Abstract
AbstractCoral reefs are restricted to warm waters, but are increasingly threatened by coral bleaching induced by sustained high sea surface temperatures. Coral endosymbiont thermal resilience has been proposed to depend, at least in part, on the lipid composition of their thylakoid membranes, which influences photosynthetic performance under sub- and super-optimal thermal conditions in photosynthetic organisms. Dinoflagellate symbionts of high-latitude coral reefs experience large seasonal changes in temperature, requiring a wide range of thermal tolerance, and so the thermal responses of their membrane lipids are of particular interest. Using gas chromatography–mass spectrometry, we investigated the composition and response to high- and low-temperature stress of thylakoid fatty acids of dinoflagellate symbionts isolated from corals of Lord Howe Island, the world’s southernmost coral reef. We detected genotype-specific differences in the quality of thylakoid fatty acids of two Cladocopium ITS2 consortia/genotypes, C100/118 and C111*, common local symbionts of the corals Pocillopora damicornis and Porites heronensis. The capability to adjust thylakoid fatty acid composition in response to temperature differed between distinct Cladocopium genotypes, and between the same Cladocopium consortium (C100/118) in the same coral species from different locations. Fatty acid adjustments were highly similar in response to short-term cold and heat stresses, with substantial increases in long-chain polyunsaturated fatty acids and a corresponding increase in the ratio of unsaturated to saturated fatty acids, but these changes did not correlate with the quantum yield of photosystem II. The response of thylakoid fatty acid composition to changes in temperature was a function of symbiont genotype, coral host species and, potentially, environmental history. Our data suggest the existence of common responses to high- and low-temperature stresses and that thylakoid fatty acid saturation is an unreliable predictor of photosystem efficiency under thermal stress in dinoflagellate symbionts.
Highlights
The mutualistic association of symbiotic dinoflagellates and their coral hosts is ecologically important but very sensitive to environmental perturbation (Davy et al 2012; Hughes et al 2018)
The primary objective of this study was to compare the composition of thylakoid fatty acids (FAs) in Cladocopium C100/C118 and C111* genotypes harboured by the corals Pocillopora damicornis and Porites heronensis, respectively, before and after exposure to short-term heat and cold stress
We found variation in the baseline FA composition of thylakoid-enriched membranes and their response to thermal stress between distinct Cladocopium genotypes
Summary
The mutualistic association of symbiotic dinoflagellates (family Symbiodiniaceae) and their coral hosts is ecologically important but very sensitive to environmental perturbation (Davy et al 2012; Hughes et al 2018). Often associated with high light intensity, can induce cellular stress in either symbiotic partner followed by bleaching, defined as reductions of symbiont pigments or symbiont density (Hoegh-Guldberg 1999; Hughes et al 2017; Oakley and Davy 2018). In the Symbiodiniaceae, acclimation to high temperature requires adjustments to the thermal stability of photosynthetic membranes (Ulstrup et al 2009). Driven changes in thylakoid membrane fluidity can uncouple light harvesting and photochemistry, suppressing NADPH and ATP synthesis and resulting in concomitant increased production of reactive oxygen species (ROS) (Tchernov et al 2004). Excess ROS are cytotoxic compounds which induce widespread cellular stress and can induce coral mortality or expulsion of symbiont cells (Oakley and Davy 2018; Rosset et al 2021)
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