Coral bleaching – Can we halt the most tragic ecosystem destruction of the 21st century?

As you peer down through your mask at the reef below, the glorious colours and pigments you’ve seen in pictures and documentaries are tragically missing. Instead, you descend into a ghostly white graveyard. This is not some far off future, this is already reality on many coral reefs around the globe.

Figure 1 – A diver traverses through an expanse of bleached Staghorn corals. Source =

This is the harrowing effect of coral bleaching, and there is no better illustration of its destructive capability than the story of the Great Barrier Reef – the world’s largest, 2300km reef. In 2016 and 2017, back to back mass bleaching events killed 50% of it’s coral(1). In February of 2020, another mass bleaching event struck Australia’s greatest treasure. However, it has been largely unnoticed due to media coverage of the Australian bushfires at the end of 2019 and the emerging Coronavirus pandemic throughout. The latest combination of surveys, carried out by James Cook University, suggest that the 2020 event affected the previously unaffected southern region of the reef and these 3 events together have bleached 98% of the reef and two thirds of it severely so(2).

Figure 2 – Bleaching records of the Great Barrier Reef for 2016 and 2017. Red circles indicate reefs which experienced severe bleaching of 60% or more coral. Green circles indicate mild or no bleaching (10% or less of corals bleached). The southern sections of the reef (illustrated here as the green section at the bottom of the reef) were most affected by the mass bleaching event of 2020. Adapted from (2).

The future of one of the seven wonders of the natural world makes for grim reading. In 2018, coral recruitment, the uptake of new coral larvae, was down 90% compared to historical figures(3). Additionally, even the fastest growing species of coral take 10-15 years to recover from severe bleaching. The occurrence of 3 major bleaching events in 5 years, combined with a median return time between pairs of bleaching events – of approximately 6 years, leaves little prospect of recovery(1).

What is the cause of mass coral bleaching events?

Human-induced climate change is steadily increasing ocean temperatures across the globe, and this is disrupting a key aspect of coral physiology, which causes bleaching.

Corals themselves exist as colonies of vast numbers of small animals called polyps. Each polyp lives in a state of symbiosis, with algae belonging to the family Symbiodiniaceae living inside their tissues(2,4). The coral provides the algae with protection and nutrients, such as nitrogen and inorganic carbon, and in return receives energy as the algae photosynthesise(2,4). It is this energy that allows for rapid calcification and the construction of the magnificent reef structures we observe.

Coral bleaching is defined as the partial or complete loss of symbiotic algae and associated photosynthetic pigments by the coral animal(4). As sea temperatures increase, the algae produce increasing quantities of tissue damaging molecules – called reactive oxygen species, and are promptly expelled by the polyps(2,4). Without their colourful photosynthetic symbionts, corals turn white and severely bleached corals starve to death.

5 Diagram of thermal coral bleaching. Note: this diagram was adapted... |  Download Scientific Diagram

Figure 3 – A diagram to illustrate the process of coral bleaching. Adapted from “A reef manager’s guide to coral bleaching” – Marshall and Schuttenberg (2016).

Coral reefs comprise 0.1-0.5% of the ocean floor, yet play host to a third of the planet’s marine fish(4). Their topographic complexity increases habitat areas and manages vital ecological interactions, allowing the coexistence of large quantities of diverse species(2). The loss of these ecosystems would damage marine biodiversity on a global scale.

Can the effects of coral bleaching be halted?

There is a pressing demand for scientists around the globe to investigate any means through which corals can gain an edge against rising sea temperatures. Researcher’s at the Australian CSIRO have made an exciting advance this year, breeding heat resistant strains of algae which can confer greater resistance to bleaching(1).

Figure 4 – Before and after photographs of an Acropora coral colony after bleaching struck Christmas Island, Kiribati in 2015 and 2016. Before photograph taken by Kieran Cox, after photograph taken by Kristina Tietjen.

Patrick Buerger and his team bred genetically identical clones of a single copy of a Symbiodiniaceae species. These strains were raised for 4 years (120 generations) with 10 strains being raised at 31°C (equivalent to a heatwave on the GBR) and 2 control strains at 27°C(1). All strains were then introduced to coral polyps and after uptake into coral tissues, heat tolerance was tested over 4 weeks. The heat-evolved strains were shown to be more thermally tolerant, as indicated by a 66% increase in cell density compared to a 79% decrease in the control strains(1). Buerger’s team believe this due to an increase in activity of genes which convert carbon into sugars and a downregulation of genes responsible for photosynthetic activity, reducing the production of reactive oxygen species which cause their expulsion(1).

Questions of how algae will interact with adult coral polyps, how they will compete with wild algae and whether this heat resistance is a short-term response that will fade in the wild with exposure to ambient temperatures have tempered the excitement of these findings. Regardless, such discoveries provide us with hope, and – most importantly, potential time to address the root cause of this issue.

Emissions and the fate of corals on a global scale

With climate change occurring globally, mass bleaching events are not limited to the Great Barrier Reef. As illustrated by 72% of World Heritage reefs being exposed to severe bleaching over the past 3 years(5). The scientific consensus, is that to retain coral communities globally – we must limit average temperature rise by 2100 to 1.5°C above pre-industrial levels, which is the goal of the Paris climate agreement(5).

Projections of global frequency of bleaching events under two emissions scenarios by UNESCO in 2017 provide further insight(5). RCP4.5 describes emissions peaking in 2040 and declining thereafter, whereas RCP8.5 describes a ‘business as usual scenario’. Their projections of a 2.4°C and 4.2°C temperature increase respectively lead to respective predictions that under RCP4.5, 48% of World Heritage reefs would experience two bleaching events per decade by 2040, whereas for RCP8.5 this figure was 86%(5). Even if the combined efforts of the current pledges made under the Paris agreement came to fruition, this would result in a temperature rise of 3.0°C and a state of play somewhere between RCP4.5 and RCP8.5(5).

The harrowing conclusion of this and other studies in recent years is the bitter acceptance that coral assemblages and their associated communities will change irreparably over the course of this century. The extent to which this represents dramatic changes to, or complete destruction, of these ecosystems will be determined directly by our combined efforts to combat climate change.


  1. Buerger, P., Alvarez-Roa, C., Coppin, C.W., Pearce, S.L., Chakravarti, L.J., Oakeshott, J.G., Edwards, O.R. and van Oppen, M.J.H., 2020. Heat-evolved microalgal symbionts increase coral bleaching tolerance. Science Advances, 6(20), pp. 2498-2498.
  2. Dietzel, A., Bode, M., Connolly, S.R. and Hughes, T.P., 2020. Long-term shifts in the colony size structure of coral populations along the Great Barrier Reef. Proceedings. Biological Sciences287 (1936), pp.20201432-20201432.
  3. Hughes, T.P., Anderson, K.D., Connolly, S.R., Heron, S.F., Kerry, J.T., Lough, J.M., Baird, A.H., Baum, J.K., Berumen, M.L., Bridge, T.C. and Claar, D.C., 2018. Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science, 359(6371), pp.80-83.
  4. Lough, J.M. and Van Oppen, M.J., 2018. Coral Bleaching: Patterns, Processes, Causes and Consequences. Springer.
  5. Heron, S.F., Eakin, C.M., Douvere, F., Anderson, K.L., Day, J.C., Geiger, E., Hoegh-Guldberg, O., Van Hooidonk, R., Hughes, T., Marshall, P. and Obura, D.O., 2017. Impacts of climate change on World Heritage coral reefs: A first global scientific assessment.

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