Photosynthesis–Respiration Coupling in Cyanobacteria: Regulation or Redox Opportunity?

Cyanobacterial photosynthesis and respiration share electron carriers, allowing respiratory and auxiliary sinks to intersect with the photosynthetic electron-transfer chain. Across ecological niches and stress conditions, these connections can redistribute electrons when dominant photosynthetic routes become limited, revealing an integrated but often underappreciated bioenergetic network. Here, we reinterpret photosynthesis–respiration coupling as a hierarchy of electron-transfer opportunities rather than as fixed, separate pathways. Midpoint-potential relationships help define which transfers are thermodynamically plausible, whereas physiological conditions determine which routes become functionally important under specific circumstances. We propose redox opportunity as a null framework for identifying the routes available within the network and for examining how physiological and regulatory changes reshape their relative contribution to electron flow.