Remember the harm, adapt to the harm, resist the harm: how to survive in extreme environments

Humanity has lost a lot in the three-year pandemic, and creatures in nature are also experiencing the harsh test of extreme climate. Those once beautiful, dead, lifeless corals are struggling to regain their vitality by "remembering" environmental pressures and passing these memories on to their offspring.

Written by | XZ

The Great Barrier Reef is the world's largest and longest coral reef. It runs along the northeast coast of Australia, from the Torres Strait to the south of the Tropic of Capricorn. It stretches for a total of 2,011 kilometers and has 2,900 coral reef islands of various sizes. The natural landscape is very special and is listed in the World Natural Heritage List. There are about 1,500 colorful tropical fish, 4,000 mollusks, 400 sponges and 300 hard corals living here. The Great Barrier Reef is Australia's most popular tourist attraction and the location for many blockbuster films, attracting about 2 million tourists every year.

Great Barrier Reef | Source: Baidu Encyclopedia

However, even the most beautiful scenery will eventually lose its luster, and the Great Barrier Reef is experiencing such a crisis. According to the latest report from the Great Barrier Reef Marine Park Authority (GBRMPA) of Australia [1], more than 90% of the corals in the Great Barrier Reef experienced bleaching in 2022. This is the sixth large-scale bleaching event in the Great Barrier Reef since 1998 and the fourth in the past seven years.

Where does coral get its color?

Coral is an invertebrate belonging to the phylum Cnidaria (also known as the phylum Cnidaria). Coral is the outer shell (also called "skeleton") secreted by coral polyps. It is mostly tree-like in shape with vertical stripes. Each single coral cross section has concentric and radial stripes. The color is usually white, but there are also a small amount of blue and black. Of course, living corals are not only these shells, but also the coral polyps living in them. Coral reefs are communities composed of individual corals. The organisms living here account for 25% of the total number of marine organisms, making it one of the largest ecosystems in the world. Therefore, the coral reef ecosystem in the ocean has a biodiversity comparable to that of tropical rainforests on land, and plays a vital role in maintaining the balance of the marine ecosystem.

Why do corals have so many colors? This starts with the symbiotic relationship of corals. There are often many symbiotic algae microorganisms in the coral body, among which there is a type of symbiotic algae called zooxanthellae, which gives corals colorful colors. Zooxanthellae produce organic matter such as glucose through photosynthesis, providing more than 60% of nutrients and energy for coral growth, and help corals metabolize carbon dioxide, produce calcium carbonate, and form coral skeletons; in turn, corals provide nutrients such as nitrogen and phosphorus to zooxanthellae to enable them to survive. In fact, most corals are white in themselves, but when zooxanthellae containing different pigments enter the coral body, the coral tissue will present a colorful and beautiful scene.

Colorful corals of the Great Barrier Reef photographed by Toby Hudson on July 24, 2010 | Source: Wikipedia

When the symbiotic algae leave or die, the colorful corals will turn white, which is the "bleaching phenomenon" of corals. As the ocean temperature continues to rise, the algae that corals rely on will decrease, making the corals more susceptible to bleaching and eventually become sick or die. When bleaching is severe, the three-dimensional spatial structure of the coral reef collapses, and the number of biological species that feed, hide, live and reproduce there will be greatly reduced. The coral reef area will change from an "oasis" to a "desert", which will cause huge damage to the marine ecosystem.

Report released by the Great Barrier Reef Marine Park Authority[1]

The bleaching of the Great Barrier Reef this year is more severe than the bleaching events that occurred in 2016, 2017 and 2020. In this report, researchers used low-altitude flight methods to survey 719 coral reefs and found that 654 (91%) had varying degrees of bleaching.

Bleached coral reefs, desolate and deadly | Image source: Baidu Encyclopedia

The Great Barrier Reef Marine Park Authority said that climate change is the biggest threat to coral reefs, and only by reducing global greenhouse gas emissions can the impact of climate change on coral reefs be limited. It is reported that the sea water temperature of the Great Barrier Reef has gradually increased since December last year, and until the end of March this year, its water temperature exceeded the historical high, which brought huge thermal stress to the coral reefs, and large areas of corals lost their color and bleached; if the bleaching phenomenon continues, then these corals are likely to die due to the long-term loss of food sources.

