快猫短视频

FEARS that global warming will kill off the world鈥檚 coral reefs may be
misplaced.

Instead of signalling impending doom, the widespread bleaching of reefs may
be a risky gamble by corals to adapt to warmer seas, according to a marine
biologist in New York. When the going gets tough, corals ditch their symbiotic
algae to find partners better suited to their new surroundings.

Reef-building corals contain single-celled algae that photosynthesise in
return for their lodging. But this partnership is precarious鈥攊f light or
temperature change for any length of time, the corals often evict their algae.
As the lodgers contain the photosynthetic pigments that give corals their
colour, reefs are left bleached.

Events such as El Ni帽o that wash warmer-than-usual water around the
oceans can trigger dramatic bleaching. The rising sea surface temperatures
predicted during the next century may also produce paler reefs
(快猫短视频, 4 November 2000, p 24).

To study the effects of bleaching, Andrew Baker of the New York Aquarium in
Brooklyn moved corals around a reef in the San Blas Islands, off Panama鈥檚
Caribbean coast. Baker took corals living around 20 metres down to spots just a
couple of metres beneath the surface, where they would have far more light and
warmth. He also moved corals from these shallow sites to the deep locations.

Roughly half of the corals shifted into shallower water were partially or
severely bleached after eight weeks, while all those moved to deeper water kept
their algae. But one year later Baker was surprised to find that all the
upwardly mobile corals survived despite the bleaching, whereas several of those
that had been moved to deeper water had died.

Some corals take on different algal lodgers, depending on the depth. Baker
found that corals moved to shallower sites switched to shallower strains of
algae after bleaching. The change could help the corals to survive in the long
term, he proposes. By jettisoning their existing algae they may have found
partners better adapted to the new conditions, he suggests.

The corals moved to deeper waters did not bleach because they were less
stressed, he believes. The bleaching strategy is risky, however, because the
corals rely heavily on their symbionts and have no guarantee of finding a better
match for their new environment.

鈥淚t is an interesting and somewhat controversial hypothesis,鈥� says Simon
Cripps of the World Wide Fund for Nature鈥檚 International Marine Programme in
Geneva. 鈥淚f correct, it suggests the corals use a very risky strategy of out
with the old and in with the new to survive.鈥� Last year, WWF and the World
Conservation Union recommended that damage from pollution and fishing should be
reduced to help corals recover. But 鈥渃limate change events are likely to cause
more serious and frequent bleaching,鈥� Cripps says. The 1997 El Ni帽o
killed many corals, for instance.

But Baker argues that as a desperate measure, bleaching may ultimately help
coral reefs survive rising sea temperatures. Although coral reefs are
ecologically fragile, they have already survived some dramatic climate changes.