Investigating the effects of environmental change on lipids of tropical reef building corals and the roles of skeletal lipids on coral biomineralization

Understanding the effects of climate change on coral biology and biomineralization is critical for projecting the future of coral reefs. Essential to this understanding are lipid biomolecules which occur both within the tissues and skeletons of corals and fulfil a variety of biological functions. Here we conducted fieldwork and culturing experiments to determine how environmental parameters influence scleractinian coral tissue and skeleton lipid compositions. Our results revealed variations in tissue lipid compositions in cultured Porites spp. due to temperature, genotype, and/or species differences, with no significant effect observed from seawater pCO2. We also observe that elevated temperatures significantly increased total fatty acid and decreased total wax ester contributions to total skeletal lipids within cultured Seriatopora spp. We find a significantly higher contribution of unsaturated fatty acids to the total fatty acid pool at 5 m water depth compared to 25 m in the tissues of Acropora retusa and Pocillopora meandrina collected from a reef site in Moorea. This work together highlights how lipids compositions within reef-building corals change according to their surrounding environment. Skeletal fatty acids in these scleractinian corals were dominated by saturated fatty acids C16:O (palmitic acid) and C18:O (stearic acid), these being the main building blocks of various lipid molecules. We also find a predominance of saturated wax esters C16:0-C14:0, C16:0-C16:0, and C16:0-C18:0 in the skeletons of Seriatopora spp. and Stylophora sp. coral colonies. To explore the role of lipids in coral biomineralization we precipitated aragonite in-vitro under various pH and dissolved inorganic carbon conditions inferred to occur at the coral calcification site, in conjunction with the absence and presence of lipids identified within coral skeletons. Lecithin (a mixture of phospholipids, the primary constituents of biological membranes) significantly inhibited aragonite precipitation and altered the morphology of the aragonite crystals produced. Stearic acid and cholesterol had no significant effects on precipitation rates, nor on aragonite structure and morphology. This research demonstrates that coral lipids are influenced by the environment and that skeletal lipids may be involved in controlling the formation of coral skeletons. Given that future climate change scenarios are likely to increase seawater temperatures and pCO2, potential variations in coral tissue and skeletal lipid compositions as observed in this research could affect the ability of reef-building corals' CaCO3 skeletons to withstand future climate scenarios.