Coral bleaching a survival ploy
2001-06-14 10:55
Paris - Coral bleaching, widely feared to be a sign of catastrophic damage
inflicted by global warming, is in fact a smart move by coral to
help it survive climate change, a researcher says.
Environmentalists have taken up bleaching as a symbol of global
warming, contending that warmer seas are killing off the
greenish-brown algae that grow within coral.
These algae are "symbionts" - they both live off and nourish the
colonies of millions of tiny creatures called coral polyps that
make up a reef's complex, beautiful plant-like structures.
If the algae disappears, the coral turns white, which is presumed
to be a sign of severe damage to the reef or its imminent death.
But Andrew Baker, of the Osborn Laboratories of Marine Science at
New York Aquarium, suggests a different reason for the bleaching.
He does not dispute that sudden, shocking increases in sea
temperature can mortally damage reefs, as evidenced among tropical
island nations during the 1997-8 El Nino cycle.
But, he suggests, bleaching may be an evolutionary attempt to cope
with such change.
He transplanted eight species of Caribbean coral at 61 sites in the
San Blas archipelago in Panama, taking them from deep water into
shallow water and vice versa.
Deep-water corals that were transplanted into shallow water showed
more signs of bleaching that those that went the other way.
This is unsurprising, as shallower water is warmer than deeper
water, and ultraviolet light, which can be damaging to
micro-organisms, is stronger at shallower depths.
But what startled Baker was that these "upward" transplants
survived far better than their "downward" counterparts in spite of
their bleaching.
None of the 24 "upward" corals had died a year after they were
transplanted, even though 11 had suffered partial or extensive
bleaching.
However, the death rate among the "downward" corals was seven out
of 37, none of which had had any bleaching.
Baker then looked at the DNA fingerprint of algae on the
transplanted coral.
He found a major redistribution and concentration of types of algae
on the "upward" transplants, but found no change at all among algal
inhabitants among the "downward" transplants.
In other words, by bleaching, the corals that had been moved to
warmer, shallower waters ditched unwanted algae so that they could
be swiftly colonised by "alternative algae" which were better able
to survive the harsher conditions, he suggests.
Bleaching as a long-term act of survival also resolves a "paradox"
about corals, says Baker, writing in Thursday's issue of Nature,
the British science weekly.
It explains how these long-lasting, complex yet fragile systems
survived previous bouts of sea warming that have occurred over a
time scale of tens of millions of years, before any climate change
was man-made.
"Bleaching may ultimately help reef corals to survive the recurrent
and increasingly severe warming events projected by current climate
models of the next half-century," Baker adds.
"Bleaching is an ecological gamble in that it sacrifices short-term
benefits for long-term advantage. This counters conventional wisdom
that bleaching is detrimental from all perspectives, and supports
the role of symbionts as adaptive agents." - Sapa-AFP
- SAPA