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Chemical Technology • May 2016
T
he Great Barrier Reef comprises thousands of
reefs, over 600 types of hard and soft coral, and
an almost endless variety of fish and sea-life. It
is also worth over US$6 billion to Australia in tourism.
The cause of the corals’ dying is coral bleaching. Here’s
how it happens. Corals are tiny marine invertebrates
living in vast compact colonies of polyps. A particular
coral group forms a coherent structure and creates a
recognisable pattern. The polyps are a few millimetres in
width and maybe a centimetre or two in length with a hard
exoskeleton secreted at its base. Over generations, these
secretions build up on themselves and become vast reefs.
The corals themselves depend for their survival on
a symbiotic relationship with photosynthetic unicellular
flagellates which live within their tissues. These are called
zooxanthellae. Corals require sunlight so that the zooxan-
thellae can photosynthesise, and water temperature that
is not too warm or too cold.
There are corals that don’t rely on this symbiotic rela-
tionship, and they can grow at much greater depths and
in the absence of sunlight. Recently, a vast and previously
unknown reef was discovered beneath the very murky wa-
ters where the Amazon river stretches out into the ocean.
The colour you see in a healthy reef is mostly as a result
of the zooxanthellae living in the tissues of coral polyps.
When the water temperature changes too much, or the
amount of light is diminished, the zooxanthellae die. This
leads to dramatic colour loss across the affected reef.
Coral bleaching indicates that the coral itself is not dead
yet, but it is starving. If the zooxanthellae do not return, then
the reef is doomed. The Great Barrier Reef has experienced
bleaching events, of increasing severity, every few years co-
inciding with the El Niño current, which raises temperatures.
However, this current has become more frequent, and of a
higher temperature, as a result of global warming.
In March of this year, Professor Terry Hughes of James
Cook University, flew over 600 km of reef. More than 60 %
of it was bleached. These bleaching events, while devastat-
ing, also lead to major structural changes across the reef.
A reef that fails to recover will be colonised by other types
of algae and sea weeds. This impacts on the fish which used
to depend on the reef and consequently leads to major
species loss. This phenomenon is known as a ‘phase-shift’
to a reef dominated by macroalgae (as compared to the
microalgae of the zooxanthellae).
This does not always happen, however, and scientists
have been attempting to figure out why. For starters, there
is a very wide range of species differentiation between cor-
als with some surviving post-bleaching events significantly
better than others. They use the space created to colonise
dying reefs and come to dominate. There are also herbivores
which eat the macroalgae and hold back growth until the
reef can recover.
The question about how to encourage coral survival is
more than just about tourists on their summer vacations.
Over 500 million people in 100 countries depend on coral
reefs for their food and livelihoods.
One organisation working on coral restoration and reme-
diation is the Coral Reef Targeted Research and Capacity
Building for Management (CRTR) Program based at the
University of Queensland. They rear nearly 10 000 nursery-
grown colonies for reef transplants every year. The problem
with the nursery-led approach of CRTR is that transplanted
corals have a higher mortality rate and grow more slowly
than do the reefs where they are cemented. Scientists’
concern is that they still know very little about the life-cycle
and biology of specific species.
In American Samoa, lagoons full of coral survive 35 °C
Rehabilitation
in a time of
coral bleaching
by Gavin Chait
Divers consider the Great Barrier
Reef, off the far northern coast of
Queensland, Australia, to be one of the
greatest destinations for viewing coral
anywhere in the world. At 2 300 km
long, the system is the largest living
thing on earth, and it is dying.