Photoperiod length paces the temporal orchestration of cell cycle and carbon‐nitrogen metabolism in C rocosphaera watsonii

Abstract

Summary We analysed the effect of photoperiod length ( PPL ) (16:8 and 8:16 h of light–dark regime, named long and short PPL , respectively) on the temporal orchestration of the two antagonistic, carbon and nitrogen acquisitions in the unicellular, diazotrophic cyanobacterium C rocosphaera watsonii strain WH 8501 growing diazotrophically. Carbon and nitrogen metabolism were monitored at high frequency, and their patterns were compared with the cell cycle progression. The oxygen‐sensitive N 2 fixation process occurred mainly during the dark period, where photosynthesis cannot take place, inducing a light–dark cycle of cellular C : N ratio. Examination of circadian patterns in the cell cycle revealed that cell division occurred during the midlight period, (8 h and 4 h into the light in the long and short PPL conditions, respectively), thus timely separated from the energy‐intensive diazotrophic process. Results consistently show a nearly 5 h time lag between the end of cell division and the onset of N 2 fixation. Shorter PPL s affected DNA compaction of C . watsonii cells and also led to a decrease in the cell division rate. Therefore, PPL paces the growth of C . watsonii : a long PPL enhances cell division while a short PPL favours somatic growth (biomass production) with higher carbon and nitrogen cell contents.

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