The biochemical energy balance of the coral symbiosis

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Over the last three decades, coral reefs around the world have declined by an estimated 51%. This has largely been caused by anthropogenic climate change resulting in increases in ocean acidification and sea surface temperatures. At their core, corals are a symbiotic relationship between a microscopic algae of the genus Symbiodinium known as “zooxanthellae”, the cnidarian coral host and associated bacterial communities. Under severe environmental stress, the coral will expel the algae. This results in the host losing its major source of organic carbon. Extensive research into tolerance of the algae, have revealed a large genetic diversity within the genus Symbiodinium and it is thought that macromolecular content (carbohydrates, proteins, lipids and phosphorylated compounds) have an effect on biochemical processes responsible for energy acquisition and repair of photosynthetic membranes within the cells. Metabolomics, the study of macromolecular compounds within a biological system, has been applied in various forms, to describe individual compounds such as fatty acids or sterols, contained within different clades of Symbiodinium. In this study, two clades of Symbiodinium sp. were chosen based on their differing tolerance to environmental stress, and analysed to investigate macromolecular changes in the face of fluctuations in light and temperature. Under normal growth conditions, clades of Symbiodinium sp. differed in protein and lipid structure. This is the first time this has been reported to date. In order to further explore these differences in macromolecular content and structure, the cells were subjected to sub-lethal light and temperature treatments. Under these conditions, it was found that both clades increased their β-sheet protein secondary structure. When exposed to elevated light, lipid was stored and carbohydrate consumed whereas the opposite was found under elevated temperature. This has further implications for nutrient exchange in hospite. Clades of Symbiodinium sp. were then exposed elevated temperature to simulate bleaching conditions. Under these high temperatures, clade A was found to exhibit the largest decline in maximum quantum yield of PSII indicating photodamage. This decline in Fv/Fm was linked to changes in lipid and protein secondary structure indicating a change in thylakoid membrane structure occurred under extreme stress. It was also proposed that the change in protein secondary structure was related to protein subunits associated with the oxygen evolving complex, and subsequently photodamage and PSII repair mechanisms. Synchrotron FTIR spectroscopic chemical imaging was also used to further analyse these changes in Symbiodinium sp. from a single-cell perspective. Macromolecular compound groups (protein, lipid, carbohydrate and phosphorylated compounds) were shown to be distributed differently across the cells. Further to this, there appeared to be a difference in the regions in which α-helix and β-sheet protein structures clustered across the individual cells.
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