The image of a once-vibrant reef bleached white by thermal stress is a haunting reminder of the threats that corals face in the wake of global warming.
However, a discovery by our scientists offers hope: selective breeding can improve the ability of corals to survive intense marine heat waves. Read on to find out more.
Contents:
- Coral reefs and climate change
- What is coral bleaching?
- Selective breeding corals for heat tolerance
- The future of coral reefs under climate change
Coral reefs and climate change
Because of their sensitivity to high ocean temperatures, what happens to reef-building corals can serve as a key indicator of the impacts of global warming and the increasing frequency of heatwave events on natural ecosystems. Coral reefs undergo a physical change when under heat stress, turning white in a process known as 'bleaching'. If heat stress does not subside, coral bleaching often leads to mortality. Corals are the ecosystem engineers of tropical reefs, and so losing them will cause significant degradation of the broader ecosystem.
In the past coral reefs have recovered from stress conditions without human intervention. However, due to the increasing frequency and intensity of marine heatwaves, there might not be enough time in between events for natural recovery to occur. It is becoming apparent that natural adaptation will likely be too slow to keep up with ocean warming of 3 °C or higher. It is with this knowledge that Dr Adriana Humanes and Dr Liam Lachs - lead authors of the study and Postdoctoral Research Associates - explored the possibility of selectively breeding corals to improve the heat tolerance of their offspring.
Video: Jesse Alpert.
What is coral bleaching?
Corals gain their colour from microscopic algae that are held within their tissue. Coral and microalgae have a mutually beneficial - or symbiotic - relationship based on the sharing of shelter, nutrients, and carbon derived from microalgal photosynthesis. While occupying coral, the microalgae is known as an endosymbiont: an organism that lives within another in a symbiotic relationship.
When water temperatures rise, the symbiosis between corals and their endosymbionts breaks down and microalgae are lost from the coral tissues. This causes corals to appear ghostly white. Bleached corals are not dead, and it is possible for the coral to recover and take up new symbionts if heat stress subsides. However, bleached corals are considerably weakened and more likely to succumb to mortality. As many as 7,000 different species of marine life are supported within coral reefs, making mass coral bleaching mortality events a considerable problem that can devastate the ecosystem.
Selective breeding corals for heat tolerance
From high-yield crops to hybridised roses and pedigree dogs, humans have practiced selective breeding on plants and animals for thousands of years. Although historically used for aesthetic or agricultural purposes, selective breeding is now being considered as a tool for nature conservation, in particular with regard to climate change adaptation. For example, wheat has been selectively bred to endure drought, while selectively breeding spruce trees has increased their cold tolerance.
Selective breeding is also being considered to boost coral heat tolerance and thus hopefully improve the chances of survival in the face of climate change. Breeding trials were conducted by the international team of scientists using two heat exposures: a short but intense heat exposure (10 days, reaching +3.5°C), and a less intense but long-term exposure (1 month, reaching +2.5°C) that is more typical of natural marine heatwaves. The team found that selecting parent colonies for high rather than low heat tolerance increased the tolerance of adult offspring. For the first time, this study shows that a single generation of selective breeding can produce a significant change in the heat tolerance of adult offspring.
This study was the culmination of a five-year project, launched by the Principal Investigator Dr James Guest with funding from the European Research Council. Dr Guest, Reader in Coral Reef Ecology at Newcastle University’s School of Natural and Environmental Sciences, explains that 'the results show that selective breeding could be a viable tool to improve population resilience. Yet, there are still many challenges that need to be overcome. How many corals need to be outplanted to benefit wild populations? Can we ensure there are no trade-offs (evidence so far suggests this is not a large risk)? How can we avoid dilution of selected traits once added to the wild? How can we maximise responses to selection?'
The future of coral reefs under climate change
Although this study provides some welcome optimism regarding the feasibility of selective breeding to improve the chances of coral survival in the face of climate change, our scientists emphasise that these levels of enhancement achieved are only moderate. As with most studies, it comes with its own set of challenges to overcome before their findings can be implemented in wild populations.
More research and development are needed to understand how to operationalise breeding interventions and maximise outcomes, but the team is hopeful that this could help some small-scale patches of corals keep pace with lower levels of global warming. Notably, there are hundreds of species of corals on many Indo-Pacific coral reefs, so while selective breeding interventions for some of these species may be possible, it is unlikely to be feasible to protect the entire ecosystem from the threat of climate change.
According to Dr Humanes, 'considerable work remains before selective breeding can be successfully implemented. A deeper understanding is needed to determine which traits to prioritize and how these traits are genetically correlated.”'
Dr Lachs agrees with his fellow lead author, saying 'this work shows that selective breeding is feasible but not a silver bullet solution and that more research is needed to maximise breeding outcomes. In parallel, rapid reductions of global greenhouse gas emissions are an absolute requirement to mitigate warming and give corals an opportunity to adapt.'
Another study by the team revealed that coral heat tolerance adaptation via natural selection could keep pace with ocean warming, but only if Paris Agreement commitments are realised, limiting global warming to two degrees Celsius.
Despite the work that needs to be done, this study acts as an important proof of concept: when paired with climate action, selectively breeding corals for heat tolerance is possible.
4-5 year old coral offspring that are part of a heat tolerance selective breeding trial being reared in an ocean nursery. Photo: Gerard Ricardo.
You might also like:
- read the studies:
- Humanes, A., Lachs, L., Beauchamp, E. et al. Selective breeding enhances coral heat tolerance to marine heatwaves. Nat Commun 15, 8703 (2024). https://doi.org/10.1038/s41467-024-52895-1
- Liam Lachs et al., Natural selection could determine whether Acropora corals persist under expected climate change. Science 0, eadl6480 DOI: 10.1126/science.adl6480
- learn more about the researchers involved in this study:
- Dr Adriana Humanes, Postdoctoral Research Associate at Newcastle University’s Coralassist Lab
- Dr Liam Lachs, Postdoctoral Research Associate at Newcastle University’s Coralassist Lab
- Dr James Guest, ERC Research Fellow
- read the related news reports:
- learn from our blog, “How we’re working to preserve and protect our coral reefs”
- explore the research projects currently underway at our School of Natural and Environmental Sciences
- discover more about how we are working towards a brighter future through our One Planet research
Header photo: Stephen Bergacker