How do I survive the heat waves

Climate change: Some corals even survive extreme heat waves

In 2015 and 2016, an extensive sea heat wave hit the Kiritimati Atoll, which is part of the Pacific island nation of Kiribati. It was one of the longest phases with above-average warm water that has been recorded by scientists. And the consequences for the coral reefs around the island were devastating, because the corals were bleached across the board: a clear sign of stress that in the long term can lead to the reefs dying. According to a study by Danielle Claar from the University of Victoria and her team in Nature Communications, however, some of the bleached corals were surprisingly resilient and recovered while the water was still heated.

In their long-term study, the scientists found more than 100 corals of the two species around the atoll Platygyra ryukyuensis and Favites pentagona marked and examined during and after the heatwave. At the beginning of the crisis they all lived in a symbiosis with heat-intolerant algae, which supplied them with nutrients. However, if the water temperatures exceed a threshold value, the corals repel such algae and thereby bleach. In the reefs, however, there are also heat-tolerant algae, with which at least some algae can enter into a new partnership, as Claar and Co have observed.

After the corals had absorbed these symbionts, they recovered, although the external conditions were actually not yet optimal again. However, this did not happen everywhere, but only in areas that were practically untouched and in which there was no further exposure to humans. In the vicinity of settlements, for example, the corals examined had heat-tolerant symbionts even before the event. representative of Favites pentagona had thereby no survival advantage, and individuals from Platygyra ryukyuensis survived even 3.3 times worse than conspecifics in other areas of the reef.

In addition to global warming, coral reefs are damaged, for example, by the entry of fertilizers or suspended matter or mechanically by ships and their anchors. Catching reef fish can change species spectra, which also has a negative impact on corals. The more such factors occur, the greater the likelihood that a reef will irreversibly die in a heatwave. Since coral bleaching will increase due to climate change, the researchers are pushing for the other influences to be pushed back as far as possible.