Investigating the Effects of High Light Versus Low Light in Two Different Types of Symbiodinium within the Reef-Building Coral, Acropora Millepora

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University of Delaware
Coral reefs are invaluable ecosystems upon which a massive number of coastal organisms rely, including human communities. The reefs themselves are composed primarily of colonial organisms, and the vast majority of their energetic budget is provided by glucose-rich photosynthate derived from their endosymbiotic dinoflagellates (genus Symbiodinium). Here, two different types of Symbiodinium were investigated, Symbiodinium C1 and D3, within the same species of Indo-Pacific coral, Acropora millepora, which was acclimated to either high- (1000 μmol quanta m-2 s-1) or low-light (100 μmol quanta m-2 s-1) conditions. Samples were analyzed by means of traditional coral metrics, biochemical composition, and targeted gene expression in order to gauge differences in photoacclimation mechanisms between the two symbiont types, as well as to better understand each alga’s respective role in holobiont health and photobiology. The coral harboring the D3 symbiont had higher levels of protein and carbohydrate, as well as increased expression for genes that encode an intracellular carbonic anhydrase, GAPDH, and glutamine synthetase, all of which were higher under the high-light treatment. In contrast, extracellular carbonic anhydrase expression was greater in the coral hosting the C1 symbiont, under highlight- acclimation. Average chlorophyll a concentration was greater in Symbiodinium C1 than D3, and the photosynthetic and respiratory data suggest that the coral hosting Symbiodinium D3 may have a higher respiratory demand. These findings, in conjunction with previous studies, provide a continued groundwork for understanding both symbiont diversity and the role that members of Symbiodinium play in the health of their host.
Marine Science