TEMPORAL CHANGES IN STATE TRANSITIONS AND PHOTOSYSTEM ORGANIZATION INTHE UNICELLULAR, DIAZOTROPHIC CYANOBACTERIUM CYANOTHECE SP. ATCC-51142

The unicellular cyanobacterium Cyanothece sp. ATCC 51142, grown under alternating 12-h light/12-h dark conditions, temporally separated N-2 fixation from photosynthesis. The regulation of photosynthesis was stu died using fluorescence spectra and kinetics to determine changes in s tate transitions and photosystem organization. The redox poise of the plastoquinone (PQ) pool appeared to be central to this regulation. Res piration supported N-2 fixation by oxidizing carbohydrate granules, bu t reduced the PQ pool. This induced state 2 photosystem II monomers an d lowered the capacity for O-2 evolution, State 2 favored photosystem I trimers and cyclic electron transport, which could stimulate N-2 fix ation; the stimulation suggested an ATP limitation to N-2 and CO2 fixa tion. The exhaustion of carbohydrate granules at around 6 h in the dar k resulted in reduced respiratory electron flow, which led to a more o xidized 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 o f darkness. In the light phase, photosystem II dimerization correlated with increased phycobilisome coupling to photosystem II (state 1) and increased rates of O-2 evolution. However, dark adaptation did not gu arantee state 2 and left photosystem I centers in a mostly monomeric s tate at certain times.