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

Abstract

Unicellular diazotrophic cyanobacteria contribute up to ~50% of marine N2 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 N2 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 N2 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 N2 fixation, beyond its established light‐harvesting function.