Mysterious neon coral may be releasing its own ‘sunscreen’

The mysterious appearance of dazzlingly colourful coral reefs is a sign that they are fighting to recover, according to researchers.

The onset of the climate crisis and warming oceans is pushing coral reefs to the brink of extinction, as seen by the appearance of white ‘bleached’ coral.

If temperatures rise just one degrees Celsius above the usual summer maximum, coral’s white limestone skeleton shines through its transparent tissue and can eventually lead to the death of the coral. Within a few years, an entire coral reef can break down and much of the biodiversity that depends on its complex structure may be lost.

However, scientists have been puzzled by the appearance of coral that, instead of going white, emits a dazzling range of bright neon colours.

Now, researchers from the University of Southampton have found that these eye-catching colours are actually the result of coral ‘sunscreen’ that it creates, possibly to encourage algae vital to its survival to return. As part of a symbiotic relationship, algae gain shelter, CO2 and nutrients, while the corals receive photosynthetic products to fulfil their energy needs.

“Our research shows colourful bleaching involves a self-regulating mechanism, a so-called optical feedback loop, which involves both partners of the symbiosis,” said Prof Jörg Wiedenmann, who heads the university’s Coral Reef Laboratory.

“However, if the coral cells can still carry out at least some of their normal functions, despite the environmental stress that caused bleaching, the increased internal light levels will boost the production of colourful, photoprotective pigments.”

Acropora corals with colourful bleaching in New Caledonia. Image: The Ocean Agency/XL Catlin Seaview Survey

A mild bleaching

“As the recovering algal population starts taking up the light for their photosynthesis again, the light levels inside the coral will drop and the coral cells will lower the production of the colourful pigments to their normal level,” Wiedenmann added.

The researchers believe corals that undergo this process are likely to have experienced only brief or mild ocean-warming events or disturbances, rather than extreme events. According to Dr Cecilia D’Angelo, this is because bleaching is not always a death sentence for the corals.

“Unfortunately, recent episodes of global bleaching caused by unusually warm water have resulted in high coral mortality, leaving the world’s coral reefs struggling for survival,” she said.

The researchers added that they are encouraged by reports of colourful bleaching in the Great Barrier Reef that occurred during the most recent mass bleaching between March and April of this year. This is because it raises the hope that at least some patches of the famous reef may have a better chance of recovery.