Coral Reef Resilience in Hot Water

When corals are stressed, they expel the symbiotic photosynthetic algae they need for survival. This is known as bleaching, because without these zooxanthellae, corals lose their bright colors. Bleaching is often linked to a rise in water temperature, as has occurred in the Hawaiian Islands. Photograph: NOAA—National Marine Sanctuaries, 2015. Reef-forming corals are struggling in warming oceans. Temperatures as little as 1 degree Celsius above normal are stressful for the delicate symbiotic relationship between corals and the tiny algae that feed them, remove their waste, and enable their beautiful colors. Heat stress can lead those algal partners to abandon their coral homes, a phenomenon called bleaching because it leaves reefs a ghostly white. Bleaching events are becoming increasingly common and prolonged. An estimated 94% of coral reefs have already experienced one or more episodes of severe coral bleaching since 1980 as a result of record-breaking temperature extremes. In 2015–2016, an El Niño triggered a global bleaching event, causing mass coral mortality. Its epicenter was Kiritimati (Christmas Island), a coral atoll in the central equatorial Pacific. There, the heatwave lasted an unprecedented 12 months, and 90% of the hard corals died. The world's media had been closely following the 2015–2016 mass bleaching event. Toward its end, journalists reached out to University of Victoria marine ecologist Julia Baum to request follow up photographs of Kiritimati's lifeless white corals. Baum's team had been following reef health over time, tagging and treating individual corals “a bit like patients in a medical study,” she says. Coral reef ecologist Julia Baum photographs corals within a transect in Kiritimati (Christmas Island), where she has followed the survival of corals during and after marine heat waves. Photograph: Trisha Stovel. Photograph: Baum Lab. So Baum's graduate student Danielle Claar headed out with her camera. Surprisingly, some before and after photos were not what they expected. “We found some types of corals that recovered from bleaching and healed themselves while they were still exposed to high temperatures,” says Baum. Coral recovery from bleaching had previously been observed only after heat stress subsidence. Their discovery, published in 2020 in Nature Communications, was “a game-changer,” says Baum. It underlined the volumes still to be learned about coral resilience. Resilience, a concept pioneered by Canadian ecologist C. S. “Buzz” Holling (1930–2019), includes resistance to and recovery from a stressor. Sampling a small cross-section of coral reef resilience research reveals work in progress in disciplines including ecology, genetics, microbiology, protected areas management, and cryobiology. Despite occupying a mere 1% of the ocean, reef-building warm water corals sustain a quarter of all marine life. Coral reef biodiversity supports fisheries, tourism, and recreation for hundreds of millions of people, with the corals’ hard structures providing coastal storm protection for millions more. About 800 reef-building species of corals exist in the phylum Cnidaria, which also includes jellyfish and sea anemones. Corals mainly reproduce both sexually and asexually. During sexual reproduction, males and females typically broadcast sperm and egg bundles into the seawater, where fertilization occurs. Embryos develop into swimming larvae about 4 days after conception. The corals then metamorphose and settle to begin sedentary polyp life, growing, over time, into a colony through asexual budding. Warm water corals form partnerships intracellularly with dinoflagellate algae in the family Symbiodiniaceae. The corals form these symbiotic partnerships as larvae or shortly after settling, explains Mary Hagedorn, senior research scientist with the Smithsonian Conservation Biology Institute and Hawaii Institute of Marine Biology. About 30% of coral species get symbionts from their parents, she says. The rest get symbionts from the environment. Although corals are able to acquire some food on their own with their harpoon-like tentacles, the vast majority of their nutrition comes from their single-celled algal symbionts. When under thermal stress, corals expel their symbionts, leading to bleaching. Bleaching events are conspicuously patchy. During some bleaching events, bleached and unbleached corals are seen side by side. Scientists around the world are actively investigating why some corals are more resistant than others to losing algal partners as temperatures rise. Variation in bleaching vulnerability is hypothesized to be due to intrinsic factors such as phenotypic and genetic differences among corals and their microalgal symbionts, plus extrinsic factors such as microhabitat differences in temperature and light. In Kiritimati, Baum established long-term reef monitoring sites in 2009 along a continuum from near villages to the remote end of the atoll. In 2014, her team tagged individual colonies of two Indo-Pacific coral species Platygyra ryukyuensis and Favites pentagona to track symbiont identities and colony fates during the heatwave. Using