Microfragmentation: A Solution for the Bleaching Reefs?
We’ve all heard of the tragic bleaching of colourful corals at the mercy of rising sea temperatures. It is easy to feel despair, not knowing if there is any way to prevent further destruction. However, scientists have discovered an effective way to regenerate coral reefs - microfragmentation.
Illustration by Caitlin Noble.
Importance of coral reefs
Coral reefs are among the most biodiverse ecosystems on the planet. They support countless plant and animal species and are essential to sustaining healthy oceans. Even though they cover less than 1% of the ocean floor, they support about 25% of marine life. What’s more, coral reefs provide ecosystem services that are vital to human communities and industries, benefiting approximately 500 million people worldwide each day. For example, the fishing and tourism industries are the driving economic forces in the Caribbean and are dependent upon healthy coral reefs. Reef-associated tourism generates over $7.9 billion for this region every year.
Reefs also protect coastal communities against some impacts of climate change, including coastal erosion, flooding and life-threatening hurricanes. It has been estimated that these habitats decrease 97% of a wave’s force, protecting coasts from destruction.
Yet, these life-giving ecosystems are under threat.
Coral reef status and threats they’re facing
In over two decades since the inaugural International Year of the Reef in 1997, which called the world to action, coral reefs have continued to deteriorate as a result of human influences. So far we’ve lost half the world’s reefs and between 70 and 90% could be gone by the middle of this century. Human activities, such as unsustainable fishing, dredging, pollution and coastal development have caused serious harm to corals. Yet, climate change has been the primary cause of coral mortality.
Increased ocean temperatures and ocean acidification have put corals under a lot of stress, pushing many species to the verge of extinction. Many people think of corals as plants or even rocks, but they’re in fact, animals - living in symbiotic relationships with algae residing in coral tissues. The algae provide more than 95% of a coral’s metabolic requirements through photosynthesis, and are also responsible for the corals’ exceptional colours. When ocean temperatures remain just a few degrees warmer above the normal for an extended period, stressed corals eject their endosymbiotic algae and become ‘bleached’. Bleached corals can recover if the water temperature decreases within a week or two - the algae can repopulate and a reef can return to its health. But if the algae doesn’t return, the bleached corals die. As a result, the reef turns into a barren landscape devoid of animals. Even if corals do survive and recover after a bleaching event, they typically show reduced growth and fecundity and are more prone to diseases. On top of that, ocean acidification decreases coral calcification and growth.
Corals can’t adapt fast enough to the new conditions of serious climate change consequences. Mass coral bleaching has increased in frequency in recent decades and the resulting loss of biodiversity and structural diversity of reefs will affect their ability to absorb wave energy, impairing their provision of coastal protection.
Coastal protection is not the only ecosystem service impaired by climate change. It is also likely to impact low-income coastal communities and developing small islands that depend on tourism financially as the white corals no longer attract tourists. Other effects of climate change include reduction in reef fish density and fisheries’ catch, exacerbating already overstretched fisheries’ resources. Losses of beach sand from coastal zones and increased bioerosion of coastal areas are also predicted. Less stable beaches will cause other organisms to become more vulnerable, such as turtles and sea birds, which depend on beach habitats for reproduction.
Additionally, local threats to coral reefs, include the deterioration of water quality as a result of sediment and nutrient runoff from coastal development and deforestation as well as the overexploitation of marine fishery stocks.
Coral reefs are disappearing quickly and this trend will continue to escalate at a dangerous pace unless urgent action is taken.
Work being done to conserve and restore corals
We often hear about this dire situation of corals, but most news outlets fail to mention coral restoration projects. Many have seen significant investment, with the first techniques developed in 1995. A few ideas have come to the forefront of coral conservation, such as human-assisted evolution, resilient reef ‘hunt’ and coral gardening. However, two recently developed approaches are especially promising - microfragmentation (assisted asexual reproduction) and facilitated larval propagation (assisted sexual reproduction).
Microfragmentation is a technique for the proliferation of massive corals pioneered by the Mote Marine Laboratory & Aquarium based in Sarasota, Florida. The aim is not to replant reefs entirely, but to provide enough new corals to allow them to reproduce themselves and ensure their resilience to a changing climate. It utilises one aspect of coral biology - its ability to reproduce asexually through budding (new polyps “bud” off from parent polyps to form new colonies) and fragmentation (an entire colony, rather than just a polyp, branches off to form a new colony, e.g. during a storm).
The first step is to select the right coral species. Scientists screen native corals for resistance or resilience to stressors such as diseases, rising water temperature and ocean acidification. They then proceed to use the resilient coral genotypes in their reef restoration work. In ensuring that high genetic diversity is maintained in the restored population, population resilience is promoted and the chances of survival are increased. For example, many massive coral (aka boulder coral) species have been selected, such as Orbicella faveolata and Montastraea cavernosa, as they are resilient to thermal stress and strong wave action. These species are also significant reef builders throughout the Caribbean and Indo-Pacific, including the Florida Keys Reef, where Mote scientists have been testing their microfragmentation method.
Stretching approximately 350 miles and providing more than $8 million to local economies, Florida’s coral reef has lost all but 2% of its living coral cover in recent decades.The reef has also been devastated by a major disease outbreak known as stony coral tissue loss disease (SCTLD), which affects the fundamental coral species, such as massive corals. No natural recovery has been observed, so Mote scientists decided to give the reef a helping hand.
