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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), the original title: "remember injury, adapt to injury, resist injury: the way to survive in extreme environments", author: XZ

Human beings have lost a lot in the three years of the epidemic, and the creatures of nature have also withstood the severe test of extreme weather. Once beautiful, dead and lifeless corals are struggling to get back to life by "remembering" environmental stress and passing on these memories to future generations.

The Great Barrier Reef (The Great Barrier Reef) is the largest and longest coral reef group in the world. It runs through the northeast coast of Australia, rising from the Torres Strait to the south of the Tropic of Cancer, stretching for a total of 2011 kilometers. There are 2900 coral reef islands of all sizes. The natural landscape is very special, so it is listed on the World Natural Heritage list. It is home to about 1500 species of colorful tropical fish, 4000 species of molluscs, 400 species of sponges and 300 species of hard corals. The Great Barrier Reef is not only the most popular tourist attraction in Australia, but also a scenic spot for many large areas, attracting about 2 million tourists every year.

Great Barrier Reef, Picture Source: Baidu Encyclopedia, however, no matter how gorgeous the scenery is, there is a moment when it loses its luster, and the Great Barrier Reef is experiencing such a crisis. According to the latest report of Australia's Great Barrier Reef Marine Park Service (GBRMPA) [1], more than 90 per cent of corals in the Great Barrier Reef were bleached in 2022, the sixth large-scale bleaching of the Great Barrier Reef since 1998 and the fourth in the past seven years.

Where does the color of coral come from? Coral polyps (Coral) are invertebrates, belonging to the phylum coelenterate (or phylum acanthozoa). Coral is the shell (also known as "bone") secreted by coral polyps, which is mostly dendritic with longitudinal stripes. Each single coral has concentric circular and radial stripes on the cross section, often white in color, and a small amount of blue and black. Of course, living corals are not only these shells, but also the coral polyps that live in them. Coral reefs are communities of corals that account for 25% of the total marine life and are one of the largest ecosystems in the world. Therefore, the coral reef ecosystem in the ocean has a biodiversity comparable to that of the terrestrial rainforest and plays a vital role in maintaining the balance of the marine ecosystem.

Why do corals have colorful colors? It starts with the symbiotic relationship of corals. There are often many symbiotic algae microbes in corals, including a class of symbiotic algae called Zooxanthellae, which give corals a variety of colors. Xanthophyta produces glucose and other organic matter through photosynthesis, provides more than 60% of nutrients and energy for coral growth, and helps corals metabolize carbon dioxide, produce calcium carbonate, and form coral bones; in turn, corals provide nitrogen, phosphorus and other nutrients for Xanthophyllum to survive. In fact, most corals themselves are white, but when Xanthophyta containing different pigments enter the coral, the coral tissue presents a colorful and beautiful scene.

The colorful coral Toby Hudson of the Great Barrier Reef was photographed on July 24, 2010. Source: wikipedia when symbiotic algae leave or die, colorful corals turn white. This is the "bleaching phenomenon" of corals. Rising ocean temperatures have reduced the amount of seaweed on which corals depend, making them more vulnerable to bleaching and eventually get sick or die. When the albinism is serious, the three-dimensional spatial structure of the coral reef collapses, the number of biological species foraging, hiding, dwelling and breeding here will be greatly reduced, and the coral reef sea area will change from an "oasis" to a "desert" state, which will cause great damage to the marine ecosystem.

The report released by the Great Barrier Reef Marine Park Service [1] is more serious this year than the albinism that occurred in 2016, 2017 and 2020. In this report, the researchers surveyed 719 coral reefs using low-altitude flight and found that 654 (91%) had varying degrees of albinism.

Albino coral reefs, deathly desolate: Baidu Encyclopedia Great Barrier Reef Marine Park Administration said that climate change is the biggest threat to coral reefs, and only by reducing global greenhouse gas emissions can we limit the impact of climate change on coral reefs. It is reported that the sea temperature of the Great Barrier Reef began to rise gradually in December last year, until the end of March this year, its water temperature exceeded the all-time high, which brought tremendous thermal pressure on the coral reef, causing large corals to lose their color and bleach; if the albinism persists, then these corals are likely to die as a result of losing their food source for a long time.

The scientific exploration in the albino crisis is not only the Great Barrier Reef. At present, coral reefs all over the world are facing more and more frequent and serious warming events. The rising sea temperature makes coral organisms expel algal symbiotes, causing catastrophic albinism. But scientists have noticed that some coral reefs repair themselves after bleaching events, and the restored reefs are more adaptable to temperature. This phenomenon is called "environmental memory", in which organisms respond to abiotic stimuli in the past, which is similar to immune memory in the immune system-when the body first encounters an antigenic substance, it remembers it. When it encounters the same antigenic substance again, it reacts quickly to maintain the stability of the body. Therefore, it seems to be a good strategy to improve coral bleaching through "environmental memory".