genetic analyses, Baum examined the influence of human disturbance on coral symbioses, bleaching timing, recovery, and symbiont community composition changes. She tested whether symbiont type mattered for bleaching and survivorship. It did. Corals on highly disturbed reefs were dominated by heat-tolerant symbionts in the genus Durusdinium. Reefs with lower disturbance levels had more heat-sensitive symbionts in the genus Cladocopium. Two months into the heatwave, as was expected, the corals with heat-tolerant symbionts were less likely to have bleached than were those with heat-sensitive symbionts. Nevertheless, many of the heat-sensitive coral colonies recovered from bleaching while they were still at elevated temperatures, a phenomenon that had not been previously observed. Their recovery was helped by the proliferation of heat-tolerant Durusdinium symbionts, which the corals had adopted instead of their formerly heat-sensitive ones, a swap known as symbiont shuffling. The corals that started the heatwave with heat-sensitive symbionts ultimately survived at rates higher than or similar to those with thermotolerant symbionts. Recovery from bleaching during this heatwave was observed only in corals at sites without high levels of local disturbance, however, underlining that corals have multiple pathways to survival—resistance and recovery. In follow-up work, Baum's team compared the fate of coral species with different life history strategies: the massive, tolerant, slow growing Porites lobata and two fast-growing but sensitive “competitive” corals Pocillopora grandis and Montipora aequituberculata. The latter two species can grow to dominate communities. With respect to survivorship at the end of the heatwave, in this case, the disturbed reefs fared better than the protected ones. That's because the disturbed sites were already composed of mainly heat-tolerant species. But the results differed between the community and species levels. For 336 coral species not eradicated by the heatwave, their survivorship declined with increasing local disturbance, underlining the complexity at different biological scales. 1. Sperm-egg clusters are released by polyps. 2. Gamete clusters ascend to the ocean surface. 3. Clusters break up, with gamete mixing and fertilization. 4. Embryos become larvae capable of settlement for 3–4 weeks. 5. Larvae settle and undergo metamorphosis. 6. Juvenile polyps develop a mouth and tentacles. 7. Zooxanthellae are incorporated, calcification increases, and budding begins at base of polyp to start a new colony. 8. Colony expands through calcification and budding. Image: CC BY-SA 4.0 Andcelano. Although Baum's work has shown that recovery is possible even in hot water, the time windows available for recovery between consecutive mass bleaching events have shrunk from 25 to 30 years a few decades ago to 1–3 years now, far shorter than the 10–15 years that even fast-growing corals need to rebound. If climate change continues unchecked, hope for coral reef resilience may rest not on recovery but on resistance. Scientists are avidly investigating intrinsic resistance—biological traits that help the corals cope in a changing climate—and extrinsic resistance conferred by living in less vulnerable locations. At Stanford University, recent doctoral student Nia Walker ran experiments subjecting corals to heat stress to study resistance. Walker notes the importance of considering resilience at different levels including that of the species, the population, and the individual coral colony, because there is so much variation. “Variation is the building block material for acclimation and adaptation,” says Walker. Walker and colleagues examined Acropora colonies in Palau. Her team collected coral fragments and exposed them to heat pulses lasting 1–9 days. Even among corals from the same species and collection site, Walker found considerable variation in time to induce to bleaching and mortality level, categorizing the samples as low, moderate, or high in resistance. After heat stress removal, she also found variable recovery time. An unbleached site within Julia Baum's Kiritimati study of corals and resilience, photographed in August 2014. Photograph: Danielle Claar. A bleached site within Julia Baum's Kiritimati study of corals and resilience, photographed in August 2014. Photograph: Danielle Claar. If heat-resistant coral genotypes are to be used in reef restoration projects, Walker notes that there is a need to consider trade-offs. Reintroducing only thermally tolerant corals may disadvantageously create a genetic bottleneck and could have disadvantages. “Heat-resistance will likely be linked to fitness trade-offs,” notes Walker. “Otherwise, all corals would be super heat resistant.” Measuring skeletal growth as a proxy for health in the recovering corals, she found that growth resumed within 4 months after bleaching in moderately resistant corals but that high- and low-resistance corals grew more slowly. High resistance but slow growth could point to a fitness trade-off, says Walker. Not all symbionts are equally resistant to heat stress, and it has been unclear to what extent corals can