| Summary: | Symbiotic relationships are very important for corals. Abiotic stressors cause the acclimatization of cell membranes in symbionts, which possess different membrane acclimatization strategies. Membrane stability is determined by a unique lipid composition and, thus, the profile of thylakoid lipids can depend on coral symbiont species. We have analyzed and compared thylakoid lipidomes (mono- and digalactosyldiacylglycerols (MGDG and DGDG), sulfoquinovosyldiacylglycerols (SQDG), and phosphatidylglycerols (PG)) of crude extracts from symbiotic reef-building coral <i>Acropora</i> sp., the hydrocoral <i>Millepora platyphylla</i>, and the octocoral <i>Sinularia flexibilis</i>. <i>S. flexibilis</i> crude extracts were characterized by a very high SQDG/PG ratio, a DGDG/MGDG ratio < 1, a lower degree of galactolipid unsaturation, a higher content of SQDG with polyunsaturated fatty acids, and a thinner thylakoid membrane which may be explained by the presence of thermosensitive dinoflagellates <i>Cladocopium</i> C3. In contrast, crude extracts of <i>M. platyphylla</i> and <i>Acropora</i> sp. exhibited the lipidome features of thermotolerant Symbiodiniaceae. <i>M. platyphylla</i> and <i>Acropora</i> sp. colonies contained <i>Cladocopium</i> C3u and <i>Cladocopium</i> C71/C71a symbionts, respectively, and their lipidome profiles showed features that indicate thermotolerance. We suggest that an association with symbionts that exhibit the thermotolerant thylakoid lipidome features, combined with a high Symbiodiniaceae diversity, may facilitate further acclimatization/adaptation of <i>M. platyphylla</i> and <i>Acropora</i> sp. holobionts in the South China Sea.
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