Written by Ben Yuen
Edited by Zachary Ferris
Feature photo from Emma Camp/UTS
Coral reefs have developed a reputation as some of the most sensitive ecosystems in the world today (Hoegh-Guldberg et al., 2007). Covering less than 1% of the seafloor, corals are generally restricted to habitats that are climatically stable, with relatively little seasonal variation in temperature and other conditions (Couce et al., 2012). Any small change in the reefs’ ambient conditions can have severe effects on the living organisms inhabiting it. As the climate continues to warm quickly in the 21st century and marine heatwaves become more frequent and severe (Oliver et al., 2018), corals worldwide are increasingly threatened (Hoegh-Guldberg et al., 2007).
However, there are examples around the world of remarkably resilient corals found in unexpected places. In a 2018 review, Camp et al. (2018) described locations where corals live under extremely harsh environmental conditions. Their findings are extraordinary—tropical coral reefs, typically considered among the most vulnerable and sensitive ecosystems in the world, can in fact thrive in some of nature’s most extreme marine environments.
Take, for instance, Montgomery Reef in the Kimberley region of Australia, a ~400 square kilometer reef system (Schoepf et al., 2015). As the largest inshore reef in the world, Montgomery Reef is located within the Camden Sound and experiences a dramatic tidal range of up to 10 meters at some times of year (Rosser & Veron, 2011). During low tide, the ocean rushes out of the entire reef structure, leaving as much as 4 meters of coral exposed to air for hours at a time. Even where corals remain submerged, they can become trapped in stagnant tidal pools where temperatures can vary by up to 7°C between low tide and high tide (Dandan et al., 2015).
Or take the corals found growing on submerged volcanic vents in Papua New Guinea (Strahl et al., 2016), where chimneys emerging from underwater volcanoes continuously emit carbon dioxide and other gases into the ocean. Some of these gases dissolve in the water, creating highly acidic conditions that most marine life would struggle to survive in. However, even in these harsh environments, a variety of hardy coral species have been found to thrive.
Coral reef ecosystems with extreme environmental conditions are found across the tropics, from mangrove forests and seagrass beds to shallow-water lagoons and back-reef tide pools. All of these locations present extreme environmental challenges that would normally hinder coral growth and survival. So what can we learn from these extraordinary survivors, clinging on at the limits of their environmental tolerance?
Extreme Conditions Create Super Corals!
Physiological examination of corals inhabiting marginal and extreme environments reveals a wide range of adaptations that enable them to survive (Camp et al., 2018). Corals found in highly acidic habitats have developed additional protection mechanisms for their cells (Strahl et al., 2016), or have modified their skeletal structure in response to higher CO2 concentrations to maintain linear skeletal growth even when calcification rates are reduced (Tambutté et al., 2015). However, increased resilience is often conferred to corals by their algal symbionts. For example, corals in the Persian Gulf endure high summer temperatures because of a heat-tolerant symbiont, Symbiodinium thermophilum, which maintains its productivity even in hot, highly saline conditions (Hume et al., 2015).
Habitats with extreme environmental conditions can represent what many reefs may look like in the future. Even if the 36 °C summer sea surface temperatures in the Persian Gulf are unlikely to be matched by the majority of the world’s coral reef habitats in the 21st Century (Riegl & Purkis, 2012), extreme sites such as this represent an environmental proxy for a future “worst-case scenario” for coral-supporting ocean basins. They also provide a stark demonstration of what corals are able to tolerate, as coral reefs persist here in the face of environmental stress. As many coral reefs are increasingly exposed to elevated temperatures and ocean acidification because of anthropogenic climate change, understanding how corals may develop new mechanisms for resilience to these threats will be essential to support their survival.
Implications for the Future of Coral Reefs
Most of all, however, these extreme environments offer us some hope in a time of global ecological despair. Around the world, some corals are defying their reputation as sensitive, vulnerable organisms by enduring extreme environmental conditions. Clearly, even amid the dire predictions of climate warming and ocean acidification for the 21st century, there remains evidence that it is possible for corals to adapt and survive.
References
- Camp, E.F., Schoepf, V., Mumby, P.J., Hardtke, L.A., Rodolfo-Metalpa, R., Smith, D.J., & Suggett, D.J. 2018. The future of coral reefs subject to rapid climate change: lessons from natural extreme environments. Frontiers in Marine Science, 5, 4. https://doi.org/10.3389/fmars.2018.00004
- Couce, E., Ridgwell, A., & Hendy, E.J. 2012. Environmental controls on the global distribution of shallow‐water coral reefs. Journal of Biogeography, 39(8), 1508-1523. https://doi.org/10.1111/j.1365-2699.2012.02706.x
- Dandan, S.S., Falter, J.L., Lowe, R.J. and McCulloch, M.T., 2015. Resilience of coral calcification to extreme temperature variations in the Kimberley region, northwest Australia. Coral Reefs, 34(4), pp.1151-1163. https://doi.org/10.1007/s00338-015-1335-6
- Enochs, I.C., Manzello, D.P., Kolodziej, G., Noonan, S.H., Valentino, L., & Fabricius, K.E. 2016. Enhanced macroboring and depressed calcification drive net dissolution at high-CO2 coral reefs. Proceedings of the Royal Society B: Biological Sciences, 283(1842). https://doi.org/10.1098/rspb.2016.1742
- Hoegh-Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenfield, P., Gomez, E., Harvell, C.D., Sale, P.F., Edwards, A.J., Caldeira, K., & Knowlton, N. 2007. Coral reefs under rapid climate change and ocean acidification. Science, 318(5857), 1737-1742. https://doi.org/10.1126/science.1152509
- Hume, B.C., D’Angelo, C., Smith, E.G., Stevens, J.R., Burt, J., & Wiedenmann, J. 2015. Symbiodinium thermophilum sp. nov., a thermotolerant symbiotic alga prevalent in corals of the world’s hottest sea, the Persian/Arabian Gulf. Scientific Reports, 5, 8562. https://doi.org/10.1038/srep08562
- Oliver, E.C.J., Donat, M.G., Burrows, M.T. et al. 2018. Longer and more frequent marine heatwaves over the past century. Nature Communications, 9, 1324. https://doi.org/10.1038/s41467-018-03732-9
- Riegl, B. and Purkis, S.J. (Eds.). 2012. Coral Reefs of the Gulf: Adaptation to Climatic Extremes. Springer Dordrecht. https://doi.org/10.1007/978-94-007-3008-3
- Rosser, N.L. and Veron, J.E.N., 2011. Australian corals thriving out of water in an extreme environment. Coral Reefs, 30(1), p.21. https://doi.org/10.1007/s00338-010-0689-z
- Schoepf, V., Stat, M., Falter, J.L., & McCulloch, M.T. 2015. Limits to the thermal tolerance of corals adapted to a highly fluctuating, naturally extreme temperature environment. Scientific Reports, 5, 17639. https://doi.org/10.1038/srep17639
- Strahl, J., Francis, D.S., Doyle, J., Humphrey, C., & Fabricius, K.E. 2016. Biochemical responses to ocean acidification contrast between tropical corals with high and low abundances at volcanic carbon dioxide seeps. ICES Journal of Marine Science, 73(3), 897-909. https://doi.org/10.1093/icesjms/fsv194
- Tambutté, E., Venn, A.A., Holcomb, M., Segonds, N., Techer, N., Zoccola, D., Allemand, D., & Tambutté, S. 2015. Morphological plasticity of the coral skeleton under CO2-driven seawater acidification. Nature Communications, 6, 7368. https://doi.org/10.1038/ncomms8368
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