In the Russian language, the problem Mattison deals with does not even have a name: this process is called either bleaching or fading of corals, copying English bleaching. This is sometimes written about in the news, but the Russian-language Wikipedia does not even have a corresponding article - and yet the problem is huge: this is one of the most noticeable - and destructive - signs of global warming. Fading (bleaching) kills coral reefs - arguably the most beautiful and diverse ecosystems on Earth.
When the color goes away
Unlike mountains or forests, coral reefs have been with us not so long ago: all existing ones are not older than ten thousand years. Their formation began at a time when the last glaciation receded and the water rose, flooding the continental shelf. At this time, people just started to develop agriculture, mammoths lived out their last days. And at the bottom of the warm tropical seas, complex structures began to grow, reaching several meters in height. They gave organic matter in such quantities that thousands of other creatures were enough. Now coral reefs occupy less than 1% of the world's oceans, but they live in a quarter of all species of marine animals.
These designs are the skeletons of coral polyps, organisms related to anemones and jellyfish. Some polyps sit separately, while others (part of anemones) even know how to crawl and dig passages in the seabed. Loners and soft-bodied polyps of reefs do not form, but colonial species with a mineral skeleton build their underwater cities. In this they are helped by their symbionts, photosynthetic algae. Polyps can catch and eat plankton, but the main part of their diet is organics, which is produced by algae living between the layers of the polyp epithelium. These same algae give living corals a characteristic brownish-greenish color.
“Coral reefs are like human cities: life is in full swing there, their inhabitants find partners, food and a home in them, and they all depend on each other.”
As soon as the water temperature rises at least a couple degrees, algae begin to produce a lot of oxygen - more than usual; in such quantities, it becomes dangerous for polyps, and they push algae out of their organisms. This is discoloration: coral with still living polyps, but without algae, becomes painfully pale. Left without symbionts, the polyps begin to starve - and if the algae does not return, the reef eventually turns into the Vereshchaginsky “Apotheosis of War” with dead polyps instead of skulls.
Disturbed balance
For the first time, global fading was noticed in the abnormally hot 1998, then in 2010. The next began four years later and became the largest in the history of observations. It continues to this day. In 2014, Pacific coral reefs north of the Mariana Trench lost their color; in the same year, bleaching affected the reefs of the Hawaiian and Marshall Islands and Florida. When summer came to the Southern Hemisphere, the coral reefs of the Indian Ocean and the Caribbean islands began to fade. By 2016, the algae had left the polyps that form part of the reefs around Madagascar and the Great Barrier Reef. A year later, Australian ecologists sounded the alarm: according to their estimates, two-thirds of all corals of the Great Barrier Reef were bleached.
Coral reefs are able to recover from episodes of discoloration - but very slowly, because imbalance entails a lot of consequences. When symbiotic bacteria go away, intensified reproduction of other microorganisms begins, which prevent young polyps from multiplying. Warming seawater absorbs more and more carbon dioxide; the concentration of carbonate ions in water increases, and mineral salts precipitate more easily; for coral polyps, this means premature mineralization, turning to stone. Together with a nutrient deficiency arising in the absence of photosynthetic algae, and growing (again due to warming) acidity of water, rapid mineralization can ultimately lead to irreversible death of coral.
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