Ainsworth, however, knew better than to count corals out. Ainsworth had witnessed colonies being destroyed by the first bleaching event, which occurred in 2016 and 2017. RecoverThe first is better than the second. She’s not the only one to notice that corals seem able to remember past heat exposures and adjust to subsequent heating.
Satellites have been used to study corals in the past. This has given researchers a better understanding of how this environmental memory manifests at a large level. Some reefs or coral colonies appear to be more resilient than others depending on what happens next. Research has shown that corals are more likely to survive high temperatures when they are exposed to sublethal stress levels. However, it is not clear how long memory might last. A very early StudyThe 1990s’ detail work suggests that corals might recall past exposures for at least a decade, but that the timescale can be quite brief, often just a few days. This supports the argument, that these changes are due to rapid acclimation of long-lived coral colonies and no adaptation of corals over many generations.
Many scientists are inspired to take action because of the speed at which reefs are deteriorating.
Increasingly, researchers are paying attention to the winners and losers of repetitive hardships to unpack the individual- and species-level differences in memory. It is evident that there is variability on the reef. [corals]Hollie Putnam is a molecular ecophysiologist from the University of Rhode Island. Some are completely pale and yellow and others are fully pigmented. This natural variability makes it a great place to ask questions. . . mechanistic questions about environmental memory. Scientists have been able to identify how heat affects corals and how exposures affect algal symbionts. This could help them prepare for harsher future conditions.
A GIANT REBORN : The Florida Keys saw record-breaking temperatures in 2014 and 2015. This led to back-to–back bleaching events at Cheeca Rocks. Cheeca Rocks is a shallow coral reef within the nearby national marine sanctuary. These star corals are mountainousOrbicella faveolataResearchers thought they had died in September 2014. But, four years later they had recovered. Perhaps, they survived the 2015 heat.
Derek Manzello, NOAA Coral Reef Watch
Scientists have already begun to explore coral memory as a tool to improve conservation strategies and restoration strategies, before corals become irreparably endangered. Coral biologists are testing corals in the laboratory to see if they can withstand mild thermal stress. This will harden them or prevent them from bleaching and heat waves. Teams are also breeding corals for a higher memory capacity or inoculating corals using heat-resistant simbionts that may play a role with memory. The ultimate goal is to eventually outplant these corals in order to rebuild reefs that are struggling against climate change.
Restored Corals Bring Hope for Reefs Worldwide
Agencies such as the National Oceanic and Atmospheric Administrations (NOAA) and National Academy of Sciences recently reorganized. Produced ReportsThese memory-based approaches to coral resilience are being promoted by the National Science Foundation (NSF), and restoration groups. Grants are also being awarded by the National Science Foundation and other restoration groups to study the genetic and epigenetic factors that influence coral memory. Jose Maria Eirin Lopez from Florida International University is an environmental epigeneticist. He says that this support highlights the relevance of this field.
He says that stress hardening can be an intervention that can be integrated into restoration. But we don’t know enough to do that efficiently. The Scientist. Its a glimmer of hope, and now we need to figure out how its working so we can actually implement this.
FEEL THE HEATThe increasing frequency of severe warming events can cause coral reefs to bleach. Coral organisms can eject their symbionts, which can lead to catastrophic bleaching. However, scientists have observed that reefs are able to bounce back from these warming events and can often do better when temperatures rise again, even if they are longer or hotter than before. This phenomenon, in which an organism modifies its response for past abiotic stimulihas come to be known as environmental memory. Researchers are currently trying to understand why certain corals may have greater memory.  CATHERINE DELPHIA |
Take it to the reefs
Barbara Brown, a British marine biologist, discovered the first evidence of environmental memories in corals in 1994. NotedOnly the eastern side of stony coral colonies is allowed (Coelastrea asperaPreviously Goniastrea aspera) bleached during a high heat event at her field site in Phuket, Thailand. A study published in NatureShe hypothesized that western sides were more sensitive to sunlight a few years later. She supported this hypothesis in the lab with coral samples she exposed to varying temperatures and irradiances.
Brown returned to Thailand in 2000 and turned several corals so the stress-tolerant sides facing east faced west. Brown returned to Thailand in 2000, and after a decade, the reef suffered another severe bleaching. However, Brown discovered that the corals facing east, due to their history of high sun exposure, did better than the eastern controls. They retained four times the number of symbionts. She and her colleagues saw this as evidence of corals having retained a memory of their past high irradiance history, despite living under lower radiation for 10 years. 2015 paperReporting the findings
Great Barrier Reef: The Ocean Heat Wave Caused Chaos
Researchers have found evidence of environmental memory in reef systems since those early experiments. In 2008, two satellite-based studies on the Great Barrier Reefone were published. Science Ainsworths and another group in 2016 Nature Climate Change Terry Hughes, a James Cook University coral-reef ecologist and her colleagues found that prior heat stress may increase bleaching during subsequent warming events.
