Acid wash helped the corals to return symbionts

Unusual bright coloration, which occurs in some corals in response to thermal stress increases the chances of the return of symbiotic algae. To such conclusion came the authors published in the journal Current Biology. They believe that “colorful discoloration” may be a useful marker indicating the areas reefs are likely to recover after heat stress. In addition, it will be possible to identify reefs are suffering from pollution with nitrogen fertilizers.

When the temperature of the water rises above a critical threshold, the corals lose endosymbiotic algae and discolored, which can lead to their death. Because of global warming it happens more and more often: for example, the Great barrier reef off the coast of Australia only over the past five years has experienced three mass bleaching event, the last of which occurred quite recently, in March-April of 2020. Many experts fear that because of rising temperatures, most coral reefs will disappear in the coming decades.

Some corals when bleaching behave unusually. Instead of just to become white, they become extremely bright color — green, red, or purple. The basis of this process is the accumulation of pigments that resemble green fluorescent protein and in normal conditions are analogous to sunscreen for corals and their symbionts. Until now, scientists could not know what has caused this “colorful discoloration” and does it some kind of adaptive function.

A team of specialists under the leadership of jörg Wideman (Jörg Wiedenmann) of the University of Southampton were able to resolve this mystery. After laboratory experiments with three species of coral and analyzing data from field observations, the scientists found that the colorful discoloration is helping damaged coral to recover after the loss of photosynthesizing symbionts.

In the absence of algae, the sunlight penetrates deep into the tissues of the coral and is reflected from its white skeleton. In response to increasing light flux of the surviving cells begin to produce large quantities of protective pigments, which makes the coral and brightly colored. Development of “sun layer” does not allow light to penetrate deeply and increases the chances of the return of symbiotic algae. If this happens, the synthesis of pigments is slow and the color coral from the unusually bright is once again becoming normal.

The analysis revealed, colorful discoloration mainly occurs in response to weak and short-lived heat stress, whereas during more severe conditions, the corals bleach white in the classic scenario and are more likely to die. However, the temperature was not the only factor that can stimulate the coral enhanced production of protective pigments. Sometimes they get unusual coloration at higher levels of nitrogen as a result of pollution by fertilizers.

According to the authors, colorful discoloration is a useful marker that will help in the preservation of coral reefs. For example, it will allow you to determine the areas which with high probability can recover from heat stress. And the presence of corals with unusual coloring on a healthy reef will point to pollution problems.

Global warming is forcing the corals to adapt, but it’s too slow process. Some scientists hope to hasten it by introducing new strains of symbiotic algae that can survive at high temperatures. Some of them feel comfortable even at 31 degrees Celsius, although it is usually already killed at 27.

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