Using assisted evolution to help corals

We have all heard about it by now. Corals are bleaching, the planet is dying, and we are all screaming. There have been great efforts in coral conservation, but the planet is warming every day. We have already hit 1.5°C above pre-industrial levels, and according to IPCC estimates, 70-90% of all corals will die at this temperature. News just came out that the current global coral bleaching event has affected 80% of corals. It seems that our efforts are in vain, as corals are doomed either way.

Figure 1: Quigley, K.M. (2024) The impacts of marine heatwaves on coral reefs and the unknown acclimation and adaptation gap

Or maybe not! 

As it turns out, global warming is NOT the only factor in coral bleaching and damage. There is also tourism, blast fishing, overfishing, pollution, etc. These are rather commonplace problems you may have heard of, and there are many ways to reduce such stressors for corals.

There are also initiatives to help corals and their ecosystem survive even in a warmer climate. In this article, I will focus on one such method: assisted evolution to develop heat resistant corals. This could help corals that would otherwise die in the 1.5°C (or even 2.0°C, at the rate we’re going) warmer world.

Assisted evolution is not some hypothetical future: the Australian Institute of Marine Science (AIMS), a statutory body of the Australian government, began trials in 2023. This coincides with our increased ability to sequence and edit genomes*, which are of great concern for the threat they could pose, but also the good they could bring. A better understanding of assisted evolution helps us consider whether it is something we wish to continue pursuing.

*To be clear, assisted evolution does NOT involve gene editing, but they are similar in being direct human interference to change genomes. Gene editing can be used alongside assisted evolution, such as identifying helpful coral genes and editing them into other corals.

If you wish to learn more about the other stressors of coral reefs, you can watch science educator Hank Green’s recent interview with the Coral Reef Alliance’s executive director, Heather Stark. The video is a little long, but honestly even just watching the first 10 minutes has a lot of value.

Corals and coral bleaching

Stony corals are animals that make coral reefs. Yes, animals. Not plants. Photosynthesis only occurs because of the algae symbiont inside their cells. Sometimes they are referred to as Zooxanthellae, and most of them are from the genus Symbiodinium. (Symbiont just means they have a close, long term relationship, like you and your gut bacteria).

Figure 2: Godinot, C. (2016) Location of zooxanthellae within the coral host

Coral bleaching involves the loss of pigments from the symbiote, caused by stressors such as high heat, high light, and/or acidification. The kaleidoscope of colours in corals actually come from the coloured pigments in algae, essential for photosynthesis. The disappearance of the pigments exposes the coral’s true colour: white, the colour of calcium carbonate secreted by the coral.

Produced as a side product of photosynthesis, reactive oxygen species (ROS) are involved in this process. The oxidative stress theory states that environmental stress -> over production of ROS -> cell damage -> loss of algae. However, this theory is not confirmed.

Figure 3: Bieri, T., et al (2016). Four possible cellular mechanisms of cnidarian bleaching under stress

Heat resistance in corals

Corals and their algal symbionts do not die immediately upon facing heat. Just like humans, they can tolerate a variety of temperatures, and individuals can adapt to tolerate various temperatures through exposure (acclimatisation). They can also undergo adaptation over many generations.

Acclimatisation occurs over one individual’s lifespan (eg, if you move to Singapore you may become more tan), as opposed to adaptation which occurs over many generations (eg, selection pressure to have darker or lighter skin). When an observed trait can change over a lifetime, it is called phenotypic plasticity, just like how our brains change once we learn new things. 

Corals, like all other living things, also undergo adaptation. Adaptation comes about when natural selection acts on (inherited) variation. That means that there must be variation - in the form of genetic diversity - in the first place. This is why genetic diversity is a key concern - if there is too little genetic diversity, natural selection cannot do its thing and no adaptations come about. This is key in any conservation efforts.

Assisted evolution

Increasing heat tolerance is the clear goal. So, how do we achieve that goal?

In increasing order of ease of implementation;

• Selective propagation: choose the corals that are already heat-tolerant and use them in ongoing restoration.

• Assisted gene flow: similar to selective propagation, except the corals are from a further place. This speeds up the natural process where sexual reproduction causes genes to flow from one population to another. 

• Assisted evolution: similar to artificial selection of crops. Take the corals, breed them many times, and select the ones with the most heat-tolerance to be bred each time. Over time, this leads to increased frequency of alleles (specific variant of genes) that lead to higher heat tolerance, and the offspring are more heat-tolerant on average. This can be combined with acclimatisation of the corals prior to transplantation.

