New research by the University of Southampton in the United Kingdom has explained an unusual process known as "colorful bleaching" that corals use as an emergency recovery strategy to fight against environmental changes.
The Ocean Agency describes the process as a "chilling, lovely and heartbreaking" final cry for help as the coral attempts to grab the algae's attention.
Other species of the coral community can display different colors during those episodes of bleaching.
United Kingdom scientists say internal light levels as a result of the bleaching triggers the production of colourful, "photoprotective" pigments.
Coral animals symbiotically coexist with tiny algae, providing them with shelter, nutrients and carbon dioxide in exchange for their photosynthetic powers.
'Bleaching is not always a death sentence for corals, the coral animal can still be alive, ' said Dr Cecilia D'Angelo, lecturer of molecular coral biology at the University of Southampton.
Warmer waters can cause bleaching in corals, turning the coral's tissue in a ghostly white color and eventually killing it.
'If the stress event is mild enough, corals can re-establish the symbiosis with their algal partner.
Once its live tissue is gone, the skeleton is exposed to the eroding forces of the environment.
This Eerie Neon Glow Coming From Bleached Coral Could Actually Be Good News
In a mass bleaching occasion, as has been occurring in latest months alongside stretches of Australia's Great Barrier Reef, inside a number of years the reef can break down inflicting vital biodiversity loss. But "colorful bleaching" has the opposite effect: the dying corals gain more pigment, and glow in shades of bright pink, purple and orange.
The researchers conducted a series of controlled laboratory experiments at the coral aquarium facility of the University of Southampton. They found that during colourful bleaching events, corals produce what is effectively a sunscreen layer of their own, showing itself as a colourful display.
These pigments offer an alternative to the glare of white light during bleaching that can deter the algae from ever returning.
"Our research shows colorful bleaching involves a self-regulating mechanism, a so-called optical feedback loop, which involves both partners of the symbiosis", lead researcher Professor Jörg Wiedenmann of the University of Southampton said in a press release. In the absence of the all-important algae to absorb light, it bounces around inside the coral tissue to give it its white appearance, But when these corals are able to continue some of their normal functions, this internal light instead boosts the production of colorful photoprotective pigments, which act a lure for the returning algae.
In healthy corals, much of the sunlight is absorbed by the photosynthetic pigments of the algae. This increased internal light level is very stressful for the symbionts and may delay or even prevent their return after conditions return to normal. The new analysis, revealed on Thursday within the journal Current Biology, discovered that some coral produces its personal "sunscreen" layer with a objective to entice algae to return.
As the recovering algae start absorbing light for photosynthesis again, light levels inside the coral drop, and so the coral stops producing as much of these colourful pigments.
The scientists believe that corals exhibiting this behavior have most likely experienced mild or short-lived warming events, rather than the type of extreme and drawn-out temperature rises that threaten much of the world's reefs.
Reports suggest that colourful bleaching occurred on some parts of the Great Barrier Reef in March and April 2020, so some patches of the world's largest reef system may have better prospects for recovery after the recent bleaching.
Scientists emphasized that while colorful bleaching is a good sign, only a significant reduction of greenhouse gases globally - in addition to improvement in local water quality - can save coral reefs beyond this century.