In the first observation of its kind, a coral
community in the southern inshore region of the Great Barrier Reef is
showing signs of adjusting to higher sea surface temperature by
quickly changing its main algal partners to types that can better cope
with the heat.
An AIMS field study near Miall Island, part of the
Keppel group of 15 islands on the southern Great Barrier Reef off the
Queensland coast near Rockhampton, has revealed a remarkable feat of
acclimatisation; the only time such an event has been observed in
natural conditions on a coral reef.
The work, which appears today in the prestigious UK
scientific journal the Proceedings of the Royal Society*, has shown
that a phenomenon known as "symbiont shuffling" took place after a
bleaching event in 2006 in the Acropora millepora coral
population studied.
According to the lead author on the paper, PhD student
Ms Alison Jones, this means that of the range of algae available to
live in partnership with the corals, the ones best suited to helping
reef-building corals beat the heat have come to dominate.
Corals survive by hosting single-celled algae known as
zooxanthellae in their tissues. It’s the heat tolerance of the algae
that determines the fate of the coral, and the range of temperatures
that can be tolerated by different kinds of zooxanthellae is quite
wide. When the tolerance threshold is reached, zooxanthellae may be
lost from the coral, causing coral bleaching and often the death of
the coral.
The AIMS researchers found that the corals in the
Keppel area now have a much higher proportion of two more thermally
tolerant strains of zooxanthellae living in them than they did before
the 2006 bleaching event, and therefore they are better able to cope
with higher sea surface temperatures.
"There has been a dramatic shift in the Miall Island
coral’s symbiotic community, mainly as a result of the change in the
predominant algal types after bleaching," Ms Jones said.
The researchers sampled and tagged colonies before and
after the bleaching event and were able to show that while before the
2006 bleaching event about 94 per cent of the algae in the corals in
the population were thermally sensitive, after the event about 71 per
cent of the surviving tagged colonies had the more heat-tolerant
strains of algae living in them.
"This work shows that the symbiont communities of
inshore corals such as those in the Keppels are much more dynamic than
we have given them credit for so far," AIMS scientist and co-author Dr
Ray Berkelmans said. "This may give them a natural advantage over
those corals without this flexibility to change predominant symbiont
type."
"We argue that if this shift is sustained and
community wide, the reefs in this area are likely to have
substantially increased their capacity to withstand the next bleaching
event."
The researchers caution against generalising from this
population to the rest of the GBR. Dr Berkelmans said that while this
work was welcome news for the Keppel Island reefs, it could not be
applied across the whole reef. The particular kind of algae examined
in this study is common on inshore reefs such as the Keppels but rare
in Acropora species elsewhere on the GBR.
Coral reefs are extremely vulnerable to climate change
and without rapid adaptation and acclimatisation over the next 50
years, they are unlikely to survive a warming planet.
The incidence of bleaching has been rising on the GBR
(and on reefs around the world) for about a decade as sea surface
temperature warms and global climate changes. Bleaching is seen by
marine scientists as one of the biggest threats to the survival of
coral reefs.
*The Proceedings of the Royal Society B paper, written
by AIMS scientists Alison Jones, Ray Berkelmans, Madeleine van Oppen
and Jos Mieog, with William Sinclair from Central Queensland
University, is titled "A community change in the algal endosymbionts
of a scleractinian coral following a natural bleaching event: field
evidence of acclimatization".
