Tackling A Coral Health Crisis: Investigating the Immunology Behind Bleaching

Tropical coral reefs are one of the most biodiverse, productive and economically important ecosystems on the planet. They are also one of the most vulnerable when it comes to the impacts of human-induced climate change. Rising ocean temperatures have resulted in the catastrophic loss of reef-building corals due to bleaching - breakdown of the vital relationship between corals and algae that they harbor within their cells. These algae provide crucial nutrients that corals need for growth and metabolism.

iStock 1069665230Bleached Acropora coral in shallow water, due to El Nino, Pacific Ocean, French Polynesia

During bleaching, algae leave the coral’s tissue, revealing its white skeleton beneath. Whilst not always fatal, the increasing frequency and severity of bleaching events means corals often do not have sufficient time or the right conditions to recover. Moreover, scientists have found that after bleaching corals are more susceptible to diseases. With global populations of reef-building corals having halved in the last thirty years, these animals are in the midst of their own global health crisis. Consequences of their death culminate in ecosystem collapse, with dire ramifications not only for marine biodiversity but also for those communities that rely on reefs for nutrition and income. The Intergovernmental Panel on Climate Change predicts ocean temperatures in all major coral reef provinces to increase until at least 2039. Thus, the survival of coral reefs over coming decades is uncertain.

Image 1 ecofeature coralDuring bleaching, corals turn white due to pigmented algae leaving their cells in response to environmental stress. (Photo by Will R from FreeImages)

In what now appears to be a race against time, conservation efforts have shifted from passive approaches to protect reef habitats to more active, interventionist strategies to save corals. These include artificially bolstering environmental resistance within coral populations. Efforts to generate stress-resistant ‘super corals’ are however complicated by our still rudimentary understanding of the bleaching process itself. At its heart, bleaching is a cellular process (algae leaving coral cells) with consequences that unfold on an ecosystem scale. Recently, scientists have tried to learn more about the molecular mechanisms involved in bleaching through studying the coral immune system, which has been implicated in mediating the coral-algae relationship.

The significance of immunity for coral biology and evolution is being increasingly recognized and researched. At the University of Oxford, a research group led by Professor Adrian Smith is beginning to investigate coral immunity and its influence on the response of corals to environmental perturbations. One of the projects leading researchers, Dr Bryan Wilson, says that new data about the coral immune system, “challenges the way we look at these ‘simple’ denizens of the worlds reefs.” He adds that corals are, “some of the oldest living creatures on Earth, and spend their long, sessile lives immersed in a microbial soup, subjected to frequent immunological challenges,” meaning their immune mechanisms have evolved to be much more flexible than was previously thought. Like our own immune systems, corals have evolved the capacity to distinguish beneficial microbes from harmful ones. This includes the recognition and maintenance of the algal partners that are lost during bleaching. Dr Wilson tells me that as part of ongoing research in Oxford, he is investigating how “ancient immunity in corals is bolstering their defenses against the environmental perturbations arising from accelerating climate change”. Such insights could be invaluable for future coral protection efforts. For instance, it could be that variation in bleaching level observed between coral populations and individuals has an immunological basis. This would enable assessment of environmental resilience of corals based on their immune profiles, which could guide prioritization of reef management schemes. Identifying particular components of immunity that make corals more or less susceptible to bleaching could even provide genetic targets for artificially enhancing resilience in corals.

Image 2 ecofeature coralAllyson DeMerlis, a member of the Cnidarian Immunity Lab, collecting corals from the Rescue the Reef Nursery off of Miami. (Photo taken by Nikki Besemer)

Also working hard to disentangle and understand coral immune mechanisms is the Cnidarian Immunity Lab at the University of Miami. The lab’s application of traditionally biomedical techniques to coral biology is revolutionizing the way corals are studied in laboratory settings. The lab’s principal investigator, Professor Nikki Traylor-Knowles, believes research into coral immunity will facilitate a better knowledge of basal coral health, which could eventually provide a much-needed metric to help key stakeholders measure reef health. She also hopes that understanding coral disease responses will shed light on possible treatment mechanisms and therapeutics for corals. However, we are, “very much at the tip of the iceberg in terms of learning about coral immunity,” she says. She also warns that although interventionist approaches to coral evolution are needed, we must not forget that, “a lot of the ways in which to save corals are policy issues,” including climate mitigation and local action to reduce anthropogenic pressures on reefs. Consequently, a multi-faceted strategy, combining the artificial bolstering of resistance in corals, along with intensified conventional management strategies is needed.

Image 3 ECOfeature coralFragments of Acropora cervicornis, a keystone species in the Caribbean being used in a laboratory experiment to test temperature tolerance in the Cnidarian Immunity Lab.

The emerging study of coral immune systems is both a worthwhile and fascinating research avenue. Not only will it shed light on these ancient creatures and how they survive, but it also offers the potential to expand the current reef management toolkit through elucidating the molecular basis of coral bleaching. With an understanding of this, we can better direct conservation efforts aiming to artificially boost environmental resistance in corals. Such interventions will be needed to ensure the continuation of coral reefs in the face of warming oceans. That said, technological solutions stemming from this knowledge must occur in tandem with changes in human behavior and cessation of ecologically destructive practices. It is only through a range of protection strategies that we can effectively address threats to coral reefs and work towards achieving a healthier, more resilient ocean.


Palmer, C.V. Immunity and the coral crisis. Commun Biol 1, 91 (2018). https://doi.org/10.1038/ s42003-018-0097-4

Anthony, K., Bay, L.K., Costanza, R. et al. New interventions are needed to save coral reefs. Nat Ecol Evol 1, 1420–1422 (2017). https://doi.org/10.1038/s41559-017-0313-5

Nikki Traylor-Knowles, Stephen R. Palumbi, Translational environmental biology: cell biology informing conservation, Trends in Cell Biology, Volume 24, Issue 5, 2014, Pages 265-267, ISSN 0962-8924, https://doi.org/10.1016/j.tcb.2014.03.001

Mansfield KM, Gilmore TD. Innate immunity and cnidarian-Symbiodiniaceae mutualism. Dev Comp Immunol. 2019 Jan; 90:199-209. doi: 10.1016/j.dci.2018.09.020. Epub 2018 Sep 27. PMID: 30268783.

This article is part of an online series dedicated to the UN Ocean Decade. One story will be published each week that is related to initiatives, new knowledge, partnerships, or innovative solutions that are relevant to the following seven Ocean Decade outcomes. Access the special digital issue dedicated to the Ocean Decade.

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