Climate Change and Coral Reefs: Understanding the Past to Act in the Present and Prepare for the Future

The Importance of Coral Reefs

In order to keep the world’s oceans healthy and functioning, healthy coral reef ecosystems are essential. Typically when one pictures a coral reef, a colourful array of stony (Scleractinian) corals comes to mind. These reefs are formed by colonies of microscopic coral individuals, called polyps, that form a characteristic hard skeleton to protect themselves (Hoegh-Guldberg et al., 2017). The bright colours associated with coral reefs are due to microscopic organisms, dinoflagellates, that live protected inside the coral skeleton, providing nutrients to the coral and allowing it to grow (Hoegh-Guldberg et al., 2017). Coral reefs are vital to humans; they provide humanity with food security, economic livelihood, and protection from coastal erosion (Eddy et al., 2021). As the current state of climate change and global emissions predict that most warm-water coral reefs will be pushed to extinction over the next 30 years, due to rising ocean temperatures and acidity, current action combatting global warming is necessary (Hoegh-Guldberg et al., 2017).

Reef Paleoclimatology

Certain long-term responses of past coral reefs to climate change reemerge frequently and can serve as an indicator of future events. The Paleocene-Eocene thermal maximum was a high temperature event that lasted through the first two epochs of the Palaeogene period, approximately 65 million years ago, and was associated with a massive carbon release to the atmosphere (McInerney and Wing, 2011; Westerhold et al., 2008). Changing oceanic conditions led to the diverse evolution of specific marine molluscs that could build small reef structures, which improved survival (Kauffman and Johnson., 1988). Fossil records suggest that migration, adaptation, and extinction were the responses of coral reefs to climatic stressors (Pandolfi and Kiessling, 2014). Living coral reefs expanded their geological ranges, migrating to cooler waters with reduced acidification. Coral reefs also developed relationships with algal symbionts, to appropriately suit environmental conditions (Webster and Reusch, 2017). Calcified marine animals with more basic exoskeletons were more resistant to increased CO₂, as the alkaline structures counteracted the increasingly acidic ocean (Taylor et al., 2015).

Today’s Coral Reefs

Coral reefs have been among the most susceptible ecosystems to the Earth’s rapidly changing climate. Increasing ocean temperatures, and acidity have adversely affected coral health, resulting in a decrease of warm-water reef coverage by over 50% in the past 70 years (Eddy et al., 2021). Increased water temperatures decrease the resilience of coral reefs, and their overall population, due to expulsion of symbiotic dinoflagellate algae (Mumby and van Woesik, 2014). This is referred to as coral bleaching, the greatest present threat to global coral populations (Souter et al., 2020) (Figure 1). Coral reefs have been among the most susceptible ecosystems to the Earth’s rapidly changing climate. Increasing ocean temperatures, and acidity have adversely affected coral health, resulting in a decrease of warm-water reef coverage by over 50% in the past 70 years (Eddy et al., 2021). Increased water temperatures decrease the resilience of coral reefs, and their overall population, due to expulsion of symbiotic dinoflagellate algae (Mumby and van Woesik, 2014). This is referred to as coral bleaching, the greatest present threat to global coral populations (Souter et al., 2020) (Figure 1).

