Fatty Acid Biomarkers of Symbionts and Unusual Inhibition of Tetracosapolyenoic Acid Biosynthesis in Corals (Octocorallia)

Seven zooxanthellae‐free species of octocorals (the genera Acanthogorgia , Acabaria , Chironephthya , Echinogorgia , Menella , Ellisella , and Bebryce ) and two zooxanthellate octocorals (the genera Paralemnalia and Rumphella ) were examined to elucidate their fatty acid (FA) composition. Arachidonic (about 40% of the total FA) and palmitic acids were predominant in all the species studied. Seven furan FA (F‐acids) (up to 9.7%) were identified in the azooxanthellate octocorals. The main F‐acids were 14,17‐epoxy‐15‐methyldocosa‐14,16‐dienoic and 14,17‐epoxy‐15,16‐dimethyldocosa‐14,16‐dienoic acids. In all specimens of Bebryce studeri , C 25–28 demospongic FA (about 20%) were identified. These FA reflect the presence of a symbiotic sponge in B. studeri and can be used as the specific markers for other corals. A significant difference ( P < 0.01) between azooxanthellate and zooxanthellate corals was found for odd‐chain and methyl‐branched saturated FA, 18:1n‐7, and 7‐Me‐16:1n‐10; that indicated the presence of an advanced bacterial community in azooxanthellate corals. The zooxanthellate species were distinguished by significant amounts of 18:3n‐6, 18:4n‐3, and 16:2n‐7 acids, which are proposed as the markers of zooxanthellae in soft corals. Contrary to the normal level of 24:5n‐6 (9.4%) and 22:4n‐6 (0.6%), unexpected low concentrations of 24:5n‐6 (0.4%) accompanied by a high content of 22:4n‐6 (up to 11.9%) were detected in some specimens. The presence of an unknown factor in octocorals, specific for n‐6 PUFA, which inhibited elongation of 22:4n‐6 to 24:4n‐6, is conjectured.