Diazotrophy affects the state transitions in unicellular nitrogen fixing cyanobacteria

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State transitions balance excitation energy between photosystem II and photosystem I and represent a key photoprotective mechanism in oxygenic photosynthesis. While extensively characterized in model cyanobacteria, the regulation of state transitions in diazotrophic species remains poorly understood. Here, we present a comparative physiological study of two unicellular nitrogen-fixing cyanobacteria, Crocosphaera watsonii WH 8501 and Crocosphaera subtropica ATCC 51142 under diazotrophic and non-diazotrophic conditions. Using growth analyses, whole-cell absorption spectroscopy, and low-temperature (77 K) fluorescence spectroscopy, we examined how nitrogen availability shapes photosynthetic energy distribution across species and conditions. Our results showed that state transitions are tightly linked to nitrogen metabolism in both species, but with distinct species-specific regulatory strategies. Diazotrophic cultures exhibited a higher capacity for state transitions than non-diazotrophic cultures, indicating enhanced flexibility in excitation energy redistribution. In C. watsonii, pronounced state transitions were observed exclusively under diazotrophic conditions, whereas non-diazotrophic cultures relied on alternative modes of excitation regulation. In contrast, C. subtropica displayed a more flexible response, with state transitions influenced by both nitrogen availability and light intensity. Fluorescence analyses revealed that state transitions involve a combination of excitation spillover between photosystems and dynamic coupling-decoupling of phycobilisomes. Together, these findings suggest that nitrogen fixation is associated with changes in photosynthetic energy regulation in marine diazotrophic cyanobacteria.