Heat-evolved microalgal symbionts increase coral bleaching tolerance

Abstract

Coral reefs worldwide are suffering mass mortalities from marine heat waves. With the aim of enhancing coral bleaching tolerance, we evolved 10 clonal strains of a common coral microalgal endosymbiont at elevated temperatures (31°C) for 4 years in the laboratory. All 10 heat-evolved strains had expanded their thermal tolerance in vitro following laboratory evolution. After reintroduction into coral host larvae, 3 of the 10 heat-evolved endosymbionts also increased the holobionts' bleaching tolerance. Although lower levels of secreted reactive oxygen species (ROS) accompanied thermal tolerance of the heat-evolved algae, reduced ROS secretion alone did not predict thermal tolerance in symbiosis. The more tolerant symbiosis exhibited additional higher constitutive expression of algal carbon fixation genes and coral heat tolerance genes. These findings demonstrate that coral stock with enhanced climate resilience can be developed through ex hospite laboratory evolution of their microalgal endosymbionts.

Fig. 1. In vitro thermal tolerance assessment of heat-evolved (SS) and wild-type (WT) algal symbiont strains at 31°C.

∆Value differences (end-beginning, 21 days) in (A) cell densities in culture and (B) maximum quantum yield of photosystem II. (C) ROS secreted into culture medium after 21 days, measured in fluorescence and normalized to cell numbers. Box colors represent (gray) heat-evolved strains and (blue) wild-type strains. Whiskers, maximum and minimum value; boxes, first and third quartile; line, median; n = 5 for each group; *Significant difference to the three heat-evolved strains SS1, SS7, and SS8 (at P_GLM < 1 × 10⁻⁶, generalized linear model, database S2).

Fig. 2. In hospite comparison of coral larvae bleaching tolerance between heat-evolved (SS) and wild-type (WT) algal symbiont strains at 31°C.

∆Values (end-beginning, 7 days) are displayed for (A) cell densities per larva and (B) maximum quantum yield of photosystem II (F_v/F_m). Box colors: red, heat-evolved strains that confer their thermal tolerance to the coral holobiont; gray, heat-evolved strains that do not confer thermal tolerance; blue, wild-type strains. Larvae biological replicates at beginning/end; cell densities: SS1, n = 5/4; SS2, n = 4/1; SS3, n = 6/6; SS4, n = 5/1; SS5, n = 5/5; SS6, n = 5/3; SS7, n = 5/3; SS8, n = 5/5; SS9, n = 5/5; SS10, n = 5/4; WT1, n = 6/6; WT2, n = 5/5. F_v/F_m: SS1, n = 4/4; SS2, n = 3/1; SS3, n = 5/6; SS4, n = 5/2; SS5, n = 4/5; SS6, n = 4/3; SS7, n = 5/3; SS8, n = 5/5; SS9, n = 4/4; SS10, n = 3/3; WT1, n = 3/6; WT2, n = 4/5. Because of mortality of SS2 and SS4 holobionts, we had only one biological replicate at the end of the heating period; strains were excluded from statistical analyses. Whiskers, maximum and minimum value; boxes, first and third quartile; line, median. *Significant difference of wild-type strains to the respective heat-adapted strains SS1, SS7, and SS8 (generalized linear model, database S2).

Fig. 3. Gene expression patterns of coral holobionts.

Multidimensional scaling plots illustrate the general transcriptomic responses of (A) the algal symbionts and (B) their respective coral hosts at rest temperatures of 27°C. *Significant differences at P < 0.001 between holobionts with heat-evolved and wild-type endosymbionts based on a permutational multivariate analysis of variance using distance matrices (adonis) with Tukey post hoc comparisons (database S3). Venn diagrams show the shared and unique DEGs for both algal symbiont (C) and host (D) in symbiosis compared to WT1. Red numbers indicate up-regulated genes, and blue numbers indicate down-regulated genes, both at <0.05 FDR, n = 6 per strain, SS8 n = 5, each replicate with 10 pooled larvae.

Fig. 4. Relative expression of genes involved in specific physiological functions.

Genes differentially expressed (FDR < 0.05) in holobionts containing laboratory-evolved symbionts versus WT1 symbionts. (A) Symbiont genes coding for parts of Calvin-Benson pathway (RuBisCO, phosphoglycolate phosphatase, fructose-bisphosphate aldolase). (B) Host genes coding for stress-related proteins (such as glutathione S-transferases, heat shock proteins, and superoxide dismutases).