Diel carbon buffering by floridoside enables source-sink resilience in the intertidal red alga Pyropia haitanensis

Diel source-sink regulation is conventionally framed by the sucrose-starch buffering system of land plants, raising the question of whether similar organizational logic operates in early-diverging photosynthetic lineages. Red algae, which rely on low-molecular-weight carbohydrates [(iso)floridoside] and cytosolic floridean starch rather than sucrose and amylopectin, provide an opportunity to test this framework. Here, we combined a 12 h 13C-NaHCO3 pulse with a 36 h chase, isotopologue-resolved LC-MS of sugar phosphates, UDP-sugars, glycerol-3-phosphate, and galactosylglycerols, quantitative floridean starch assays, and time-series transcriptomics in Pyropia haitanensis. Newly assimilated carbon rapidly entered central nodes (glycerol-3-phosphate, hexose-phosphate and UDP-sugars) and subsequently accumulated more slowly in (iso)floridoside and floridean starch. Despite higher basal isofloridoside, 13C was preferentially incorporated into floridoside. During the chase, labelled carbon was diluted and turned over with limited redistribution. These fluxes corresponded with diel transcriptional programs: light-phase up-regulation of carbonic anhydrase, Rubisco and genes directing carbon into galactosylglycerols and starch, and dark-phase induction of α-galactosidase and starch-remodelling enzymes. Together, these results show that low-molecular-weight carbohydrates function as metabolic gatekeepers that relay photosynthate into transient and polymeric sinks, establishing a red-algal analogue of the sucrose-starch buffer and providing a comparative framework for source-sink biology across photosynthetic lineages.