PHOTOSYSTEM-II CYCLIC HETEROGENEITY AND PHOTOACTIVATION IN THE DIAZOTROPHIC, UNICELLULAR CYANOBACTERIUM CYANOTHECE SPECIES ATCC-51142

The unicellular, diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 demonstrated important modifications to photosystem II (PSII) centers when grown under light/dark N-2-fixing conditions. The properties of PSII were studied throughout the diurnal cycle using O-2-flash-yield a nd pulse-amplitude-modulated fluorescence techniques. Nonphotochemical quenching (q(N)) of PSII increased during N-2 fixation and persisted after treatments known to induce transitions to state 1. The q(N) was high in cells grown in the dark, and then disappeared progressively du ring the first 4 h of light growth. The photoactivation probability, e psilon, demonstrated interesting oscillations, with peaks near 3 h of darkness and 4 and 10 h of light. Experiments and calculations of the S-state distribution indicated that PSII displays a high level of hete rogeneity, especially as the cells prepare for N-2 fixation. We conclu de that the oxidizing side of PSII is strongly affected during the per iod before and after the peak of nitrogenase activity; changes include a lowered capacity for O-2 evolution, altered dark stability oi PSII centers, and substantial changes in q(N).