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The unicellular Cyanobacterium Cyanothece sp. ATCC 51142, grown under alternating 12-h light/12-h dark conditions, temporally separated N2 fixation from photosynthesis. The regulation of photosynthesis was studied using fluorescence spectra and kinetics to determine changes in state transitions and photosystem organization. The redox poise of the plastoquinone (PQ) pool appeared to be central to this regulation. Respiration supported N2 fixation by oxidizing carbohydrate granules, but reduced the PQ pool. This induced state 2 photosystem II monomers and lowered the capacity for O2 evolution. State 2 favored photosystem I trimers and cyclic electron transport, which could stimulate N2 fixation; the stimulation suggested an ATP limitation to N2 and CO2 fixation. The exhaustion of carbohydrate granules at around 6 h in the dark resulted in reduced respiratory electron flow, which led to a more oxidized PQ pool and produced a sharp transition from state 2 to state 1. This transient state 1 returned to state 2 in the remaining hours of darkness. In the light phase, photosystem II dimerization correlated with increased phycobilisome coupling to photosystem II (state 1) and increased rates of O2 evolution. However, dark adaptation did not guarantee state 2 and left photosystem I centers in a mostly monomeric state at certain times.

Temporal Changes in State Transitions and Photosystem Organization in the Unicellular, Diazotrophic Cyanobacterium Cyanothece sp. ATCC 51142