switch symbionts when conditions change. Mariana Rocha de Souza, a recent PhD graduate at the University of Hawaii, investigated this in corals from Kāne'ohe Bay, Hawaii, a body of water with variable water quality, temperature, and acidity. Tagging and collecting coral fragments across the bay and logging data on temperature, she used DNA analysis to identify coral microbes, creating a symbiont DNA library. To model how the symbiont community might be affected by future ocean conditions, Rocha de Souza created a mesocosm of eight corals from three genera in four treatment groups that were heated, acidified, heated and acidified, or kept in controlled conditions for 2.5 years. Exploring vulnerability to bleaching and the influence of symbiont species, Mariana Rocha de Souza and Shreya Yadav record data on coral bleaching in Kāne'ohe Bay, Hawaii. Prior to treatment, she found symbiont communities quite similar within genera. After treatment, the symbiont composition was species specific. “Even corals from the same genus didn't have the same symbiont composition,” she says. Some corals had interactions with multiple symbiont species. Others were more specialized. She found that heat stress influenced symbiont composition, whereas acidification did not. The site of collection also had a major impact on symbiont composition after experimental treatments. Even after being exposed to stress for 2.5 years in the experimental setting, “corals from specific locations still had something imprinted,” she says, responding differently according to their collection location. Her work contributes to broader efforts to disentangle whether local and regional variation in heat-stress vulnerability is due to acclimation, genetic adaptation, or heritable epigenetics. Adrienne Correa and colleagues, Carsten Grupstra and Alex Veglia, set up an experiment to examine the potential role of fish feces in disseminating coral symbiotic algae. Photograph: Lauren Howe-Kerr. Adrienne Correa and her colleagues, Lauren Howe-Kerr and Carsten Grupstra process Pocillopora coral fragments in Moorea, French Polynesia, to sample viruses and assess their potential role in coral bleaching. Photograph: Rebecca Maher/Correa Lab. In the middle of Rocha de Souza's study, in 2019, Kāne'ohe Bay experienced a heatwave and bleaching. Heat-tolerant symbionts increased in frequency, and corals with heat-tolerant symbionts bleached less. In the north of the bay, little changed, underlining the quirks of geography. In light of suggestions by some that we infect corals with more heat-resistant symbionts to help them weather warmer conditions, “Maybe that's not going to work,” says Rocha de Souza. We can try to infect corals with “preferred” symbionts, but their site of origin matters. New symbionts, she says, may not “stick.” Indeed, many questions remain regarding the basic biology of how corals and symbionts pair up, says Adrienne Correa, at Rice University. How do host and symbiont recognize each other? What triggers break ups? These are important questions in light of efforts to breed more heat-tolerant symbionts, which might evolve faster than corals because of their shorter generation times and greater abundance. In this effort, one knowledge gap has been understanding when and how symbionts have sex. “Genetic recombination could facilitate faster generation of heat tolerance,” says Correa. So she has been studying meiosis and sexual reproduction to ascertain the conditions that trigger more frequent symbiont sex. How symbionts interact with corals and the environment is another research vein. During bleaching, there is a mass loss of symbionts to the environment, but under normal conditions, “symbionts are entering and exiting healthy hosts on a daily basis,” says Correa, sparking many questions: What is the symbionts’ fate if they are expelled from a host? Do they float off and die? Were they expelled because they were already dying? Are they taken up by another host? And does that change under a stressful versus normal or healthy context? A 6-month-old assisted gene flow elkhorn coral colony produced from cryopreserved sperm crossing Florida sperm with Curaçao eggs. Photograph: Cody Engelsma, Mote Marine Laboratory, Florida. PhD student Ines Raimundo, working in Raquel Peixoto's lab at KAUST, applies probiotics to corals in the Red Sea. Photograph: KAUST/Morgan Bennett-Smith. Correa is also investigating whether symbiont availability for corals is aided by fish. Following work by others, Correa's group worked in Mo'orea, French Polynesia, comparing the feces of fish with different diets, such as butterflyfish and parrotfish, which eat corals, with those of surgeonfishes, which eat algae. Examining the proportion of living to dead symbionts in feces, her team found that the coral-eating fish had live symbiont cell concentrations five to seven orders of higher than their concentrations in and Correa's team that fish feces might be an important of thermotolerant symbiont during bleaching of Correa's is coral reef “We are to what an of viruses are with coral she says. can infect corals or their symbionts, so Correa is whether viruses to bleaching. She