Figure 1: (A) The amount of hard coral cover globally from 1978-2019 (solid orange line). The darker and lighter shades of orange correspond to an 80% and 95% confidence interval, respectively (Souter et al., 2020). (B) The amount of algal cover on global corals between 1986-2019 (solid blue line). The darker region of blue corresponds to an 80% confidence interval, and the outermost light blue region represents a 95% confidence interval (Souter et al., 2020). (C) Scleractinian corals in the American Samoa before (left), during (middle), and after (right) bleaching events (Credit: Ocean Agency / Ocean Image Bank). </p> The negative effects on coral reef organisms then go on to impact entire ecosystems, with changing climatic conditions altering the overall community structure of reef environments (Araújo-Silva, Sarmento and Santos, 2022). This makes it increasingly difficult for reef ecosystems to respond to future disturbances, and recover from the negative impacts of climate change (Hoegh-Guldberg et al., 2017). ### Emergent Technologies and Future Reef Research Evolution, relocation, and acclimatization are the three main suggested ways for coral reefs to cope with the changing temperature and pH of the ocean, and various methodologies are emerging to support corals in those ways (Hoegh-Guldberg et al., 2017). Although there has been evidence to show corals can remember and react accordingly to past stress, this ability could use further research to develop ways to utilise it effectively (Hackerott, Martell and Eirin-Lopez, 2021). Recently, cryopreserved sperm of one Acropora palmata coral population was used to fertilize a different coral population, which resulted in promising offspring survival (Hagedorn et al., 2021). Last but not least, CRISPR-Cas9, a gene editing technology, was used with single-guide RNA to induce mutations in different Acropora millepora genes with encouraging success (Cleves et al., 2018). ### References Abelson, A., 2020. Are we sacrificing the future of coral reefs on the altar of the “climate change” narrative? ICES Journal of Marine Science, 77(1), pp.40–45. https://doi.org/10.1093/icesjms/fsz226. Allemand, D. and Osborn, D., 2019. Ocean acidification impacts on coral reefs: From sciences to solutions. Regional Studies in Marine Science, 28, p.100558. https://doi.org/10.1016/j.rsma.2019.100558. Anthony, K., Bay, L.K., Costanza, R., Firn, J., Gunn, J., Harrison, P., Heyward, A., Lundgren, P., Mead, D., Moore, T., Mumby, P.J., van Oppen, M.J.H., Robertson, J., Runge, M.C., Suggett, D.J., Schaffelke, B., Wachenfeld, D. and Walshe, T., 2017. New interventions are needed to save coral reefs. Nature Ecology & Evolution, 1(10), pp.1420–1422. https://doi.org/10.1038/s41559-017-0313-5. Araújo-Silva, C.L., Sarmento, V.C. and Santos, P.J.P., 2022. Climate change scenarios of increased CO2 and temperature affect a coral reef peracarid (Crustacea) community. Marine Environmental Research, 173, p.105518. https://doi.org/10.1016/j.marenvres.2021.105518. Berner, R.A. & Beerling, D.J., 2007. Volcanic degassing necessary to produce a CACO3 undersaturated ocean at the triassic–jurassic boundary. Palaeogeography, Palaeoclimatology, Palaeoecology, 244(1-4), pp.368–373. Bostock, H.C., Tracey, D.M., Currie, K.I., Dunbar, G.B., Handler, M.R., Mikaloff Fletcher, S.E., Smith, A.M. and Williams, M.J.M., 2015. The carbonate mineralogy and distribution of habitat-forming deep-sea corals in the southwest pacific region. Deep Sea Research Part I: Oceanographic Research Papers, 100, pp.88–104. https://doi.org/10.1016/j.dsr.2015.02.008. Cleves, P.A., Strader, M.E., Bay, L.K., Pringle, J.R. and Matz, M.V., 2018. CRISPR/Cas9-mediated genome editing in a reef-building coral. Proceedings of the National Academy of Sciences, 