Effects of Increasing CO2 Levels on Coral Reefs

The Great Barrier Reef, Australia, covers an area of over 340,000 square kilometers.
Image credit: agrosse / Shutterstock.com

Corals are marine invertebrates of the phylum Cnidaria that live in colonies consisting of thousands of genetically identical polyps (~10 mm) anchored together by a calcium carbonate exoskeleton excreted at their base.

Since the last dramatic shift in sea level during the glacial period nearly 10,000 years ago, the exoskeleton remains of generations of sessile coral colonies have formed large deposits that compact and crystallize, forming large structures known as coral reefs.

Coral reefs form around volcanic islands and certain continental divides in warm, shallow ocean waters less than 150 m in depth, but as the sea levels rise and the reef structure shifts and compacts, the layer of mineralized calcium carbonate can grow to more than 1,500 m in depth and stretch across vast areas of the ocean floor, as long as 2,600 km in the case of Australia’s Great Barrier Reef.

Coral reefs are capable of breaking the rapid ocean currents and provide calm, structured habitats that serve as oceanic centers of biodiversity, supporting 25% of all sea life and housing 32 of the 34 phyla in the animal kingdom.

Coral reefs and their ecosystems are also vital to humans and the global economy offering food, tourism, medicines and protection of shorelines with a total estimated value of $30 billion USD per year.¹

Despite the grandeur of coral reefs, both the living coral and the fossilized coral reefs are susceptible to rapid changes in the marine environment. The greatest threats to coral reefs is the rising temperature of the oceans as a result of climate change and the acidification of the oceans due to increased levels of carbon dioxide in the atmosphere which dissolves in the ocean and forms carbonic acid.²

While these threats are a result of anthropogenic greenhouse gas emissions, coral reefs are also damaged by fertilizer and chemical run-off carried by rivers into the ocean, and natural and human-related changes to the ocean environment and ecosystems that favor large populations of natural coral predators, such as sea urchins and starfish.

Climate change Causing Great Barrier Reef Irreversible Damage
Video credit: Greenshack Dotinfo / YouTube

Coral Bleaching as a Result of Climate Change

Coral reefs cover only 0.1% of the ocean floor since reef-building corals have evolved to live only within a narrow range of environmental conditions, requiring adequate sunlight, alkaline waters, and temperatures of ~18^oC, and are highly sensitive to fluctuations in the surrounding environment.²

The light and temperature requirements of corals are imposed by symbiotic, photosynthetic dinoflagellates, commonly known as zooxanthellae, that live within the tissue of the majority of reef-building coral and serve as the only source of energy to the host.

As ocean temperatures rise, the population of zooxanthellae in the tissue of the corals declines and the tissue becomes transparent, revealing the white exoskeleton in a process referred to as coral bleaching.

Mass coral bleaching events have been observed worldwide since global temperatures started to increase in the early 1980s. While corals can recover even after large bleaching events such as the loss of approximately 16% of global reef-building corals populations during 1997 and 1998, an increase in the frequency and severity of coral bleaching events as a result of rapid climate change could overwhelm the capacity of coral reefs to recover between such events.³

Warm sea surface temperatures cause corals to begin to bleach. Bleaching occurs when symbiotic algae, known as zooxanthellae, are expelled from coral tissue.
Image credit: Ethan Daniels / Shutterstock.com

Based on calculations of the thermal stress to corals under climate change scenarios proposed by the UN based Intergovernmental Panel on Climate Change (IPCC ), there will likely be significant heat stress to nearly all of the world’s coral populations and long-term degradation of two-thirds of coral reefs even under the most conservative scenarios requiring the greatest effort to curb GHG emissions and maintain global temperature change below a maximum of 2^oC.² However, the possible adaptation of corals to the thermal conditions over the next century could reduce the amount of coral reefs susceptible to long-term degradation to nearly one-third.

Although corals have historically demonstrated resilience during heating and CO­₂ cycles, successful adaption by both the host coral and the symbiont zooxanthellae requires concurrent and complementary adaptation mechanisms over thousands of years under long-term equilibrium and has never been observed over periods as short as 100 years.

The Impact of CO₂ Emissions on Ocean Acidification and Coral Reefs

Although temperature has been identified as the strongest variable to induce coral bleaching, ocean acidification is also known to drive bleaching but estimates of its impact vary widely causing high levels of uncertainty in model predictions.

Ocean acidification exacerbates coral bleaching by reducing the productivity of the symbiotic association of coral and dinoflagellates, likely as a result of the pH dependent efficiency of photosynthetic water oxidation, 2H₂O → O₂ + 4e^- + 4H⁺, and the pH sensitivity of the photoprotective mechanisms of photosystem proteins.⁴

In addition, the rate of calcium carbonate production by corals is dependent on the concentration of carbonate ions (CO₃^2–); however, as the pH of the oceans decrease, carbonate concentrations will decrease due to a shift in the equilibrium towards the formation of bicarbonate (HCO₃^-) ions.

A shallow area that once housed a coral reef has been reduced to rubble reducing biodiversity in the area.
Image credit: Ethan Daniels / Shutterstock.com

Although most corals have the capacity to convert HCO₃^- to CO₃^2-, the high energy costs of this process limits utilization of bicarbonate to well-nourished corals while bleached or partially bleached corals demonstrate a considerably reduced capacity to utilize bicarbonate. Without an adequate calcium carbonate exoskeleton to anchor the corals, they are susceptible to erosion by ocean currents and complete devastation during storms.

Given the inevitable rise in atmospheric CO₂ levels and global surface temperatures through the 21^st century, the ability of corals to survive and maintain current reef structures will be dependent on successful mutualistic co-evolution of corals and dinoflagellates. The selection of favorable genetic traits often results in trade-offs that decrease other desirable genetic attributes.

Trade-offs are generally less tolerated by mutualistic organisms due to their potential impacts on either symbiont or the holobiont (host and micro-symbiont).⁵ Successful adaptation of corals to climate change will require concurrent adaption to both thermal and pH-induced stresses while excluding any genetic trade-off that lowers tolerance to either.

Conservation of Coral Reefs

The long-term survival of coral reefs will require management techniques that maintain large, well-connected coral populations which generally show greater genetic variation and evolutionary capacity.

Thus, regulations that protect coral populations now from fragmentation by human activities such as over-fishing, pollution, and habitat destruction may be critical to give corals an opportunity to adapt and survive the impacts of climate change.

References

1. Cesar, H.J.S., Burke, L., and Pet-Soede, L. The Economics of Worldwide Coral Reef Degradation. Cesar Environmental Economics Consulting, Arnhem, and WWF-Netherlands, Zeist, the Netherlands. 2003, pp. 23.
2. Frieler, K., Meinshausen, M., Golly, A., Mengel, M., Lebek, K., Donner, S.D., and Hoegh-Guldberg, O. Limiting global warming to 2°C is unlikely to save most coral reefs. Nat. Clim. Change 2012, 3, 165–170.
3. Wilkinson, C. Status of Coral Reefs of The World: 2004. Global Coral Reef Monitoring Network, 2004.
4. Pandolfi, J.M., Connolly, S.R., Marshall, D.J., and Cohen, A.L. Projecting Coral Reef Futures under Global Warming and Ocean Acidification. Science 2011, 333, 418–422.
5. Stockwell, C. A. et al. Contemporary evolution meets conservation biology. Trends Ecol. Evol. 2003, 18, 94.