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 N₂-fixing conditions. The properties of PSII were studied throughout the diurnal cycle using O₂-flash-yield and pulse-amplitude-modulated fluorescence techniques. Nonphotochemical quenching (q_N) of PSII increased during N₂ 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 during the first 4 h of light growth. The photoactivation probability, ε, 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 heterogeneity, especially as the cells prepare for N₂ fixation. We conclude that the oxidizing side of PSII is strongly affected during the period before and after the peak of nitrogenase activity; changes include a lowered capacity for O₂ evolution, altered dark stability of PSII centers, and substantial changes in q_N.