On the trail of a mysterious phenomenon: Researchers have uncovered why damaged corals sometimes develop a brilliant color instead of fading. It is a sign that they are fighting deadly coral bleaching. The pigments are said to provide sun protection for the stressed algae partners of the coral polyps so that these microsymbionts remain or return.
The earth’s coral gardens are fascinating natural wonders and form key elements in the complex communities of the oceans. Builders of the filigree underwater worlds are tiny things: the lime frameworks are the work of myriads of small coral polyps that capture plankton with their fine arms. However, many types of coral also feed on a symbiosis with algae. The unicellular organisms live in the body of the cnidarians and gain energy there from sunlight. They give the polyps some of the carbon compounds formed and in return they get everything else they need to live.
A ghostly spectacle
The destruction of this partnership is reflected in the so-called coral bleaching: If the algae are affected by excessive water temperatures or other stress factors, they leave the coral polyps. As a result, the white limestone skeleton of the corals begins to shine through its transparent fabric. If this process is strong and long-lasting, the coral polyps will eventually die, leaving only the pale “bones”. The coral bleach is now laying more and more like a shroud over the reefs of the earth, warn researchers. This is due to the rising water temperatures as a result of global warming.
However, coral death still poses some riddles for researchers. A mysterious phenomenon is an astonishing color development, which scientists have found in some coral stocks affected by bleaching: instead of increasingly losing their normal color, these corals develop various, even more intense colors. The researchers led by Jörg Wiedenmann from the University of Southampton have now devoted a study to this effect. To do this, they carried out a series of experiments in aquariums. They subject corals to various stress factors, just as they do in the nature of bleaching, to investigate when the phenomenon occurs. They then analyzed which pigments the color reactions are based on.
Sun protection for sensitive partners
As they report, their results indicate that the phenomenon occurs in corals, which, in contrast to extreme damage, have experienced episodes of mild or brief sea warming or other disturbances. The polyps then react with the formation of the color pigments. According to the researchers, it is apparently a sunscreen that is supposed to prevent additional stress on the algae and possibly encourage them to return. “In healthy corals, a large part of the sunlight is absorbed by the photosynthetic pigments of the algal symbionts. When the corals lose their symbionts, however, the light is increasingly reflected from the white coral skeleton into the tissue of the polyps, ”explains Wiedenmann. This effect thus increases the internal light level and is very stressful for the symbionts. It can therefore further impair their living conditions or delay or even prevent their return after normalization of the conditions, the scientists say.
However, if the coral cells still have some of their normal abilities despite the environmental stress that caused the bleaching, they can form the sunscreen pigments. If the algae population can then recover and the symbionts use the light for their photosynthesis again, the excessive radiation inside the coral polyps decreases and the cells reduce the production of the colored pigments to their normal level, the scientists explain.
It is therefore an encouraging sign when it suddenly becomes colorful in a damaged reef. Wiedenmann and his colleagues also report on current observations in this context: During the large-scale coral bleaching in some areas of the Great Barrier Reef in March-April 2020, the colored phenomena also occurred in some areas. So there is hope that at least some spots on the world’s largest reef system will have a chance of recovery, the scientists say. However, they emphasize that ultimately only a significant reduction in greenhouse gases at a global level and a sustainable improvement in water quality at a regional level can save the coral reefs.
Source: University of Southampton, technical article: Current Biology, doi: 10.1016 / j.cub.2020.04.055