Heat‐evolved algal symbionts enhance bleaching tolerance of adult corals without trade‐off against growth

Abstract

Ocean warming has caused coral mass bleaching and mortality worldwide and the persistence of symbiotic reef‐building corals requires rapid acclimation or adaptation. Experimental evolution of the coral's microalgal symbionts followed by their introduction into coral is one potential method to enhance coral thermotolerance. Heat‐evolved microalgal symbionts of the generalist species, Cladocopium proliferum (strain SS8), were exposed to elevated temperature (31°C) for ~10 years, and were introduced into four genotypes of chemically bleached adult fragments of the scleractinian coral, Galaxea fascicularis. Two of the four coral genotypes acquired SS8. The new symbionts persisted for the 5 months of the experiment and enhanced adult coral thermotolerance, compared with corals that were inoculated with the wild‐type C. proliferum strain. Thermotolerance of SS8‐corals was similar to that of coral fragments from the same colony hosting the homologous symbiont, Durusdinium sp., which is naturally heat tolerant. However, SS8‐coral fragments exhibited faster growth and recovered cell density and photochemical efficiency more quickly following chemical bleaching and inoculation under ambient temperature relative to Durusdinium‐corals. Mass spectrometry imaging suggests that algal pigments involved in photobiology and oxidative stress were the greatest contributors to the thermotolerance differences between coral hosting heat‐evolved versus wild‐type C. proliferum. These pigments may have increased photoprotection in the heat‐evolved symbionts. This is the first laboratory study to show that thermotolerance of adult corals (G. fascicularis) can be enhanced via the uptake of exogenously supplied, heat‐evolved symbionts, without a trade‐off against growth under ambient temperature. Importantly, heat‐evolved C. proliferum remained in the corals in moderate abundance 2 years after first inoculation, suggesting long‐term stability of this novel symbiosis and potential long‐term benefits to coral thermotolerance.

Assisted evolution of algal symbionts to enhance coral reef bleaching tolerance: A success story

“…In contrast to previous studies reporting failure in Symbiodiniaceae inoculation, Chan et al. (2023) successfully established a persistent symbiosis between adult corals and heat‐evolved Symbiodiniaceae .…”

“…The significance of heat‐evolved C. proliferum was further emphasized by Chan et al. (2023) demonstrating that heat‐evolved C. proliferum corals perform better than wild‐type C. proliferum corals under thermal stress. This highlights the crucial role of heat‐evolved symbionts in helping coral to withstand heatwave events.…”

“…Chan et al. (2023) demonstrate the success of heat‐evolved symbionts to enhance thermal tolerance in adult corals. In contrast to previous studies reporting failure in Symbiodiniaceae inoculation, Chan et al.…”

“…To investigate the effect of SS8 on adult corals, Chan et al. (2023) selected Galaxea fascicularis , a coral species naturally hosting both Cladocopium and Durusdinium symbionts. Two genotypes of chemically bleached G. fascicularis successfully acquired a 10‐year heat‐evolved strain of the generalist species C. proliferum (referred to SS8‐corals), which persisted for the 5 months of the experiment.…”

“…Chan et al. (2023) is the first study to demonstrate that heat‐evolved symbionts can associate with adult coral and persist long‐term while enhancing coral thermotolerance without a trade‐off against growth. This study paves the way for utilizing heat‐resistant coral associated with heat‐evolved Symbiodiniaceae in reef restoration projects.…”