Scientific exploration of the albinism crisis

It's not just the Great Barrier Reef. Currently, coral reefs around the world are facing more frequent and more severe warming events. Rising sea temperatures cause coral organisms to expel algal symbionts, causing catastrophic bleaching. But scientists have noticed that some coral reefs will repair themselves after experiencing bleaching events, and the repaired coral reefs are more adaptable to temperature. This phenomenon is called "environmental memory", that is, organisms respond to past abiotic stimuli, which is similar to immune memory in the immune system - when the body encounters a certain antigen for the first time, it will remember the substance, and when it encounters the same antigen again, it will respond quickly to maintain the stability of the body. Therefore, it seems to be a good strategy to improve the bleaching of corals through "environmental memory".

The Florida Keys experienced high summer temperatures in 2014 and 2015, which resulted in a series of bleaching events on nearby shallow coral reefs. These star corals were deemed dead by researchers in September 2014, but four years later, these corals survived the high temperatures of 2015 because they had experienced the high temperatures of 2014, and recovered four years later. 丨Image source: NOAA coral reef watch

When corals experience heat stress, they will bleach; after they recover, they will have a certain resistance to heat stress.[2]

The first evidence of coral "environmental memory" appeared in 1994, when Barbara Brown, a marine biologist at Newcastle University in the UK, noticed in the wild that a colony of stony coral (Coelastrea aspera) exposed to high temperatures bleached on the east side, but not on the west side. Brown believed that this might be because the west side had a higher sunlight coverage than the east side, and therefore had a stronger ability to withstand high temperatures. Brown later published his research results in Nature [3], confirming that this hypothesis was correct.

In 2000, in order to further verify the authenticity of the environmental memory hypothesis, Brown changed the side of the coral that originally faced west to face east. Ten years later, when bleaching occurred again, Brown found that the degree of bleaching on the east side of these corals that changed their orientation was much lower than that of the control group, and there were more symbiotic algae microorganisms. This further shows that although the corals on the east side have lived in a low-light environment for 10 years, they still retain their "memory" of high light and have a certain tolerance to high temperatures [4].

With these early experimental data, researchers are beginning to gather evidence of “environmental memory” in coral reef systems on a larger scale. Two studies based on the Great Barrier Reef found that pre-exposing corals to heat stress can reduce their severity of bleaching in future high-temperature environments.

In the first study[5], researchers analyzed nearly three decades of sea surface temperature data and found that by giving corals thermal stimulation in advance, coral mortality and loss of symbiotic algae could be reduced by 50% when seawater temperatures exceeded the bleaching threshold.

In a second study[6], researchers found that after the bleaching event in 2002, the 3,000 individual coral reefs that make up the Great Barrier Reef did not show any abnormalities in the following 14 years, but in 2016, large-scale bleaching occurred. However, just one year later, in 2017, rising global temperatures only caused bleaching of 50% of the coral reefs, which was significantly lower than the large-scale bleaching that occurred in 2016. This shows that the protective effect of pre-exposure to high temperatures is time-limited, that is, the "environmental memory" of corals is effective within a certain period of time, and if there is a gap of 14 years, then this memory of corals may no longer exist.

The mechanism of “environmental memory”

So far, most studies have been limited to the observation of coral bleaching and have not explored its "memory" characteristics. In order to explain the molecular mechanism of "environmental memory", scientists have conducted research on corals at the cellular and molecular levels. In 2014, researchers found [7] that the heat tolerance of Acropora hyacinthus was closely related to the variation of more than 100 gene loci on its chromosomes. In addition, within a few hours to a few weeks after being heated, corals can increase the expression levels of hundreds of genes, and the expression of these genes can significantly enhance the body's ability to resist heat stress. Steve Palumbi, a marine geneticist at Stanford University, said that this is a typical "adaptive response", that is, heat stress changes the transcription level of coral heat-resistant genes, triggers changes in protein levels, and ultimately leads to changes in physiological performance.

A 2017 study [8] also confirmed the existence of adaptive responses. The researchers found that in diverse environments, the gene expression of the mustard coral Porites astreoides is plastic. In other words, they can change gene expression in order to adapt to environmental changes. The researchers exchanged 15 nearshore coral colonies with 15 offshore coral colonies and observed their growth in the new habitat (Note: the environmental pressure faced by nearshore corals is greater than that of offshore corals). A year later, the researchers found through gene expression map analysis that the gene expression of the previous nearshore corals had changed and they grew well in the new environment; while the related stress resistance genes of the previous offshore corals were almost not upregulated and could not adapt well to the new environment, and eventually large-scale bleaching occurred.

