Mapping carbon fate during bleaching in a model cnidarian symbiosis: the application of 13 C metabolomics.

Coral bleaching is a major threat to the persistence of coral reefs. Yet we lack detailed knowledge of the metabolic interactions that determine symbiosis function and bleaching-induced change. We mapped autotrophic carbon fate within the free metabolite pools of both partners of a model cnidarian-dinoflagellate symbiosis (Aiptasia-Symbiodinium) during exposure to thermal stress via the stable isotope tracer (13 C bicarbonate), coupled to GC-MS. Symbiont photodamage and pronounced bleaching coincided with substantial increases in the turnover of non13 C-labelled pools in the dinoflagellate (lipid and starch store catabolism). However, 13 C enrichment of multiple compounds associated with ongoing carbon fixation and de novo biosynthesis pathways was maintained (glucose, fatty acid and lipogenesis intermediates). Minimal change was also observed in host pools of 13 C-enriched glucose (a major symbiont-derived mobile product). However, host pathways downstream showed altered carbon fate and/or pool composition, with accumulation of compatible solutes and nonenzymic antioxidant precursors. In hospite symbionts continue to provide mobile products to the host, but at a significant cost to themselves, necessitating the mobilization of energy stores. These data highlight the need to further elucidate the role of metabolic interactions between symbiotic partners, during the process of thermal acclimation and coral bleaching.