A recent study from the Coralassist lab has produced heat-tolerant corals via assisted evolution, showing that such a method has promise. This is made possible by the fact that heat-tolerance is inherited via genetics. (Humanes et al, 2024) See: the Yale E360 article by Sofia Quaglia, as well as the scientist’s own comments.

In the study mentioned above, “demonstrating a substantial genetic basis of heat tolerance”, “narrow-sense heritability (h²) estimates are between 0.2 and 0.3”. This is used to calculate the breeder’s equation, R= h²S, where R is the response to selection. It is commonly used in agriculture and means that we can actually calculate how much the trait of a population will change using selective breeding! It also means that there was only some improvement in heat tolerance, since the heritability index is not that high.

You may recall that the coral also has an algal symbiont. We can use similar methods on the algae to get more heat-tolerant corals, which in one study has been shown to work in adult corals without any trade-offs. (Chan, 2023)

There are issues associated with all of these. 

• Firstly, the heat-tolerant corals may not be heat-tolerant enough. 

• Secondly, there could be trade offs, where increasing heat tolerance could lead to a decrease in other beneficial traits. Already, it has been shown that heat tolerant corals tend to grow slower, and are less resistant to cold - an issue for winter. 

• Thirdly, being heat resistant does not mean being resistant to other stressors, such as light, acidity, and disease, which are also common problems faced by corals.

• Fourthly, these methods may lower genetic variation, making them less able to adapt, as natural selection requires genetic variation.

• Lastly, there could be various ways that combinations of alleles could lead to heat tolerance, and they can lead to different aspects or types of heat tolerance, complicating selection

We need to ensure that the corals we currently have and want to restore can withstand the heat. Since we cannot build AC units in the sea, heat resistant corals are therefore important in coral restoration and conservation, and it gives me (and hopefully you?) much hope that corals can indeed thrive in this warmer world.

Also, human manipulation of genetic material has been a hot topic for a while. For a more productive discussion, it is good to have a better understanding of various methods used (beyond only gene editing) and the ways they can be used.

Works Cited

Humanes, A., et al. (2024) ‘Selective breeding enhances coral heat tolerance to marine heatwaves’ Nature Communications, 15. doi: https://doi.org/10.1038/s41467-024-52895-1

Chan, W.Y., et al. (2023). ‘Heat‐evolved algal symbionts enhance bleaching tolerance of adult corals without trade‐off against growth’ Global Change Biology, 29 (24). doi:https://doi.org/10.1111/gcb.16987

Introduction, Conclusion, Linked in text

(image) Quigley, K.M. (2024) The impacts of marine heatwaves on coral reefs and the unknown acclimation and adaptation gap., (2024) ‘Breeding and Selecting Corals Resilient to Global Warming’‌ Annual Review Animal Biosciences, 12. doi:https://doi,org/: 10.1146/annurev-animal-021122-093315

Hoegh-Guldberg, O., et al. (2018) ‘Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty’ Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 175-312. doi: https://doi.org/10.1017/9781009157940.005

Readfearn, G. (2025). ‘More than 80% of the world’s reefs hit by bleaching after worst global event on record’, The Guardian, 23 April 2025. Available at: https://www.theguardian.com/environment/2025/apr/23/coral-reef-bleaching-worst-global-event-on-record. (Accessed: 23 April 2025)

Australian Institute of Marine Science (2022). Breeding temperature tolerant corals for reef restoration and adaptation. Available at: https://www.aims.gov.au/research-topics/featured-projects/reef-spawning-research-aims/breeding-temperature-tolerant-corals-reef-restoration-and-adaptation. (Accessed: 10 April 2025)

Kolbert, E. (2021). ‘Assisting Evolution: How Far Should We Go to Help SpeciesAdapt?’’, Yale E360, 9 February 2021. Available at: https://e360.yale.edu/features/assisting-evolution-how-far-should-we-go-to-help-species-adapt (Accessed: 13 April 2025)

Quaglia, S. (2024). ‘As Ocean Waters Warm, a Race to Breed Heat-Resistant Coral’, Yale E360, 14 November 2024. Available at: https://e360.yale.edu/features/selective-breeding-coral-climate-change (Accessed: 10 April 2025)

Liam Lachs, Humanes, A. and Guest, J. (2024). ‘We’ve bred corals to better tolerate lethal heatwaves, but rapid climate action is still needed to save reefs’, Down To Earth, 15 October 2024. Available at: https://www.downtoearth.org.in/climate-change/weve-bred-corals-to-better-tolerate-lethal-heatwaves-but-rapid-climate-action-is-still-needed-to-save-reefs (Accessed: 24 April 2025)

