• Bianca Jussen

Coral Reefs Could Be the Next Major Casualty in the Fight against Climate Change


Imagine a fantastical scenario where we press a button and terminate all anthropogenic causes of greenhouse gas emissions. From now until eternity. It would still take years for Earth’s atmosphere to stabilize because of an effect known as ‘climate lag’. Even in the most optimal of conditions, we would still see the worsening effects of climate change for years to come.

One of these effects is the rise of ocean temperatures. In 2020, global oceanic temperatures had already increased by an average of 0.76℃ compared to the last century. We expect this trend to continue with warming fluctuations of greater frequency and severity. A 0.76℃ average change in temperature may seem inconsequential, but a warming fluctuation of 1-2℃ for just a few weeks is enough to cause coral bleaching– a process that often initiates coral death. In 2016, such an event occurred in Australia’s iconic Great Barrier Reef. Ocean temperatures were elevated for nine months, at times exceeding 1-2℃ above average. Catastrophically, over 90% of corals experienced bleaching, leaving 30% of the Great Barrier Reef’s coral population dead.

Figure 1

A basic understanding of corals is necessary to comprehend the mechanisms behind coral reef threats and restoration methods. Corals belong to the phylum ‘Cnidaria’ which also includes the jellyfish and sea anemone subgroups. They are sessile soft-bodied polyps that form colonies and reproduce both sexually and asexually (via fragmentation or budding). These polyps form the coral reef structure by secreting a calcium carbonate “skeleton”. This “skeleton” allows for more polyps to attach and also harbors a variety of other marine animals, making them an indispensable keystone species. Coral reefs get their familiar striking colors from a vital symbiotic relationship with photosynthetic algae called ‘zooxanthellae’. Corals provide habitat and compounds utilized in photosynthesis while zooxanthellae provide nutritious photosynthates that provide a significant portion of the coral’s energy needs.

Climate-caused ocean acidification and coral bleaching interfere with coral function by impairing their ability to build skeletons and removing their zooxanthellae partners. Ocean acidification occurs when the ocean, the world’s greatest carbon sink, uptakes so much carbon dioxide that the water becomes more acidic. On a molecular level, acidification occurs due to the excess hydrogen ions (H+) produced from the union of carbon dioxide (CO2) and water (H2O). Carbonate ions (CO3-2) then bond with these excess hydrogen ions, ultimately limiting reef growth by reducing the carbonate ions available for corals to construct their calcium carbonate skeletons.

Coral bleaching is the other major consequence of climate change afflicting coral reefs. As greenhouse gasses trap more energy from the sun, the ocean absorbs excess heat, increasing ocean temperatures and initiating coral bleaching. This occurs when zooxanthellae become so heat-stressed that they produce free radicals, forcing corals to expel these symbiotic partners in response to tissue damage. If ocean temperatures remain elevated for too long, the corals starve to death before the zooxanthellae can repopulate.

Figure 2

Corals first emerged about 500 million years ago and are subsequently no stranger to the effects of climate change. Our climate has changed dramatically at different points throughout Earth’s history, occurring slowly over hundreds of thousands of years. However, in this context of geological time, the climate is now warming at an unprecedented rate– about ten times faster than that observed during the Ice Age’s warming period. Corals have not yet evolved to withstand the stressors associated with modern climate change and are already at the end of their temporal rope with up to 90% of coral reefs estimated to die within the next twenty years. If we fail to mitigate and eventually reverse the major oceanic effects of climate change, we can count on coral reefs to be the next major casualty of the Anthropocene.

The focal point of coral reef restoration should be on the dual-threat of oceanic warming and acidification considering their contributions to coral death. However, because climate change mitigation takes time - if not from human inaction then from atmospheric lag effects - we must also focus on acclimating corals to their progressively warmer environments. Active coral restoration aims to fulfill this goal through methods including asexual and sexual propagation.

