While speculation continues over the impact of global climate change on
the world’s tropical reefs, Dr Anya Salih, marine biologist with
Australia’s University of Sydney, is working to identify the capability
of the Great Barrier Reef to adapt to the warming of the world’s oceans.
“When corals are stressed by changes like increased water temperature or
exposure to sunlight, they can lose their colours or ‘bleach’,” says
Anya. “Bleaching, is a sign that the coral is unwell. It may recover but
if the bleaching is too severe it will die.”
Over the last 20 years, a rise in mass coral bleaching throughout the
world has been linked to the increased frequency of El Nino Southern
Oscillation - a phenomenon that, in turn, has been linked to global
warming.
“As a result, the world’s mean ocean temperature has increased by about
0.5 degrees,” says Anya, “and it is predicted that it will warm by
another one to three degrees worldwide by the middle of this century.
“For corals, a change as little as one to three degrees can lead to
bleaching so we are greatly concerned about their survival.”
When stressed, a coral will expel the zooxanthellae - the microscopic
algae or tiny plants that live in its tissue - causing the loss of
colour known as “bleaching”. Zooxanthellae use photosynthesis to
transform sunlight into energy and help corals to survive by creating an
extra source of carbohydrates or energy.
“We had already noticed that some corals didn’t bleach and some did
so only partially,” said Anya, “[and] during night dives, I had also
observed that some coral colours had glow-in-the-dark or fluorescent
properties.”
“I wondered if there could be a relationship between the two phenomena.”
Dr Anya Salih
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To investigate, Anya and her team joined the crew of Undersea Explorer - a
combined scientific research and adventure dive vessel based in Port
Douglas, Queensland - for an expedition to the remote outer regions of
the Great Barrier Reef.
Anya estimates that about 70 to 90 per cent of corals in shallow waters
have the fluorescent colour pigments with the number decreasing to about
50 per cent at lower depths.
“These coral’s fluoresce continually under sunlight but the sunlight is
so strong you can’t see it,” she says.
“We believe that the fluorescent pigments in some coral tissues act like
a kind of sunscreen by dissipating excessive sunlight as fluorescence
before it damages the zooxanthellae.”
By placing special light filters on their underwater torches at night,
Anya and her team identified a range of fluorescent coral shades from
intense blues to brilliant reds, and most significantly, they found that
fluorescent corals did seem to survive bleaching better.
Back in the lab, Anya explains: “During fluorescence, the electrons
inside the coral’s colour pigment molecules become excited. They jump
higher and emit energy. The blue colours come from high energy
emissions, then as the energy decreases the colours change to greens,
then yellows and reds. Basically, the coral pigment cells break up solar
radiation into the same energy wavelengths seen in the colours of the
rainbow and by absorbing higher energy - UV and blue light - and
emitting it in green to red fluorescent colours, they transform damaging
light into less energetically damaging wavelengths.”
In the long term, Anya hopes their research will be used to help protect
coral from the damaging effects of continued global warming.
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