Membrane lipids of symbiotic algae are diagnostic of sensitivity to thermal bleaching in corals

TLDR

Massive bleaching events of zooxanthellate corals have been documented worldwide over the past three decades, yet the underlying physiological mechanism remains unknown despite its correlation with modest sea‑surface temperature rises and increased light intensity. The study demonstrates that thylakoid membrane lipid composition determines thermal‑stress sensitivity in symbiotic algae of cnidarians. The authors show that a fraction of photosynthetically produced oxygen is reduced by photosystem I via the Mehler reaction, generating reactive oxygen species that accumulate under high irradiance and trigger algal death and expulsion. Analyses reveal that lipid saturation establishes a critical temperature threshold separating thermally tolerant from sensitive zooxanthellae, making lipid composition diagnostic of bleaching patterns; fluorescence kinetics indicate damaged membranes remain capable of water splitting yet are energetically uncoupled, and phylogenetic evidence suggests symbiotic algae evolved reduced heat tolerance in the late Cenozoic.

Abstract

Over the past three decades, massive bleaching events of zooxanthellate corals have been documented across the range of global distribution. Although the phenomenon is correlated with relatively small increases in sea-surface temperature and enhanced light intensity, the underlying physiological mechanism remains unknown. In this article we demonstrate that thylakoid membrane lipid composition is a key determinate of thermal-stress sensitivity in symbiotic algae of cnidarians. Analyses of thylakoid membranes reveal that the critical threshold temperature separating thermally tolerant from sensitive species of zooxanthellae is determined by the saturation of the lipids. The lipid composition is potentially diagnostic of the differential nature of thermally induced bleaching found in scleractinian corals. Measurements of variable chlorophyll fluorescence kinetic transients indicate that thermally damaged membranes are energetically uncoupled but remain capable of splitting water. Consequently, a fraction of the photosynthetically produced oxygen is reduced by photosystem I through the Mehler reaction to form reactive oxygen species, which rapidly accumulate at high irradiance levels and trigger death and expulsion of the endosymbiotic algae. Differential sensitivity to thermal stress among the various species of Symbiodinium seems to be distributed across all clades. A clocked molecular phylogenetic analysis suggests that the evolutionary history of symbiotic algae in cnidarians selected for a reduced tolerance to elevated temperatures in the latter portion of the Cenozoic.