modifications of the photosynthetic efficiency are proposed as an adaptation mechanism to adjust the intracellular o xygen concentration to the needs for nitrogenase activity.