Shift in trophic strategy of coral holobionts exposed to internal waves in the Andaman Sea may contribute to higher bleaching resilience

Abstract

Large amplitude internal waves (IW) of high frequencies break at the continental shelf in the Andaman Sea. These breaking waves travel upslope and periodically introduce cold, nutrient-rich, hypoxic subpycnocline water into shallow-water reefs while increasing organic matter flux. Locally, western island shores are exposed to IW-induced physicochemical variations, while eastern island shores are sheltered from these variations. Exposed reefs harbor corals with increased heat resistance, but the underlying mechanisms remain yet undiscovered. Heterotrophic feeding can play an important role in coral bleaching resilience, thus, a shift in trophic strategy through IW-enriched nutrients and organic matter flux may contribute to thermal resistance. Here we analyzed the fatty acid (FA) profiles of hosts and symbionts of two crucial reef-building species, Porites sp. and Pocillopora sp. from both shore sides. Preliminary results show that host and symbiont FA profiles were coral speciesspecific despite similar total lipid concentrations. Coral hosts and symbionts can transfer lipids from one to another. Nevertheless, in both species, symbiont FA compositions were distinct from host FA. This difference was considerably larger in the more heterotrophic coral Porites, likely because they exchange fewer lipids with their symbionts. Furthermore, Porites coral hosts on IW-exposed reefs showed a FA composition with a more heterotrophic signature than those on IW-sheltered sites. This suggests coral holobionts on IW-exposed reefs were likely able to utilize enriched nutrients and plankton delivery by IW and this could contribute to their improved bleaching resistance. Ongoing stable isotope analysis will provide further insights into IW-induced shifts in trophic strategy