115(20), pp.5235–5240. https://doi.org/10.1073/pnas.1722151115. Eddy, T.D., Lam, V.W.Y., Reygondeau, G., Cisneros-Montemayor, A.M., Greer, K., Palomares, M.L.D., Bruno, J.F., Ota, Y. and Cheung, W.W.L., 2021. Global decline in capacity of coral reefs to provide ecosystem services. One Earth, 4(9), pp.1278–1285. https://doi.org/10.1016/j.oneear.2021.08.016. Figuerola, B., Hancock, A.M., Bax, N., Cummings, V.J., Downey, R., Griffiths, H.J., Smith, J. and Stark, J.S., 2021. A Review and Meta-Analysis of Potential Impacts of Ocean Acidification on Marine Calcifiers From the Southern Ocean. Frontiers in Marine Science, 8, p.24. https://doi.org/10.3389/fmars.2021.584445. García Molinos, J., Halpern, B.S., Schoeman, D.S., Brown, C.J., Kiessling, W., Moore, P.J., Pandolfi, J.M., Poloczanska, E.S., Richardson, A.J. and Burrows, M.T., 2016. Climate velocity and the future global redistribution of marine biodiversity. Nature Climate Change, 6(1), pp.83–88. https://doi.org/10.1038/nclimate2769. Goldberg, W.M., 2013. Biology of reefs and reef organisms, Chicago, Illinois: The University Of Chicago Pres. Gooday, A.J., 2001. Benthic foraminifera. Encyclopedia of Ocean Sciences, pp.274–286. Hackerott, S., Martell, H.A. and Eirin-Lopez, J.M., 2021. Coral environmental memory: causes, mechanisms, and consequences for future reefs. Trends in Ecology & Evolution, 36(11), pp.1011–1023. https://doi.org/10.1016/j.tree.2021.06.014. Hafezi, M., Stewart, R.A., Sahin, O., Giffin, A.L. and Mackey, B., 2021. Evaluating coral reef ecosystem services outcomes from climate change adaptation strategies using integrative system dynamics. Journal of Environmental Management, 285, p.112082. https://doi.org/10.1016/j.jenvman.2021.112082. Hagedorn, M., Page, C.A., O’Neil, K.L., Flores, D.M., Tichy, L., Conn, T., Chamberland, V.F., Lager, C., Zuchowicz, N., Lohr, K., Blackburn, H., Vardi, T., Moore, J., Moore, T., Baums, I.B., Vermeij, M.J.A. and Marhaver, K.L., 2021. Assisted gene flow using cryopreserved sperm in critically endangered coral. Proceedings of the National Academy of Sciences, [online] 118(38). https://doi.org/10.1073/pnas.2110559118. Hennige, S.J., Wicks, L.C., Kamenos, N.A., Perna, G., Findlay, H.S. and Roberts, J.M., 2015. Hidden impacts of ocean acidification to live and dead coral framework. Proceedings of the Royal Society B: Biological Sciences, 282(1813), p.20150990. https://doi.org/10.1098/rspb.2015.0990. Hoegh-Guldberg, O., Pendleton, L. and Kaup, A., 2019. People and the changing nature of coral reefs. Regional Studies in Marine Science, 30, p.100699. https://doi.org/10.1016/j.rsma.2019.100699. Hoegh-Guldberg, O., Poloczanska, E.S., Skirving, W. and Dove, S., 2017. Coral Reef Ecosystems under Climate Change and Ocean Acidification. Frontiers in Marine Science, 4, p.158. https://doi.org/10.3389/fmars.2017.00158. Hoegh-Guldberg, O., Kennedy, E.V., Beyer, H.L., McClennen, C. and Possingham, H.P., 2018. Securing a Long-term Future for Coral Reefs. Trends in Ecology & Evolution, 33(12), pp.936–944. https://doi.org/10.1016/j.tree.2018.09.006. IPCC, 2019. Chapter 5: Changing Ocean, marine ecosystems, and ... Special report: special report on the ocean and cryosphere in a changing climate. Available at: https://www.ipcc.ch/srocc/chapter/chapter-5/ [Accessed November 21, 2021]. Kauffman, E.G. & Johnson, C.C., 1988. The morphological and ecological evolution of middle and Upper Cretaceous Reef-building rudistids. PALAIOS, 3(2), p.194. Kroeker, K.J., Kordas, R.L., Crim, R., Hendriks, I.E., Ramajo, L., Singh, G.S., Duarte, C.M. and Gattuso, J.-P., 2013. Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming. Global Change Biology, 19(6), pp.1884–1896. https://doi.org/10.1111/gcb.12179. Li, D.J., 2012. The tectonic cause of mass extinctions and the genomic contribution to biodiversification. arXiv.org. Available at: https://arxiv.org/abs/1212.4229 [Accessed November 21, 2021]. Maier, C., Hegeman, J., Weinbauer, M.G. and Gattuso, J.