The mosaic of aquatic photorespiration: evolution, diversity, and global biogeochemical impacts

Summary Aquatic phototrophs contribute over half of global carbon fixation, yet their photorespiratory metabolism remains a critical, underexplored component of the Earth's carbon cycle. First reported by Otto Warburg in the green alga Chlorella , Rubisco oxygenase activity produces the inhibitory metabolite 2‐phosphoglycolate (2PG), requiring energetically demanding salvage pathways that release carbon and nitrogen. For decades, photorespiration in aquatic phototrophs was deemed negligible due to the widespread occurrence of CO 2 ‐concentrating mechanisms (CCMs) suppressing oxygenation. However, it is now clear that a basal yet essential photorespiratory flux persists even with active CCMs. This flux is dynamically regulated and integrated with central metabolism, rather than serving as a mere detoxification circuit. Across aquatic phototrophs, photorespiratory pathways exhibit remarkable diversity and lineage‐specific subcellular organization, reflecting complex endosymbiotic histories and ecological adaptations. The photorespiratory intermediate glycolate can be excreted in many lineages, allowing the pathway to function as a metabolic safety valve that stabilizes redox poise under fluctuating conditions. At the ecosystem scale, glycolate excretion – approaching one gigaton of carbon annually – fuels heterotrophic microbial turnover and drives oceanic carbon cycling. Consequently, aquatic photorespiration emerges as a highly responsive process under environmental change and a dynamic hub within global biogeochemical fluxes.