How did this adaptive response come about? The answer is: changes in gene expression levels.

An important factor affecting gene expression is epigenetics. That is, the DNA sequence does not change, but gene expression undergoes heritable changes. In a study published in 2018[9], scientists planted Acropora millepora in warm and cold areas of the Great Barrier Reef and found that there were different DNA methylation patterns between the corals in the two areas - DNA methylation can shut down the activity of certain genes, while demethylation induces the reactivation and expression of genes. Those corals growing in high-temperature environments downregulated the methylation levels of genes related to the body's response to environmental changes, making their expression more active, and the corals were better able to cope with environmental changes; while those corals growing in low-temperature environments, because they do not need to cope with high-temperature environments, upregulated the expression of housekeeping genes (genes necessary to maintain basic biological functions). This further confirms the role of DNA methylation in adapting to the environment.

In addition to the corals themselves responding to environmental changes, the corals' algal symbionts may also play a role. For example, the shift in dominant algae may make the corals more heat-resistant. Research by Ross Cunning, a coral biologist at the Shedd Aquarium in Chicago, shows [10] that when the star coral (Montastraea cavernosa) symbionts are dominated by the algae Durusdinium, the coral has a lower degree of bleaching. Durusdinium algae can survive at temperatures above 30°C and have strong heat resistance. After transplanting Durusdinium algae to three other corals, the heat resistance of these corals was significantly enhanced. Cunning believes that after screening by heat stress, the survival rate of heat-resistant algae is much higher than that of other algae, which also improves the adaptability of corals with heat-resistant algae to heat stress.

So how do symbiotic algae microorganisms improve the heat tolerance of corals? A 2022 study found [11] that symbionts can affect the methylation level of the host, thereby changing the host's gene expression. Specifically, when the researchers replaced the heat-sensitive Cladocopium symbiont in the coral with the heat-resistant Durusdinium symbiont, certain regions of the coral genome became methylated. Therefore, scientists believe that there is a very complex relationship between the external and internal environments of corals, mediated by symbionts, which may drive epigenetic modifications in corals, but the specific mechanism remains to be explored.

If memory could be inherited

Do corals pass on their environmental memories to their offspring? Hollie Putnam of the University of Rhode Island in the United States has studied the transgenerational inheritance of coral "environmental memory." Putnam used the hermaphroditic branching coral (Pocillopora damicornis) as her research object: this coral can hatch its young internally. Before the young were born, the researchers exposed the parent branching corals to a high temperature and acidic environment for six weeks and found that their young were more adaptable to the high temperature and acidic environment than the parental young that did not experience environmental stress [12]. In addition, there are also studies showing that the algal symbionts of corals may also have transgenerational inheritance characteristics [13].

Four mechanisms of coral environmental memory[14]

So far, the research on the mechanism of "environmental memory" can be summarized into the following four points:

1. Symbiont evolution: When some corals are subjected to heat stress, their symbiotic algae will evolve into heat-resistant algae.

2. Transcription enhancement: When corals are subjected to heat stress, they promote the transcriptional activity of heat-resistant genes.

3. Epigenetic modification: Under heat stress, the DNA methylation pattern in the coral genome will change; in addition, symbiotic algae will also change the DNA methylation in the coral genome.

4. Heritability: Offspring of parents that have been subjected to heat stress are more adaptable to high temperature environments.

Restoring corals, a race against time

Many coral researchers say the ultimate goal of studying environmental memory is to incorporate it into coral restoration efforts. For example, coral restoration could be achieved by stressing the corals in a controlled manner before transplanting them, or by implanting heat-resistant symbiotic algae microorganisms. Scientists are just beginning to try these methods, and while no stress-resistant corals have been grown yet, that moment will come soon.

It’s a race against time. The world has lost 14% of its coral since 2009, and the United Nations predicts that by 2034, all coral reefs will experience at least one bleaching event per year.

The speed of coral bleaching has prompted many scientists to take action in the absence of complete information. In this regard, coral reef ecologist Serena Hackerott said: "From a restoration perspective, there are a lot of studies that only need to expose corals to environmental stress (such as high temperature) to complete the restoration, without really studying the mechanism." However, implementing new transplantation strategies will also be challenging because most communities around coral reefs simply do not have the infrastructure to produce stress-resistant corals on a large scale or in a timely manner. In addition, even if corals that have been acclimated to environmental stress are transplanted, they cannot make up for the loss of coral colonies. Since 2016, the Great Barrier Reef alone has lost more than 1 billion coral colonies over nearly 35 million hectares of ocean, and researchers can only plant about 30,000 coral colonies each year at most, which is far from enough. But in any case, coral restoration work must continue. If nothing is done, we will lose the coral reefs permanently.

