A dual role of phycoerythrin in the nitrogen‐fixing cyanobacterium Crocosphaera watsonii : From light‐harvesting to nitrogen reservoir

Unicellular diazotrophic cyanobacteria contribute up to ~50% of marine N 2 fixation, and Crocosphaera watsonii , characterized by high phycoerythrin (PE) content, is among one of the key species in tropical and subtropical oceans. Although recent observations have revealed heterogeneous N 2 fixation activity within C. watsonii populations, the mechanisms behind its heterogeneity remain unresolved. Here, we hypothesize that PE's structural state is associated with the regulation of N 2 fixation. Using single‐cell fluorescence spectroscopy, we observed that photosystem‐uncoupled free PE was exclusively present under diazotrophic conditions and localized to the central cytoplasmic region. While mean PE fluorescence remained unchanged, PE : allophycocyanin ratios declined at the periphery in the night. These findings suggest an additional role for PE, potentially as a temporary nitrogen reservoir to buffer cellular N levels during active N 2 fixation, beyond its established light‐harvesting function.