Ainsworths team analyzed nearly three decades worth of sea surface temperature data and found that bleaching events on Great Barrier Reef occur within a week or so of a period of cooling below the bleaching threshold. This initial exposure to sublethal heat proved protective and reduced coral mortality and symbiont losses by 50% compared to stress events in which temperatures rose above the bleaching threshold but without any sublethal priming.
Hughes, on the other hand, focused on the magnitude and length of the warming in the wake of back-to-back bleaching episodes in 2016 and 2017, which was a devastating two-punch to the massive reef system. Speaking to The Atlantic Hughes stated that the Great Barrier Reef had approximately 2 billion corals prior to 2015. By 2018, half were dead.
Hughes’ team discovered that the severity and heat of the damage did not necessarily correlate. The results showed that the Great Barrier Reef’s 3,000 reefs had not bleached in 14 year. 2016’s bleaching was the most severe. NOAA forecasts bleaching based typically on the number of weeks within a 12-week span corals spend in warm water. This is also known as the degree heating weeks (DHWs). Researchers consider bleaching probable after four DHWs, with severe bleaching and death after eight. Hughes claims that these thresholds are not very reliable and rely on history.
Eight DHWs increased reefs’ chances of severe bleaching (defined in NOAA as the bleaching that occurs to more than 30% of corals) by 90 percent. This is far greater than NOAA had anticipated. A year later, however, another warming event caused severe bleaching in only 50% of the reefs, even though most reefs experienced more and longer-lasting heat.
Hughes says that the study suggests that there is a limit to the protective effects of previous heat exposure. There is a memory when the bleaching events are one-year apart. However, the behavior of corals is quite different when they are 14 year apart.
Scientists also supported Brown’s initial observations about environmental memory at the level individual colonies. Derek Manzello, a coral ecologist and federal coordinator for NOAAs Coral Reef Watch used cameras carried by divers to track the fate of more that 4,000 colonies of 15 coral species in Florida Keys during bleaching events of 2014 and 2015. Manzello says that each species has its own response to heat. However the team did find that the second event did more damage than the first. It was less DHWs caused by the first are more severe than those caused by the second, even though there were more DHWs in 2015 than in 2014.
See a Coral to Overlast Climate Change
Manzellos team also noted the resilience of slow-growing, mounded corals to heat damage over fast-growing, branching corals. These findings were also echoed in Other studies. He says that the corals, which are massive, stay around because their physiology is better suited for stress. The Scientist. They have more lipids, tend to be more efficient at feeding from the water column, and so are the ones that capture light well. [to bleaching]When the temperature rises. It remains to be seen if memory plays a role in the relative success of mounded Corals compared with other types, he says.
Are Memories Inheritable?Hollie Putnam from the University of Rhode Island continues to investigate the mechanisms of environmental memory to determine how long it lasts. Hollie is also researching the possibility for corals to transmit their memories to the next generation. The branching coral Pocillopora damicornisPutnam and her colleagues exposed adult corals for six weeks to high temperature and low pH conditions. This mimicked the simultaneous stressors of ocean acidification (OA) and warming. The researchers found that although the conditions caused corals to partially blemish, the young were more prepared to be exposed to stressful conditions than larvae from parents who were not subject to the same stressors. The larvae of OA- and heat-exposed corals grew slower but had a higher respiration, which is a proxy for metabolism. Their PaperThe authors suggested that corals may adapt to heat stress through trans-generational climate change. Recent research studiesSimilar studies have shown that corals and algal symbionts may also be heritable. Putnam says, “Whenever I talk about it, I get really excited.” The ScientistCorals are undoubtedly capable of resisting, responding and evolving in ways that have yet to be discovered. |
Memory based on molecular basis
Scientists are studying corals in the laboratory to understand the mechanisms by which individual coral colonies encode their experiences at the molecular and cellular levels. This helps explain observations of environmental memories in the field. One example is a 2014 study. revealedMore than 100 genetic loci were associated with heat resistance in tabletop Corals (Acropora Hyacinthus) that were repeatedly exposed to heat. Researchers have also discovered that corals can boost the expression of hundreds more genes over a short time after heat exposure. FrontloadingTranscripts and proteins involved ApoptosisResponses to oxidative stress, Heat shock, and the Response to unfolded proteins. That’s the classic form of acclimation, according to Steve Palumbi (marine geneticist at Stanford University) who studies transcriptional changes in corals following heat stress. The environment triggers a transcriptional change that then leads to a change of proteins that in turn causes a change of physiology.
The memory of corals could also be perpetuated by the algal symbionts. For example, a shift within the dominant algal clade could make corals heat-resistant. Ross Cunning, Chicago’s Shedd Aquarium coral biologist, previously showed that Caribbean corals were more heat resistant than those from the Caribbean. Montastraea cavernosa Bleached less oftenWild when its symbionts became dominated by an algal genus DurusdiniumIt can withstand temperatures up to 30 C, which is higher than what would kill other strains. M. cavernosa, Along with two other coral species, BetterWhen inoculated with, laboratory stress tests DurusdiniumIt is more resistant to heat than when it is inoculated by clades that are less heat-tolerant. Only a quarterCunning suggests that coral species can be host to multiple clades. However, those that can, Cunning claims, have symbiont associations as the key to heat tolerance. He suggests that changes in these communities due to heat, such as the outcompetition of heat-tolerant clades, might serve as one basis to environmental memory.
