Laboratory-Induced Evolution of Temperature Tolerance in Coral Symbionts

Written by Tim Bateman

Upon first entering grad school, a lab mate mentioned that some miraculous evolution of thermal tolerance would not reverse the downward trajectory of coral reefs because corals aren’t waiting for us to measure them before they decide to adapt. He said that if they were going to evolve, then they would have done so. This seemed like pretty sound logic at the time. But then, I was introduced to the concept of laboratory-induced adaptation. This concept asks the question, ‘If corals cannot adapt on their own fast enough, can we help them out?’ Specifically, several recent studies have investigated how increased selective pressure affects the evolution of thermal tolerance in coral’s algal symbionts and how well these adapted symbionts perform in symbiosis with corals. There are many questions to answer before this can become a serious consideration for conservation, including questions related to the timescale, limits, and efficacy of adapting thermal tolerance in symbiotic partners, but there is certainly potential for this to become an avenue for conservation.

The laboratory of Dr. van Oppen at the Australian Institute of Marine Science has been leading the charge to investigate the induced evolution of coral symbionts. They have conducted numerous studies that utilized a ratchet design (Figure 1) to evolve algal symbionts to elevated temperature and then measured different sets of parameters to assess the extent and mechanisms of thermal tolerance in the algae as well as infecting them into coral juveniles to see if these symbionts allow their coral host to tolerate higher temperatures. 

To induce adaptation artificially quickly, researchers used an experimental ratchet design to increase selective pressure on symbionts (Figure 1). This design exposes populations to increasingly elevated temperatures in steps paired with measurements of growth at each temperature step. Cultures maintain large population sizes and are only moved up to the next temperature if they have a positive growth rate. This increase the chance that beneficial mutations will enter the gene pool. Once they have reached their highest temperature, cultures are maintained for long-term adaptation over multiple generations to allow beneficial mutations to become fixed in the population.

Figure 1: Sample ratchet design schematic indicating temperature steps and maintenance of large population sizes to increase the chance of beneficial mutations entering the gene pool (left). Water baths with heaters containing culture test tubes used for individual temperature steps in a ratchet experiment (right).

One critical aspect of these investigations revolves around the amount of time required for these beneficial adaptations to become fixed in the populations, as this dictates how long cultures must be maintained before there is a significant increase in thermal tolerance. Algal symbionts have shown evidence of increased thermal tolerance after as few as 40 generations, which took approximately one year. Crucially, however, when infected into coral juveniles, these same evolved symbionts did not increase the overall thermal tolerance of the coral despite displaying increased thermal tolerance in culture.

Figure 2: Successful infection of a coral recruit with lab-evolved symbionts. (a) Raw image of a coral recruit, (b) same recruit image with the region of interest (ROI) selected (recruit base area) and the area pigmented highlighted after applying red, green blue (RGB) thresholds. From Chakravarti et al. 2017.

While inducing the evolution of increased tolerance in culture as fast as possible is important to produce these symbionts quickly, the ultimate measure of success is increased tolerance of corals. Successful infection of these symbionts into coral juveniles is a good first step (Figure 2) but additional studies must assess the success of the symbiotic relationship. Current coral symbioses have evolved for millions of years and so creating new corals will likely come with some tradeoffs potentially resulting in less beneficial relationships. To investigate how to improve this, another study investigated different mechanisms that support thermal tolerance in culture and in symbiosis in an attempt to begin to pinpoint which biological processes are important for symbionts to confer their evolved thermal tolerance to their coral hosts once in symbiosis.

Directly investigating all possible biological processes important for symbiosis and thermal tolerance would be a monumental task. Rather than direct measurements, researchers measured gene expression of evolved and non-evolved (wild-type) symbionts because gene expression can be used to infer the activity of particular biological processes in an organism and the environmental stress the organism is experiencing. Unsurprisingly, evolved symbionts had stable gene expression at higher temperatures while wild-type symbionts had unstable expression and increased expression of genes related to DNA repair, protein repair, and the universal stress response. All of these point to extreme stress in the wild-type symbionts and outline specific biological processes involved in thermal tolerance. A similar study also investigated gene expression in symbionts evolved for a much longer time, approximately four years, and correlated this with thermal tolerance of coral juveniles infected with these evolved symbionts. Not only did the corals have increased thermal tolerance when infected with these newly evolved symbionts, but researchers were also able to narrow down some critical processes that were evident in the gene expression data. Photosynthesis and host heat stress response genes were the best predictors of thermal tolerance, which stands to reason considering that photosynthesis provides the energy corals require to survive at any temperature, and heat stress response genes allow corals to continue essential biological functions at elevated temperature.

These studies are investigating a potential avenue for conservation that can rapidly increase thermal tolerance in populations. There are, however, still many questions to be answered before this idea can become feasible at scale. Fitness tradeoffs associated with tolerant symbionts could hamper the growth and survival of corals hosting evolved symbionts. Further direct investigation of tolerance and symbiosis mechanisms could provide support for this. Additionally, the survival and efficacy of these evolved symbionts have only be tested in laboratory settings and must still be tested in the field. Finally, considerations of invasive species and ethics could complicate these evolved symbionts’ deployment as a conservation mechanism. Overall, laboratory-induced evolution of thermal tolerance in coral symbionts has the potential to provide biological insight and a conservation pathway for corals in the future, but much work is still required before this becomes a reality.

Buerger, P., Alvarez-Roa, C., Coppin, C. W., Pearce, S. L., Chakravarti, L. J., Oakeshott, J. G., Edwards, R. and van Oppen, M. J. H. (2020). Heat-evolved microalgal symbionts increase coral bleaching tolerance. Science Advances 6, 8.

Chakravarti, L. J., Beltran, V. H. and van Oppen, M. J. H. (2017). Rapid thermal adaptation in photosymbionts of reef-building corals. Global Change Biology 23, 4675-4688.

Chakravarti, L. J. and van Oppen, M. J. H. (2018). Experimental Evolution in Coral Photosymbionts as a Tool to Increase Thermal Tolerance. Frontiers in Marine Science 5.

Chakravarti, L. J., Buerger, P., Levin, R. A. and van Oppen, M. J. H. (2020). Gene regulation underpinning increased thermal tolerance in a laboratory-evolved coral photosymbiont. Molecular Ecology 29, 1684-1703.

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