Complex food sources aid physiological compensation of bleached corals

Climate change induced coral bleaching threatens the survival of coral reefs. The disruption of the symbiosis between corals and their photosymbionts during bleaching inhibits photosynthesis, the main source of energy of the coral holobiont. Yet, corals can supplement their metabolic demand through heterotrophic feeding and may partially compensate for the lack of autotrophic energy. However, the potential of different types of heterotrophic food to compensate for productivity loss during bleaching is not yet known. Therefore, we evaluated the effect of different food types on the physiology of symbiotic and bleached corals of three species ( Galaxea fascicularis , Porites lobata , and Stylophora pistillata ). Symbiotic and bleached fragments were exposed to five feeding treatments in a 21-week aquarium experiment that included combinations of dissolved and particulate feeds composed of thawed plankton, Artemia salina nauplii, three phytoplankton species, honey, yeast, and amino acids. Here we show that in symbiotic G. fascicularis and P. lobata , growth was not affected by the different food types, while growth increased 1.5- to 2-fold in the high complexity feed in symbiotic S. pistillata . In contrast, all bleached corals benefitted from richer diets. Complex feeds doubled to tripled growth parameters relative to dissolved or low-complexity feeds, and bleached S. pistillata fragments fed only thawed plankton perished. Food treatments did not alter respiration or photosynthetic rates, indicating that growth gains stemmed from enhanced heterotrophic nutrient supply. Physiological rates were consistently higher in symbiotic fragments than in their bleached clones across all species and the differences increased with the baseline productivity of the species from G. fascicularis over P. lobata to S. pistillata . However, the food treatments did not have a clear effect on the differences between bleached and symbiotic fragments. Our results demonstrate that incorporating diverse particulate and dissolved feed components into restoration, aquaculture, or field supplementation protocols could bolster coral resilience to the increasing frequency of mass-bleaching events.