What is killing reefs and how people are trying to stop it?

Coral polyps cannot exist without single-celled symbiodinium algae to feed them - however, due to global warming, the symbiosis is disintegrating, and the previously colorful reefs are bleaching and dying. Is there a chance of survival for the planet's brightest underwater ecosystems?

So, coral polyps are representatives of the class Anthozoa (Anthozoa) from the type of cnidaria (Cnidaria). The latter means that corals are close relatives of, for example, jellyfish and sea anemones. However, unlike them, some corals have a strong exoskeleton made of calcium carbonate. Such species are called hard, or reef-building, in contrast to soft ones that do not have external "armor". Inside such a “house” there are polyps, which, in fact, represent the body of the organism. Reef-building corals obtain most of their energy from single-celled symbiont algae (Symbiodinium) living in their soft tissues. Symbiodiniums receive protection in exchange for giving their owners the right to use the products of their photosynthesis.

This is a very important feature of reef-building corals, and we will definitely return to it later.

"Classic" tropical reefs are very demanding on environmental conditions. They need a temperature of about 26-27 ° C, clear water that is not desalinated by rivers flowing into the sea or precipitation, as well as a sufficient amount of light so that the algae can photosynthesize. The latter limits the spread of corals in depth - most species are not able to exist below 50 meters. Of course, there are exceptions - some species that do not have zooxanthellae grow even at depths of about 300 meters in the coastal waters of Norway - but most species still cannot survive in the dark.

Due to their selectivity, reefs occupy only 0.1% of the area of ​​the oceans - but, despite this, they are one of the most important ecosystems in the world. About 25% of all existing ocean fauna is found here - from starfish to sharks. These are colorful oases in the blue sea desert. They also benefit people - the fish we eat spawn and feed here, tourists dive with scuba diving, contributing to the GDP of coastal countries, and the reefs themselves shield the coast from the destructive elements - hurricanes and tsunamis. In recent years, however, these humble ocean engineers have suddenly found themselves on the brink of extinction. And along with them, the whole complex mosaic of ocean ecosystems is in danger of collapsing.

Imagine a dead city. Peeling paint on the walls of houses. Empty streets, once full of people, now completely deserted. A place that life left forever. This is exactly the picture that scientists began to observe almost simultaneously on coral reefs around the world in the 1980s. The first and most important symptom, immediately striking, was the complete absence of bright colors in the affected areas: the once colorful corals seemed to be washed with bleach. White "skeletons", between which defenseless fish scurried in confusion, stretched for many kilometers. The phenomenon has received a simple but capacious name - coral bleaching, that is, bleaching.

It didn't take long to solve the riddle as a first approximation. Remember symbiodiniums - the very algae that settle in the tissues of a polyp in exchange for the products of their own photosynthesis? They provide corals not only with food: the bright color of the latter is also a consequence of cohabitation with microscopic symbionts. However, with an increase in the average annual water temperature, discord begins to occur in interspecific relations. Coral exposes algae "out the door" of his own house, becoming white as chalk. For some time he is still able to live on his own - however, not for long. Over millions of years of living together, reef-building corals have typically lost the stinging cells needed to catch prey on their own. Left without an algae-nurse, such a "domestic disabled person" is doomed to starvation. Moreover, the development of many pathogens - for example, the bacterium Vibrio shiloi, which infects the polyps of the species Oculina patagonica - also accelerates with increasing temperatures, finishing off reefs that have not yet recovered from the “parting”. And, as you might guess, the main role here was played by global warming, which has already set everyone on edge.

Identifying the cellular mechanisms responsible for the bleaching process has proven to be much more difficult. Of all the proposed versions, the so-called oxidative stress has become the most convincing. Laboratory studies have shown that high temperatures disrupt the enzymes involved in the photosynthetic cycle. This not only made photosynthesis less efficient, but also caused the accumulation of so-called ROS, reactive oxygen species, in the cell. Under this phrase, oxygen ions (oxygen atoms with an electrical charge), peroxide molecules (for example, H2O2) and free radicals (particles that have one or more unpaired electrons on the outer electron shell) are hidden.


All ROS easily enter into chemical reactions. They are formed in living cells all the time, as products of normal oxygen metabolism, but are held back by antioxidants. Moreover, they may even play an important role in some of the normal life processes of the cell. But in response to stress, they begin to accumulate in excessive amounts, damaging the cell, and often even leading to its death.
Algae, whose photosynthetic systems were disrupted by heat, began to actively produce these most reactive oxygen species, and then release them into the environment, that is, into the tissue of the coral that sheltered them. Polyps, shocked by the sudden toxicity from the symbionts, were forced to put them "out the door", thereby driving themselves into a trap and dooming them to starvation. Of course, there have been cases when algae, “thinking about their behavior”, re-populated the tissues of bleached coral - but this is more of a rarity than the norm. Scientists agree that it is impossible to let the situation take its course - without human help, most reefs will disappear very soon. Hope for salvation came from the Eilat Gulf of the Red Sea. Despite the fact that the average annual temperatures there are rising as rapidly as elsewhere, no one has seen cases of bleaching on the reefs there. The perplexed scientists decided to test the heat-resistant corals in the laboratory, and the results were stunning - polyps did not die even at temperatures four degrees above normal! What's more, these "super corals" were doing better than usual in extreme heat! It is believed that this is a consequence of a complex evolutionary history: by colonizing the Red Sea from south to north, polyps have adapted to both colder and hotter conditions. Considering that by the end of the 21st century, global temperatures will rise by two or three degrees, it is quite possible that by about 2100 the Red Sea reefs will become the last surviving "bastions". But it's not just that: if scientists can figure out exactly how algae avoid oxidative stress, they might be able to "introduce" them to bleaching-prone corals elsewhere in the world. Another option is the creation of transgenic symbionts that combine the resistance of Red Sea species with the best properties of algae from other regions. However, this is still far away: so far the world is warming up, and experts, caring people and reef residents are watching with excitement how the most important relationships in the ocean are falling apart. While bleaching is a serious enough problem, it is not the only thing that threatens reefs. Decreased biodiversity, accumulation of carbon dioxide in the atmosphere and chemical pollution also contribute to the death of the Earth's most important ecosystems. Some of these problems are more local, others are more global, but people are to blame for all of them in one way or another. many pathogens - for example, the bacterium Vibrio shiloi, which infects the polyps of the species Oculina patagonica - also accelerates with increasing temperatures, finishing off reefs that have not yet recovered from the “parting”. And, as you might guess, the main role here was played by global warming, which has already set everyone on edge.



Take at least the crown of thorns (Acanthaster planci) - large starfish that live on the reefs of the Indo-Pacific. These echinoderms, which look like a frying pan studded with poisonous thorns, are the main natural enemies of reef-building corals. One crown can destroy up to 13 m2 of reef per year. Due to the low nutritional value and toxic thorns, the injection of which is dangerous even for humans, almost no one eats the stars themselves. The only gourmet is the large charonia clam (Charonia tritonis). Unfortunately, charonias were not lucky to have large beautiful shells, which were once highly valued by collectors and tourists. Due to uncontrolled fishing in the last century, mollusks were on the verge of extinction, which immediately took advantage of crowns of thorns. They began to multiply uncontrollably, causing huge damage to hundreds of reefs - even the famous Great Barrier off the coast of Australia suffered. In order to curb the occurrence of starfish, the charonium was taken under protection in most countries. And Australian divers have made it a rule to take syringes with formalin, which is deadly for crowns, into diving. Even the Queensland University of Technology did not stand aside and developed a special underwater drone with a syringe called COTSBot (from the English name for the Crown-of-thorns Starfish crowns).