The Florida reef group experienced high summer temperatures in 2014 and 2015, leading to a series of bleaching events on nearby shallow coral reefs. The star-shaped corals were determined to be dead by researchers in September 2014, but four years later, the corals survived the high temperatures of 2015 and returned to health four years later. ? source: NOAA coral reef watch

After heat stress, corals are bleached; after recovery, the first evidence of coral "environmental memory" appeared in 1994, when Barbara Brown, a marine biologist at Newcastle University in the UK, noticed that the east side of the stony coral community (Coelastrea aspera) in the high temperature environment was bleached, while the west side was not. Brown believes that it may be that the sun coverage on the west side is higher than that on the east side, and its high temperature tolerance is stronger. Since then, Brown published his findings in Nature [3], confirming that this hypothesis is correct.

By 2000, to further verify the environmental memory hypothesis, Brown changed the westward side of the coral to the east. Ten years later, when the albinism occurred again, Brown found that the albinism on the eastern side of the reef was much lower than that of the control group, and there were more symbiotic algae microbes. This further shows that although the eastern corals have lived in low light for 10 years, they still retain the "memory" of high light and have a certain high temperature tolerance [4].

With these early experimental data, researchers began to gather evidence of "environmental memory" in coral reef systems on a larger scale. Two studies based on the Great Barrier Reef have found that pre-exposure of corals to heat stress can reduce their whitening in future high temperatures.

In the first study [5], the researchers analyzed sea surface temperature data over the past three decades and found that after thermal stimulation of corals in advance, when the sea water temperature exceeded the whitening threshold, the coral mortality and symbiotic algae loss rate could be reduced by 50%.

In the second study [6], the researchers found that the 3000 separate coral reefs that make up the Great Barrier Reef had no anomalies in the following 14 years after the 2002 albinism event, while large-scale albinism occurred in 2016. But just a year later, in 2017, rising global temperatures caused only 50% of coral reefs to bleach, significantly lower than the massive bleaching that occurred in 2016. This shows that the protective effect of pre-exposure to high temperature is time-limited, that is, the "environmental memory" of corals is effective for a certain period of time, and if 14 years apart, such memories may no longer exist.

So far, most of the studies on the mechanism of "environmental memory" are limited to the observation of coral bleaching, and the characteristics of "memory" have not been explored. To explain the molecular mechanism of "environmental memory", scientists have studied corals at the cellular and molecular levels. In 2014, researchers found that the heat tolerance of hyacinth antler coral (Acropora hyacinthus) was closely related to variations in more than 100 loci on its chromosome. In addition, corals can increase the expression of hundreds of genes within hours to weeks after heating, and the expression of these genes can significantly improve 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 transcriptional level of heat-resistant genes in corals, causing changes in protein levels, resulting in changes in physiological properties.

A 2017 study [8] also confirmed the existence of adaptive responses. The researchers found that the gene expression of mustard shore coral (Porites astreoides) was malleable in a variety of environments. In other words, they can change gene expression in order to adapt to environmental changes. The researchers swapped 15 inshore coral communities with 15 offshore coral communities to observe their growth in new habitats. A year later, through the analysis of gene expression profiles, the researchers found that the gene expression of the previous inshore corals had changed and grew well in the new environment, while the related stress genes of the previous offshore corals were hardly up-regulated and could not adapt well to the new environment, resulting in large-scale albinism.

How is this adaptive response formed? The answer is: the level of gene expression has changed.

An important factor affecting gene expression is epigenetic. That is, the DNA sequence did not change, but the gene expression changed heritable. In a study published in 2018 [9], scientists planted porous antler coral (Acropora millepora) in both warm and cold parts of the Great Barrier Reef and found different DNA methylation patterns between the two corals-DNA methylation shuts down the activity of certain genes, while demethylation induces gene reactivation and expression. Corals that grow at high temperatures downregulate methylation levels of genes related to environmental changes, making them more active, making corals more responsive to environmental changes, while corals that grow at low temperatures, because they do not need to cope with high temperatures, up-regulate the expression of butler genes (genes necessary to maintain basic biological functions). This further confirmed the role of DNA methylation in adaptation to the environment.