See Corals Show Genetic Plasticity
Epigenetic alterations is another possibility that could underlie corals’ environmental memories. Researchers have also observed changes in gene expression. In 2018, researchers Outplanted Acropora milleporaWe compared corals in different parts of the Great Barrier Reef to determine if their methylation patterns were different in stressful environments. The genes responsible for responding to environmental stressors were more active in those who were exposed to them, while those living in less stressful environments had higher housekeeping genes. Outplanted corals that exhibited methylation patterns that were more like those of local corals were more healthy, based on weight gain and lipid and protein, as well as carbohydrate content. This supports methylations role in acclimation.
Cunning has teamed up with Eirin Lopez on an earlier project. experimentThis suggests that symbionts may cause epigenetic changes within their host. In particular, the researchers changed the dominant symbiont in laboratory animals. M. cavernosaFrom the heat-sensitive CladocopiumTolerant to heat DurusdiniumThe methylation of coral genomes was twice as common. These included intergenic and introns genes, as well transposable elements, which the authors believe may play a part in transcriptional silencing. Cunning believes that there is a complex relationship between the coral’s external environment and its internal environment, which could lead to epigenetic modifications.
MECHANISMS OF MEMORYCorals have shown a remarkable ability to recall their previous exposure to heat stress. This helps them to survive subsequent bleaching events. Scientists are currently studying this phenomenon in the laboratory. They have discovered multiple ways corals can recall previous heat stress exposures. Although the mechanism behind this process is not yet known, corals may be able to pass on their memories to their offspring.  CATHERINE DELPHIA MODIFIED ABY THE SCIENTISTSTAFF |
SYMBIONT SLIPIn response to heat stress, some coral species shift towards a more tolerance clade of symbiotic algae. |
TRANSCRIPTION BOOSTCorals can frontload genes involved in apoptosis and the heat shock response as well as oxidative stress. |
EPIGENETIC CHANGESHeat can sometimes alter the level of DNA methylation within a coral’s genome. |
PREPARED OFFSPRINGPreliminary evidence suggests coral larvae that have experienced bleaching are more able to withstand heat stress. |
Can memory be used to improve restoration goals?
Many coral researchers have a common goal: The ScientistIt is possible to integrate environmental memory into restoration projects. This may be done by stressing corals in a controlled way prior to outplanting them, by planting false memories of stress in the form of durable symbionts, or by breeding species with enhanced capacities to form and retain memory. One day, restoration and conservation scientists may include all three in their coral restoration plans.
Some of these efforts are only beginning. Palumbi explains The ScientistAlthough he hasn’t yet outplanted any stress-hardened corals he has, it is something he hopes to do soon. Erinn Muller is the coral restoration program manager at Floridas Mote Marine Laboratory. She has successfully bred and outplanted thousands of corals each season. She recently gave corals to a colleague, who is now trying out outplanting artificially stressed-primed individuals. We are happy to provide biological material to people who want to learn more about how we do it. [stress-priming]She says that this could be a useful tool.
Experts agree that it is literally a race against the time to improve scientists’ knowledge of coral’s environmental memories. The world is losing 14 percentIts corals have been protected since 2009 and the United Nations predictedBy 2034, all reefs are expected to experience bleaching at least once per year. Ainsworth remembers a time in which studying bleaching meant simulations in the lab, as natural bleaching was not common enough. She says that we are unable to work in the field bleaching fast enough for the events to keep up.
See Image of The Day: Stress-Resistant corals
Many scientists are motivated to act despite not having all the information. There are many studies that can only be done by stress exposure. However, it is possible to study mechanisms in the restoration aspect, according to Serena Hackerott who is a coral reef ecologist at Eirin Lopezs lab. Muller warns that outplanting new strategies will be difficult as many communities are not equipped to raise stress-hardened corals. Muller states that it is always a nightmare to prepare corals for projects that require them to do so months in advance. If we can find a way to do it while still being flexible, that’ll be the key.
Other researchers also tell The ScientistEven if the corals have been stress-hardened, it is impossible to compensate for coral losses by outplanting. Muller is capable of outplanting approximately 30,000 coral colony per year. Hughes, however, notes that the Great Barrier Reef has lost over a billion coral colonies in just 35 million hectares. He also estimates that the cost to restore those reefs will be in the millions of dollars per hectare. He says that the current literature on memory is valuable for what it can teach coral biologists, but not as a way to artificially re-grow coral reefs. [its] feasible.
Researchers agree that conservation efforts must be made quickly, regardless of how they progress. Palumbi says that it is a constant reminder to me that the cost of doing nothing keeps going up. You can view this as a scientific exercise. We want to know all the details before we act. . . If we do not do something, coral reefs will disappear for a long time.
