Host-to-symbiont transfer of fatty-acid-derived carbon in the Paramecium bursaria-Chlorella variabilis photosymbiosis

Metabolic exchange stabilizes photosymbioses, but carbon transfer is usually framed as exporting photosynthate from algal symbionts to heterotrophic hosts. Previous work showed that Paramecium bursaria reorganizes its lipid droplets during symbiosis and that perturbing host lipid metabolism reduces endosymbiont abundance. Here, we investigated reciprocal organic-carbon exchange between P. bursaria and its intracellular Chlorella symbionts. Symbiosis was associated with extensive lipid remodeling in both partners and marked triacylglycerol accumulation in endosymbiotic algae. Following host supplementation with 13C-oleic acid, NanoSIMS and EA-IRMS detected 13C enrichment in intracellular algae. Isotopologue-resolved lipidomics further showed incorporation of host-acquired fatty acid carbon into algal storage and membrane lipids. These findings reveal an unexpected host-to-symbiont direction of organic-carbon transfer and expand the conventional model of metabolic exchange in photosymbiosis.