Integral Light-Harvesting Complex Expression in Symbiodinium Within the Coral Acropora aspera under Thermal Stress

Abstract

Coral reef success is largely dependent on the symbiosis between coral hosts and dinoflagellate symbionts belonging to the genus Symbiodinium. Elevated temperatures can result in the expulsion of Symbiodinium or loss of their photosynthetic pigments and is known as coral bleaching. It has been postulated that the expression of light-harvesting protein complexes (LHCs), which bind chlorophylls (chl) and carotenoids, are important in photobleaching. This study explored the effect a sixteen-day thermal stress (increasing daily from 25-34 °C) on integral LHC (chlorophyll a-chlorophyll c 2-peridinin protein complex (acpPC)) gene expression in Symbiodinium within the coral Acropora aspera. Thermal stress leads to a decrease in Symbiodinium photosynthetic efficiency by day eight, while symbiont density was significantly lower on day sixteen. Over this time period, the gene expression of five Symbiodinium acpPC genes was quantified. Three acpPC genes exhibited up-regulated expression when corals were exposed to temperatures above 31.5 °C (acpPCSym-1:1, day sixteen; acpPCSym-15, day twelve; and acpPCSym-18, day ten and day sixteen). In contrast, the expression of acpPCSym-5:1 and acpPCSym-10:1 was unchanged throughout the experiment. Interestingly, the three acpPC genes with increased expression cluster together in a phylogenetic analysis of light-harvesting complexes.