Massive coral species have been an obvious target of restoration projects, but their recovery has been limited due to their slow growth and recruitment - they expand only a couple of millimetres per year. The coral gardening concept, which utilises few large (greater than 6 cm²) fragments of massive corals, has struggled to produce substantial growth and survival of these species to restore a damaged reef. Here’s where the microfragmentation technique comes in - corals grown via this method produce more than 4 times the amount of new tissue grown from coral gardens and grow up to 10 times faster than the large fragments.
It’s all about the size - the smaller the pieces you break the corals into, the faster their growth. In the lab, corals are therefore cut into very small pieces, about 1 cm² or less in size, only a few centimetres smaller than in coral gardening. This accelerates their growth dramatically, up to 40 times the normal rate. Normally, it can take several years for a single larva to grow into the size of a golf ball, but “if you take a piece of coral the size of a golf ball and cut it into 20 pieces, each the size of a pencil eraser, those grow into a golf-ball sized chunk in a few months instead of a few years”, says Dr David Vaughan of the Mote Lab, who has been working on microfragmentation.
Microfragmented corals are grown in land-based or in-water nurseries to ~6 cm² prior to outplanting onto reefs, where coral clones can fuse together to form new, healthy colonies. If they do so, it can create a coral head the size of a basketball in just two years, when normally it would take around 75 years.
Since 2008, Mote scientists have planted more than 100,000 corals. A promising sign is that outplanted fragments of O. faveolata, a boulder coral species, have survived a couple of bleaching events in the Florida Keys in 2014 and 2015. Additionally, in Summer 2020, the scientists documented that the restored O. faveolata corals had spawned, engaging in sexual reproduction to produce new generations of corals. The restored branching corals (staghorn coral Acropora cervicornis) were sexually mature and capable of spawning too. Reefs restored by Mote began to recover within a year and more fish and invertebrates moved in.
To maintain genetic diversity in our changing climate, Dr Vaughan and his team have also crossbred corals to produce new genetic combinations, which are tested against high temperatures and high acidity. There may be only 25 genotypes in the wild, but the team has created 5,000 new ones in the lab. Such diversity within species might protect coral reefs from emerging stressors, including prolonged heat and new pathogens.
Facilitated sexual reproduction
Scientists from other parts of the world are participating in coral restoration too. For example, SECORE and other organisations, e.g. the Nature Conservancy, are facilitating sexual reproduction in corals to create more diverse populations. Also known as facilitated larval propagation, the technique involves collecting coral reproductive cells during spawning events and fertilizing them in a lab as quickly as possible, as these cells are only viable for a few hours. The embryos are then grown in land-based or in-water nurseries until they are ready for outplantation. Millions of embryos can be created from a single dive mission, which makes this method perfect for large-scale restoration. It is particularly important, because many populations of foundational coral species are experiencing reproductive failure in the wild. The technique has already been tested in reefs off the Caribbean, Mexico, Florida, the Bahamas, the Dominican Republic, Bonaire, Australia, Palau and Guam. Around 24 years ago, a cruise ship ran aground, severely damaging the Cuevones Reef in North Cancun. It reduced coral cover from 14% to 1% within a 480 m² area. SECORE has been using the larval propagation technique to restore the area since 2010, with the first corals being outplanted in 2015. These corals had a maximum size of 10 to 15 cm in diameter back then. In four years, they grew to at least 40 cm and are looking healthy and strong.
Conclusion
The magnitude of reef degradation today is far greater than the ability to restore them using traditional methods, so these novel techniques offer some hope for the future. Microfragmentation and assisted larval propagation have already been successful. But scientists are not stopping here. These methods are continuously being revised and tested under different environmental conditions. They’re also being upgraded, as done by Frank Mars, who has designed hexagonal steel structures, known as “coral spiders”, to create suitable surfaces for planting corals and structures attractive to fish. Mars’s Coral Reef Restoration Project has already installed over 8,600 spiders covering more than 8,000 m² of the ocean floor in Indonesia. Two years after the initial construction, one rehabilitation zone has recovered so much that it attracted turtles, sharks and fish to reestablish in this area.
Biorock technology is taking this approach even further. Facilitating the flow of low voltage direct current electricity into the coral spiders, an increased rate of minerals are taken from seawater by growing corals, which accelerate their growth by 3-4 times. Indonesia has 400 of the 500 Biorock constructions worldwide.
The variety of strategies available for coral restoration is encouraging. The solutions are at our fingertips. The situation might seem catastrophic, but consider that we’ve been in a relatively similar situation before - the hole in the ozone layer. We identified the problem, found an answer to it and the hole was closed. It shows that we can mitigate our problems, we just need to use our ingenuity, science, manpower and money, and act now.
WERONIKA PASIECZNA
Ron is a zoology student at the University of Sheffield and has a diploma in environmental ethics from the University of Helsinki. She is a bird ringer in training, Student Hedgehog Ambassador for the Hedgehog Friendly Campus campaign, and she volunteers for BTO, River Stewardship Company and Don Catchment River Trust. In her spare time, she enjoys reading, spending time in nature and participating in citizen science projects. Her main interests are animal behaviour, conservation, ecology and biological recording.
You can find her on Instagram @rongoeswild and on Twitter @rongoeswild.