Green, H. (2025) Coral Reefs Are Not Doomed. 23 April 2025. Available at:https://www.youtube.com/watch?v=hxZDyV-E5WY&t=1s (Accessed: 23 April 2025)

Corals and coral bleaching

(image) Godinot, C. (2016) Location of zooxanthellae within the coral host, in Godinot, C., et al. (2016) ‘On the use of 31P NMR for the quantification of hydrosoluble phosphorus-containing compounds in coral host tissues and cultured zooxanthellae’ Scientific Reports, 6, 21760 (2016). https://doi.org/10.1038/srep21760

(image) (2016) Four possible cellular mechanisms of cnidarian bleaching under stress, in Bieri, T., et al. (2016). ‘Relative Contributions of Various Cellular Mechanisms to Loss of Algae during Cnidarian Bleaching’ PLOS ONE, 11(4), p.e0152693. doi:https://doi.org/10.1371/journal.pone.0152693

Douglas, A.E. (2003). ‘Coral bleaching––how and why?’ Marine Pollution Bulletin, 46(4), pp.385–392. doi:https://doi.org/10.1016/s0025-326x(03)00037-7.

Bieri, T., et al. (2016). ‘Relative Contributions of Various Cellular Mechanisms to Loss of Algae during Cnidarian Bleaching’ PLOS ONE, 11(4), p.e0152693. doi:https://doi.org/10.1371/journal.pone.0152693

Downs, C.A., et al. (2002). ‘Oxidative stress and seasonal coral bleaching’ Free Radical Biology and Medicine, 33(4), pp.533–543. doi:https://doi.org/10.1016/s0891-5849(02)00907-3

‌

Doering, T., et al. (2023). ‘Comparing the Role of ROS and RNS in the Thermal Stress Response of Two Cnidarian Models, Exaiptasia diaphana and Galaxea fascicularis’ Antioxidants, 12(5), p.1057. doi:https://doi.org/10.3390/antiox12051057

‌

(evidence against the oxidative stress theory, this does not necessarily mean that the theory is false and these are more of outliers.)

Nielsen, D.A., Petrou, K. and Gates, R.D. (2018). ‘Coral bleaching from a single cell perspective’ The ISME Journal, 2(6), pp.1558–1567. doi:https://doi.org/10.1038/s41396-018-0080-6

Dungan, A.M., et al. (2022). ‘Lack of evidence for the oxidative stress theory of bleaching in the sea anemone, Exaiptasia diaphana, under elevated temperature’ Coral Reefs. Volume 41, pp.1161–1172, doi:https://doi.org/10.1007/s00338-022-02251-w

Heat resistance in corals

Schoepf, V., et al. (2015). ‘Limits to the thermal tolerance of corals adapted to a highly fluctuating, naturally extreme temperature environment’ Scientific Reports, 5(1). doi:https://doi.org/10.1038/srep17639

Palumbi, S.R., et al. (2014). ‘Mechanisms of reef coral resistance to future climate change’ Science, 344, pp.895–898. doi:https://doi.org/10.1126/science.1251336

‌Berkelmans, R. and van Oppen, M.J.H. (2006). ‘The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change’ Proceedings of the Royal Society B: Biological Sciences, 273(1599), pp.2305–2312. doi:https://doi.org/10.1098/rspb.2006.3567

Assisted evolution

Caruso, C., Hughes, K. and Drury, C. (2021). ‘Selecting Heat-Tolerant Corals for Proactive Reef Restoration’ Frontiers in Marine Science, 8. doi:https://doi.org/10.3389/fmars.2021.632027

Quigley, K.M. (2024) ‘Breeding and Selecting Corals Resilient to Global Warming’‌ Annual Review Animal Biosciences, 12. doi:https://doi,org/: 10.1146/annurev-animal-021122-093315

Humanes, A., et al. (2024) ‘Selective breeding enhances coral heat tolerance to marine heatwaves’ Nature Communications, 15. doi: https://doi.org/10.1038/s41467-024-52895-1

Buerger, P., et al. (2020). ‘Heat-evolved microalgal symbionts increase coral bleaching tolerance’ Science Advances, 6(20). doi:https://doi.org/10.1126/sciadv.aba2498

‌

Chan, W.Y., et al.(2023). ‘Heat‐evolved algal symbionts enhance bleaching tolerance of adult corals without trade‐off against growth’ Global Change Biology 29 (24). doi:https://doi.org/10.1111/gcb.16987

Special thanks to

University of Sheffield (no date) Harvard referencing. Available at: https://librarydevelopment.group.shef.ac.uk/referencing/harvard.html (Accessed: 25 April 2025)