The effects of different light-dark cycles on the metabolism of the diazotrophic, unicellular cyanobacteria Cyanothece sp. ATCC 51142, and Cyanothecesp. PCC 7822

The diazotrophic unicellular cyanobacterium Cyanothece sp. ATCC 51142 demonstrates circadian patterns in nitrogenase activity, H2 production and glycogen storage when grown under nitrogen-fixing, 12:12 light:dark (L:D) conditions. In this study, we grew Cyanothece sp. ATCC 51142, and another strain in this genus, Cyanothece sp. PCC 7822, under long-day (16:8 L:D) and short-day (8:16 L:D) nitrogen-fixing conditions to determine if they continued to display circadian rhythms. Both strains demonstrated similar circadian patterns for all three metabolic parameters when grown under long-day conditions. However, the strains responded differently to short-day growth conditions. Cyanothece sp. ATCC 51142 retained reasonable circadian patterns under 8:16 L:D conditions, whereas Cyanothece sp. PCC 7822 had quite damped patterns without a clear circadian pattern. In particular, glycogen storage changed very little throughout the day and we ascribe this to the difference in the type of glycogen granules in Cyanothece sp. PCC 7822 which has small β-granules, compared to the large, starch-like granules in Cyanothece sp. ATCC 51142. The results suggested that both mechanistic and regulatory processes play a role in establishing the basis for these metabolic oscillations.