Is Experimental Evolution the Key to Saving our Coral Reefs?
By: Seth Marshall
We usually talk about evolution as occurring over vast periods of time, but experimental evolution — that is, experimentally selecting for desired traits — could occur in as little as a few years! The work of Chan et al. (2023) Global Change Biology applies experimental evolution to coral reef restoration, and is a landmark study exploring the relationships between adult corals and their symbionts. What does this mean? How did they do it? Why does this matter? Continue reading to learn more.
Why care about coral?
Coral reefs are among the most productive biodiversity hotspots globally, hosting 25% of known marine species. Corals form mutualistic relationships with microalgae (known as zooxanthellae) which rely upon one another for food and protection. On top of this, they provide goods and ecosystem services to over 500 million people through food, employment, coastal protection, and more. Unfortunately, they are very vulnerable to the effects of anthropogenic climate change due to the delicate balance of their inner-workings.
Coral reefs are threatened by both marine heat waves and ocean acidification, both of which result in coral bleaching, characterized by corals appearing white and lifeless. During the process of bleaching, corals expel their symbionts (the zooxanthellae living in a coral), depriving corals of their food source. Coral bleaching can result in death, and is expected to increase as marine heat waves increase in frequency, duration, and intensity, and as ocean acidification shows no signs of stopping.
A New Approach to Reef Restoration
Faced with drastic coral declines in the past 2 to 3 decades, many teams have worked on restoring coral reefs. They have primarily focused their efforts on “coral gardening” (aka coral farming, coral aquaculture), which is the cultivation of adult coral fragments for future establishment in reefs. While this method is sufficient in getting more coral onto reefs, it does not help corals survive oceanic conditions. Chan et al. 2023 addresses this concern on a smaller scale.
The team out of the University of Melbourne sought to investigate the viability of introducing symbionts to corals that help them handle the rising ocean temperatures. A decade prior to the study, a generalist species of microalgal symbiont, Cladocopium proliferum, had undergone experimental evolution, selecting for heat tolerance. What resulted is the “SS8” strain of C. proliferum, which is particularly good at handling warmer conditions.
In order to test the effectiveness of this symbiont introduction (known as inoculation), we need a coral and a baseline measurement to compare to. The coral Galaxea fascicularis was chosen as the symbiont host. As for a baseline, the enhanced heat tolerance of SS8 is similar to that of the naturally occurring genus Durusdinium, providing a good comparison.
But wait, why use SS8 when Durusdinium already exists naturally? This is because the heat tolerance of Durusdinium comes at the price of reduced coral growth rate at ambient temperatures, among other factors. While it can survive higher temperatures than other symbionts, it is a less ideal candidate for reef restoration due to this drawback.
In addition to SS8 and Durusdinium, Chan and colleagues prepared 3 other inoculation treatments: a wild strain (not heat-evolved) of C. proliferum and two groups of control symbionts obtained from the Great Barrier Reef. The corals were chemically bleached with menthol and allowed time to recover before being inoculated. The successfully inoculated corals were later subjected to a heat stress test, and their physiological responses were recorded.
A Novel Symbiosis, and Future Implications
Following a series of analyses ranging from coral size, photochemical efficiency, and even metabolite profiling, the results were in: the SS8 strain of C. proliferum inoculated the coral G. fascicularis, resulting in increased heat tolerance. What is the significance of these findings?
One of the reasons G. fascicularis was chosen as the host coral for this study is because it is not the typical host of C. proliferum. The success of this inoculation marks the first time a coral has been inoculated with a heterologous symbiont, holding important implications for future coral reef restoration. While more research is needed, this implies that SS8 can be inoculated into multiple species, meaning we do not have to experimentally evolve strains on a coral-to-coral basis based upon their microbiome. Just as importantly, the SS8 inoculation conferred heat tolerance, which holds similarly important implications for the future of reef restoration.
Throughout the heat stress test, the SS8 inoculated corals outperformed or were on par with the other groups across all measurements. Notably, the SS8 corals exhibited similar heat tolerance to Durusdinium inoculated corals while not having a negative effect on ambient temperature coral growth, indicating progress in the right direction.
Furthermore, Chan and colleagues assessed the corals 2 years after the first inoculation and found the SS8-inoculated coral still thriving. This long-term stability suggests that a symbiont population could self-sustain, holding implications for the longevity of reef restoration. With our current understanding of these relationships, it is believed that the employment of heat-evolved symbiont strains like SS8 can buy corals time while humans work towards lowering carbon emissions.
References
Chan, W. Y., Meyers, L., Rudd, D., Topa, S. H., & van Oppen, M. J. H. (2023). Heat-evolved algal symbionts enhance bleaching tolerance of adult corals without trade-off against growth. Global Change Biology, 29, 6945–6968. https://doi.org/10.1111/gcb.16987
