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

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.

Fig. 4.

LSU rDNA-based evolution of the Symbiodinium species complex (SSC) and phylogenetic position of the zooxanthellae isolates analyzed in Figs. 1, 2, 3. Heat-sensitive and resilient phylotypes are shown in red and blue, respectively. Clades A–G are the seven recognized Symbiodinium phylogenetic groups (35), with A and B (shaded yellow) being typically considered as bleaching-resistant, shallow-water types, and C (shaded pink) as bleaching-sensitive, deeper-living types. Our analysis suggests that at least 13 clades can be recognized based on genetic distances (thick branches in the tree) and that thermal sensitivity is not clade-specific. The ultrametric, linearized tree shown here allowed us to apply a crude clock and calibrate the evolution of the SSC in time. The sea-surface temperature curve, based on tropical planktonic foraminifera δ¹⁸O, serves as an approximate time scale for SSC evolution. Note that two to three DNA substitutions in the LSU rDNA correspond to 1 million years of evolution; thus, speciation events in the last 500,000 years may not be detectable by using this genetic marker. Neighbor-joining (1,000 replicates) and Bayesian (1 million generations) statistical values are indicated on the main internal branches.

Fig. 1.

Effects of elevated temperatures on the structure of thylakoid membranes in zooxanthellae. Transmission electron micrographs of thin sections of Symbiodinium spp. isolated from Tridacna spp. [Provasoli–Guillard National Center for Culture of Marine Phytoplankton (CCMP) (West Boothbay Harbor, ME) no. 828] (A and B), the sea anemone Aiptasia sp. (CCMP no. 831) (C and D), the coral M. samarensis (E), and the coral S. pistillata (F). Samples were incubated at 26°C(A and C) and 32°C(B and D–F). All cultures were grown in F/2 medium (36) under a 12/12-h light/dark cycle. The corals were grown in a closed system supported by a biological filtration system under a 10/14-h light/dark cycle. Note the degradation of the thylakoid membranes within the plastids of the heat sensitive strains.

Fig. 2.

Maximum quantum yields of fluorescence (F_v/F_m, dimensionless) and electron-transfer rates (τ, μs) from the primary electron acceptor in PSII, Q_A, to the secondary quinone, Q_B, for all clones of zooxanthellae. Fluorescence parameters were derived from measurements with a custom-built fast repetition-rate fluorometer (14, 24). All cultures were grown in F/2 medium; cultures were incubated for up to 224 h (to verify resilience and nonreversibility of thermally damaged cultures) under a 10/14-h light/dark cycle at 26 and 32°C for each species tested. Maximum quantum yields of photochemistry (F_v/F_m) of the thermally tolerant clones averaged 0.57 ± 0.05 at 26°C and 0.55 ± 0.01 at 32°C; the corresponding electron-transfer rates (τ) were 318 ± 24 and 341 ± 9 μs. In heat-sensitive clones, the maximum quantum yields averaged 0.50 ± 0.07 at 26°C and 0.31 ± 0.03 at 32°C; the corresponding electron-transfer rates were 304 ± 54 and 200 ± 46 μs.

Fig. 3.

Ratios of Δ9-cis-octadecatetraenoic (18:1) acid to Δ6,9,12,15-cis-octadecatetraenoic acid (18:4) for seven clones of Symbiodinium spp. ANOVA of the log-transformed data indicates a statistically significant difference between heat-sensitive and heat-tolerant clones.