In addition to corals' own responses to environmental changes, corals' algal symbiotes may also play a role, such as the transformation of dominant algae that may make corals more heat-resistant. Research by Ross Cunning, a coral biologist at the Shepherd Aquarium in Chicago, shows that when the symbiote of star coral (Montastraea cavernosa) is dominated by algal Durusdinium, the coral is less bleached. Durusdinium can survive at a temperature higher than 30 ℃ and has strong heat tolerance. After the transplantation of Durusdinium algae into three other corals, the heat tolerance of these corals was significantly enhanced. Cunning believes that after heat stress screening, 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 study in 2022 found that symbiotes can affect the level of methylation in the host, resulting in changes in gene expression in the host. Specifically, when the researchers replaced the heat-sensitive Cladocopium symbiote in the coral with the heat-resistant Durusdinium symbiote, some areas of the coral's genome were methylated. Therefore, scientists believe that there is a very complex relationship between the external environment and the internal environment of corals, which is mediated by symbiotes, which may drive the epigenetic modification of corals, but the specific mechanism remains to be explored.

If memory can be inherited, will corals pass on environmental memory to future generations? Hollie Putnam of the University of Rhode Island (University of Rhode Island) specializes in the intergenerational inheritance of coral "environmental memory". Putnam studied hermaphroditic branched corals (Pocillopora damicornis), which can hatch young internally. Researchers exposed parental branched corals to high temperature and acid for six weeks before the cub was born, and found that their cubs were more adaptable to high temperatures and acidity than their parents who had not experienced environmental stress [12]. In addition, some studies have shown that the algal symbionts of corals may also have intergenerational genetic characteristics.

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. Symbiotic evolution: when some corals are subjected to heat stress, their symbiotic algae evolve to heat-resistant algae.

two。 Transcriptional enhancement: corals promote the transcriptional activity of heat-resistant genes when subjected to heat stress.

3. Epigenetic modification: DNA methylation patterns in coral genomes change under heat stress; in addition, symbiotic algae also change DNA methylation in coral genomes.

4. Heritability: the offspring of parents under heat stress are more adaptable to high temperature environment.

Many coral researchers say the ultimate goal of the Environmental memory study is to integrate it into the coral restoration operation. For example, coral repair can be achieved by putting pressure on coral in a controlled manner before transplanting it, or by implanting heat-resistant symbiotic algae microbes. Scientists have just begun to try these methods, and although they have not yet planted any stress-resistant corals, the moment will come soon.

This is a race against time. The world has lost 14% of its corals since 2009, and the United Nations predicts that all coral reefs will experience bleaching at least once a year by 2034.

The rate of bleaching of coral reefs has prompted many scientists to take action in the absence of complete information. In response, coral reef ecologist Serena Hackerott said: "from a restoration point of view, there are many studies that only need to expose corals to environmental pressure (such as high temperature) and do not really need to study the mechanism." However, implementing the new transplantation strategy will also face challenges, as most communities around coral reefs simply do not have the infrastructure to produce pressure-resistant corals on a large scale or in a timely manner. In addition, even if the coral is domesticated under environmental pressure, it cannot make up for the loss of the coral population. The Great Barrier Reef alone has lost more than 1 billion corals in nearly 35 million hectares of sea area since 2016, while researchers can grow up to about 30000 corals a year, which is far from enough. But in any case, the coral restoration work still has to go on, and if we do nothing, we will lose the coral reef forever.

Marine rainforest coral reefs are experiencing more and more serious albino crises, and we have an unshirkable responsibility on land. The climate change caused by Greenhouse Effect is the direct factor leading to coral bleaching, so it is urgent to solve the climate problem. Therefore, in order to protect the beautiful coral landscape and the entire marine ecosystem, we must raise environmental awareness and formulate corresponding environmental protection strategies to solve the climate change brought about by Greenhouse Effect. The colorful world in the Great Barrier Reef, whether witnessed or viewed through photos or images, must be amazed and fascinated by its colorful natural scenery. If we want our children, our children's children, to have a chance to feel the grandeur of the Great Barrier Reef, we have to act immediately, starting with the little things that save energy, before we have a chance to reverse this endangered situation. recreate the splendor of the ocean and the Great Barrier Reef.

reference

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

2.Hackerott S, Martell H A, Eirin-Lopez J M. 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, B.E., Dunne, R.P., Edwards, A.J. 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, ScienceAdvances, 5, 10, (2019).

6.Hughes, T.P., Kerry, J.T., Connolly, S.R. 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 Ting 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 U S A. 2018 bot 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 TX 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 Bing 218 (Pt 15): 2365-2372.

13.Quigley, K.M., Willis, B.L. & Kenkel, C.D. 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=email&_hsmi=213217506&_hsenc=p2ANqtz--BANWdSc4seOEDA11aDxG_ATegEP1ONZdGGKBET7ro3IOqwI7P1R_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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