The coral reefs in the marine rainforest are experiencing increasingly severe bleaching crises, and we on land have an unshirkable responsibility. Climate change caused by the greenhouse effect is the direct cause of coral bleaching, and solving the climate problem is urgent. Therefore, in order to protect the beautiful coral landscape and the entire marine ecosystem, we must raise environmental awareness, formulate corresponding environmental protection strategies, and solve the climate change caused by the greenhouse effect. The colorful world in the Great Barrier Reef, whether seen in person or viewed through photos or images, must be marveled and fascinated by its colorful natural landscape. If we want our children and our children's children to have the opportunity to experience the magnificence of the Great Barrier Reef, we must take immediate action and start with small energy-saving things, so that we have a chance to reverse this endangered situation and restore the magnificent scenery of the ocean and the Great Barrier Reef.

References

1.https://www2.gbrmpa.gov.au/learn/reef-health

2. Hackerott S, Martell HA, Eirin-Lopez JM. Coral environmental memory: causes, mechanisms, and consequences for future reefs[J]. Trends in Ecology & Evolution, 2021.

3.Brown, B., Dunne, R., Goodson, M. et al. Bleaching patterns in reef corals. Nature 404, 142–143 (2000)

4.Brown, BE, Dunne, RP, Edwards, AJ et al. Decadal environmental'memory'in a reef coral?. Mar Biol 162, 479–483 (2015).

5. Diego K. Kersting, Cristina Linares, Living evidence of a fossil survival strategy raises hope for warming-affected corals, Science Advances, 5, 10, (2019).

6.Hughes, TP, Kerry, JT, Connolly, SR et al. Ecological memory modifies the cumulative impact of recurrent climate extremes. Nature Clim Change 9, 40–43 (2019).

7.Bay RA, Palumbi SR. Multilocus adaptation associated with heat resistance in reef-building corals. Curr Biol. 2014;24(24):2952-2956.

8.Kenkel, C., Matz, M. Gene expression plasticity as a mechanism of coral adaptation to a variable environment. Nat Ecol Evol 1, 0014 (2017).

9.Dixon G, Liao Y, Bay LK, Matz MV. Role of gene body methylation in acclimatization and adaptation in a basal metazoan. Proc Natl Acad Sci US A. 2018;115(52):13342-13346.

10.Silverstein RN, Cunning R, Baker AC. Tenacious D: Symbiodinium in clade D remain in reef corals at both high and low temperature extremes despite impairment. J Exp Biol. 2017;220(Pt 7):1192-1196.

11. Rodriguez-Casariego JA, Cunning R, Baker AC, Eirin-Lopez JM. Symbiont shuffling induces differential DNA methylation responses to thermal stress in the coral Montastraea cavernosa. Mol Ecol. 2022;31(2):588-602.

12.Putnam HM, Gates RD. Preconditioning in the reef-building coral Pocillopora damicornis and the potential for trans-generational acclimatization in coral larvae under future climate change conditions. J Exp Biol. 2015;218(Pt 15):2365-2372.

13. Quigley, KM, Willis, BL & Kenkel, CD Transgenerational inheritance of shuffled symbiont communities in the coral Montipora digitata. Sci Rep 9, 13328 (2019).

14.https://www.the-scientist.com/infographics/infographic-how-corals-remember-the-past-prepare-for-the-future-69646?_ga=2.28750398.369808563.1663584145-1709206126.1655523097

Related links

1.https://www.gbrmpa.gov.au/the-reef/reef-health

2.https://www.the-scientist.com/news-opinion/great-barrier-reef-suffers-sixth-mass-bleaching-in-two-decades-70018?utm_campaign=TS_DAILY_NEWSLETTER_2022&utm_medium=emai l&_hsmi=213217506&_hsenc=p2ANqtz--BANWdSc4seOEDA11aDxG_ATegEP1ONZdGGKBET7ro3IOqwI7P 1R_XAxiuo2yn16XENoIz_YlEhoMCrkVp9kRS-aJaog&utm_content=213217506&utm_source=hs_email

3.https://www.aims.gov.au/news-and-media/reef-recovery-window-after-decade-disturbances

4. https://www.the-scientist.com/news-opinion/corals-show-genetic-plasticity-32563

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