-P., 2009. Calcification of the cold-water coral Lophelia pertusa, under ambient and reduced pH. Biogeosciences, 6(8), pp.1671–1680. https://doi.org/10.5194/bg-6-1671-2009. Mallela, J., 2007. Coral Reef encrusted communities and carbonate production in cryptic and exposed coral reef habitats along a gradient of terrestrial disturbance. Coral Reefs, 26(4), pp.775–785. Martindale, R.C., Foster, W.J. & Velledits, F., 2019. The survival, recovery, and diversification of metazoan reef ecosystems following the end-permian mass extinction event. Palaeogeography, Palaeoclimatology, Palaeoecology, 513, pp.100–115. McInerney, F.A. & Wing, S.L., 2011. The paleocene-eocene thermal maximum: A perturbation of carbon cycle, climate, and biosphere with implications for the future. Annual Review of Earth and Planetary Sciences, 39(1), pp.489–516. McLachlan, R.H., Price, J.T., Solomon, S.L. and Grottoli, A.G., 2020. Thirty years of coral heat-stress experiments: a review of methods. Coral Reefs, 39(4), pp.885–902. https://doi.org/10.1007/s00338-020-01931-9. Mumby, P.J. and van Woesik, R., 2014. Consequences of Ecological, Evolutionary and Biogeochemical Uncertainty for Coral Reef Responses to Climatic Stress. Current Biology, 24(10), pp.R413–R423. https://doi.org/10.1016/j.cub.2014.04.029. Pandolfi, J.M., Connolly, S.R., Marshall, D.J. and Cohen, A.L., 2011. Projecting coral reef futures under global warming and ocean acidification. Science (New York, N.Y.), 333(6041), pp.418–422. https://doi.org/10.1126/science.1204794. Pandolfi, J.M. & Kiessling, W., 2014. Gaining insights from past reefs to inform understanding of coral reef response to global climate change. Current Opinion in Environmental Sustainability, 7, pp.52–58. Putnam, H.M., Barott, K.L., Ainsworth, T.D. and Gates, R.D., 2017. The Vulnerability and Resilience of Reef-Building Corals. Current Biology, 27(11), pp.R528–R540. https://doi.org/10.1016/j.cub.2017.04.047. Souter, D., Planes, S., Wicquart, J., Logan, M., Obura, D. and Staub, F., 2020. Status of Coral Reefs of the World: 2020. 6th ed. [PDF] UNEP, p.17. Available at: <https://gcrmn.net/wp-content/uploads/2021/10/Executive-Summary-with-Forewords.pdf>. Sully, S., Burkepile, D.E., Donovan, M.K., Hodgson, G. and van Woesik, R., 2019. A global analysis of coral bleaching over the past two decades. Nature Communications, 10(1), p.1264. https://doi.org/10.1038/s41467-019-09238-2. Taylor, J.R.A., Gilleard, J.M., Allen, M.C. and Deheyn, D.D., 2015. Effects of CO2-induced pH reduction on the exoskeleton structure and biophotonic properties of the shrimp Lysmata californica. Scientific Reports, 5, p.10608. https://doi.org/10.1038/srep10608. UNESCO, U.W.H., 2021. Assessment: World Heritage coral reefs likely to disappear by 2100 unless CO2 emissions drastically reduce. [online] UNESCO World Heritage Centre. Available at: <http://whc.unesco.org/en/news/1676/> [Accessed 10 Nov. 2021]. Vanwonterghem, I. and Webster, N.S., 2020. Coral Reef Microorganisms in a Changing Climate. iScience, 23(4), p.100972. https://doi.org/10.1016/j.isci.2020.100972. Webster, N.S. & Reusch, T.B., 2017. Microbial contributions to the persistence of coral reefs. The ISME Journal, 11(10), pp.2167–2174. Westerhold, T. et al., 2008. Astronomical calibration of the Paleocene Time. Palaeogeography, Palaeoclimatology, Palaeoecology, 257(4), pp.377–403. Willis, B.L., van Oppen, M.J.H., Miller, D.J., Vollmer, S.V. and Ayre, D.J., 2006. The Role of Hybridization in the Evolution of Reef Corals. Annual Review of Ecology, Evolution, and Systematics, 37(1), pp.489–517. https://doi.org/10.1146/annurev.ecolsys.37.091305.110136.