has observed that viruses to a more during coral heat Nevertheless, some viruses can be is even for viruses to heat such as in a on a to high temperatures in are all of different viruses do that we have to for as we says Correa. Although the only for corals in hot water is to the in the a better understanding of a healthy coral That's the of Raquel at the University of and in have a like says an that includes and viruses that to the relationship between corals and their symbiont algae. Her team is to how coral and algal on a research in the in 2014. Photograph: used for for and more for and might probiotics treatment with in a this case, are to the she says. of concept in the her team is a experiment in the Red to on the reef How will corals they up treatment during species it be These are a few of the also marine Mary to begin coral of the coral time is leading the Recovery with Smithsonian to facilitate a cryopreserved of coral and To says Hagedorn, the of coral for is on the her is that a of many coral species, like might to in a more future from human she has also of concept for efforts used as assisted gene by in fertilization to coral Her team increased genetic in elkhorn coral by crossing coral from and them in at Mote Marine and Her corals are by But experimental restoration coral is already in progress on some reefs in coral for is says Corals to on a but their is Her team had to for to coral The team for coral sperm and larvae without like on she says. “We this of to them and is such as coral into a without growth of new corals from tiny coral at the reef in one of the few sites healthy and of this coral species which has severe Photograph: change is not the only to coral and efforts to create marine protected areas have been to them from local such as coastal and for the But do protection during marine In with her doctoral student at the time University marine ecologist published a in Biology in which they examined whether for coral reef coral reef resilience to climate phenomenon over time is that coral species composition between protected and Corals such as Acropora and Montipora are thermally and found that can both the of species within a community and the to climate and colleagues tested the that the resilience of coral to data in coral and after including and ocean warming their results that have on coral loss or recovery. is that local such as and get by the much greater of ocean Their analysis them to that reefs be by local protected areas are not more resistant to climate and if they can be says of coral reef at the Conservation in Her PhD research in that coral communities in and of a reef in an local a more community of corals to But species in are the same sensitive to climate change. does not at the of that says Reefs experienced with stress and disturbance, have sensitive species and or tolerant ones. does that “We need marine protected says we need them in the samples water off on the Photograph: so far the of bleaching is the a coral at the de reef lab in has been studying in coral species composition, and in this Although coral reefs have not seen mass mortality from thermal stress, there are of such as coastal and says linked to climate change and are another recent the coral loss on coral was of coral says. was like in a Although hard to be about the of coral we need to with coral reefs is we have multiple says with one common human efforts in such as have recovered reefs over but up is “We are a few orders of what we need to that the loss of corals is not Not all corals are created and it is important that coral reef all species corals do not much or whereas corals the important are the being at the says. for knowledge that we need to better the conditions coral resistance and recovery. Two coral are providing for biodiversity and reef But the of corals to hard them in from the of warming When coral reefs they out of the water to their reefs do not grow and can break explains a at for Marine calcification rates on coral In a study by and published in Nature Communications, she and her colleagues at coral reef sites in reef calcification is by that reefs could become around calcification and going says a from to marine acidification will likely have on calcification she says, but the is ocean is that we have to be says She on remote to identify and produced a global analysis known as the Reefs that are the of global for Coral But as and in Conservation in in of reefs will be if are not At the Coral in coral reef of the of in in her that are but not that a on without to not building a biological to reef local conditions, and climate there is hope for the future for coral If it is that the majority of coral reefs will be by so that's and that's says But she is is Nature is And a more about these types of climate and how corals have in we would have she says. an she colleagues her to a reef in the of The in a a She was that a coral reef could exist But as she through the the water the corals she had was like the of a coral she says. not have been and they like that that about the quirks of is a and in her on

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.