Asexual propagation represents the bulk of restoration projects by exploiting the clonal nature of coral colonies. Cloned fragments of healthy “donor” corals are transplanted onto damaged “recipient” corals or artificial reef structures to restore the degraded reef. In some projects, there is a nursery-grown intermediate phase where the fragmented coral is cultured prior to transplantation to improve survival. Ultimately, asexual propagation may not be a viable long-term solution because genetic diversity is inadvertently compromised. Because the coral fragments are genetically identical to their donors, there are fewer unique genotypes from which advantageous traits (such as heat tolerance) can arise.

Genetic diversity is an important consideration because coral populations would greatly benefit from advantageous traits as they face intense selective pressures from worsening ocean conditions. Corals that lack the biological means to survive in this new environment perish while those remaining could lead to novel climate-resilient corals if given enough time. Sexual propagation is an emerging strategy that advances this goal by promoting genetic diversity naturally.

Sexual propagation utilizes seasonal mass spawning events in which corals synchronously release their gametes into the ocean in a strikingly colorful display. These gametes float to the surface where they may fuse to create genetically unique planktonic embryos which eventually settle onto the reef. Ultimately, threats such as predation, misdirection through ocean currents, and inadequate settlement locations prevent a substantial number of coral larvae from reaching adulthood. Through human intervention, these early life mortalities can be significantly reduced. This is achieved by collecting and rearing embryonic corals until they are ready to be settled directly onto the reef or artificial structure. Besides the greater contribution to genetic diversity, sexual propagation is much more scalable than asexual propagation because corals can be settled en masse rather than being manually attached to the reef.

Active coral restoration methods may differ in application, but they share one ambitious goal – developing a stable reef ecosystem equipped to withstand the wave of environmental stressors to come. Despite these conservation efforts ramping up within the last few decades, the future of coral reefs remains uncertain. It certainly calls into question whether our current methods are doing enough. In response to these concerns, new strategies have been proposed to accelerate coral adaptation beyond the more “natural” means discussed.

Selective breeding programs in which heat tolerance is bred into vulnerable coral populations are being researched through organizations such as the Reef Restoration and Adaptation Program. Corals from naturally warmer environments are crossed with more heat-sensitive corals to improve their resilience against rising ocean temperatures. Other researchers achieve the same goal by focusing on coral’s symbiotic relationship with zooxanthellae. A modified virus has been created that can insert heat tolerance genes into the genomes of zooxanthellae. These genetically-modified symbionts embody another strategy that prevents corals from expelling their symbiotic partners in response to heat stress. These scientific developments are promising, but coral ecology is complex and further research is needed to address the efficacy and economic scalability of proposed solutions like these.

So why are we going through the trouble of saving these creatures in the first place? This is a valid question in an anthropocentric world where non-human life is devoid of worth unless it proves some human-serving purpose. Perhaps surprisingly, coral reefs provide a wealth of benefits for humans and animals alike. Healthy coral reefs provide food and shelter for about 4,000 different fish species and are therefore integral to 50% of federal commercial fisheries. It is also estimated that there may be millions of undiscovered organisms inhabiting coral reefs, some of which may have medicinal uses. Medicines are already being developed from known coral reef organisms to treat cancer, bacterial infections, arthritis, and other ailments. Coral reefs also protect coastlines by buffering 97% of energy from wave action, preventing erosion and damage to coastal communities, especially during storm events. If we allow corals to go extinct, we can expect to lose up to $375 billion annually.

Coral reefs possess a colorful history. They have endured 5 mass extinction events entailing asteroid impacts, volcanic eruptions, falling sea levels, oxygen-starved oceans, and other catastrophes of equally biblical proportions. Tenacious as they are, the future of coral reefs hangs in the balance. This time, the cause is human. The effects of ocean warming and acidification compound with destructive fishing practices, sedimentation, and pollution to create a multi-headed monster. Our hope is that solutions such as active restoration, improved water quality, and global fisheries management will slow coral reef decline. However, we will not reach the ultimate achievement of a self-sustaining coral reef system until anthropogenic climate change is addressed.

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