Effect of experimental thermal stress on lipidomes of the soft coral Sinularia sp. and its symbiotic dinoflagellates

A complete or partial loss of symbiotic dinoflagellates (SD) by the corals in response to thermal stress has been described as coral bleaching. A stress damages the photosynthetic apparatus of SD and their thylakoid membranes, and symbiotic alga cells disappear via exocytosis and/or apoptosis resulting in a disturbance of host cell membranes. Lipids play an essential role in coral metabolism and constitute the basis of membranes. At present, studies of coral bleaching typically deal with long-term lipid indices, such as total lipids and fatty acids (FAs), while short-term dynamics of lipidome (full range of all lipid molecular species) has not been considered in the context of bleaching mechanism. To evaluate the lipidomic approach to characterizing coral bleaching, the soft coral Sinularia sp. was exposed to a short-term experimental thermal stress (33 °C). The dynamics of chlorophyll level, the composition of total FAs, the amounts of total lipids and major lipid classes (such as wax esters (WE), triacylglycerols (TAG), and monoalkyldiacylglycerols (MADAG), glycolipids, ethanolamine-, choline-, serine-, inositolglycerophospholipids (PE, PC, PS, PI), and ceramide aminoethylphosphonate (CAEP)) and the variations in the content of lipid molecular species were studied during a 36-h heat exposure. >160 molecular species were identified by tandem high-resolution mass spectrometry. Under the short-term stress, a considerable decline in the levels of chlorophylls, glycolipids, and FA markers of SD was detected, whereas the variations in the content of total lipids, total phospholipids, and major FAs were not significant. The heat exposure immediately disturbed the daily cycle of the composition of glycolipid molecular species. Among the storage lipids, the content of TAG and MADAG decreased, whereas the content of polyunsaturated WE molecular species increased under the stress. The following effects were recorded for the structural lipids: a depletion in the PE, PC, and PI amounts, a conversion of PE into their lyso and oxidized derivatives, an appearance of diacyl form of PE, and a disturbance of transfer of some polyunsaturated FAs from SD to the host. The content of PS was relatively stable. Probably, PE molecular species are a primary target of thermal stress in the Sinularia host. The most dramatical changes in the Sinularia lipidome were detected between 24 h and 36 h of the exposure. A lipidomic approach is important for studies of dynamics of the coral symbiotic association and a resistance of different coral taxa during short-term bleaching events.