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A VIRUS could be to blame for the bleaching of coral reefs around the world,
explaining why some are more susceptible than others to rising sea
temperatures.

Warmer waters damage corals鈥� symbiotic algae, which normally provide energy
for their hosts by photosynthesis. The El Ni帽o climate fluctuation of
1997 and 1998 caused the most extensive coral bleaching yet recorded,
devastating reef systems in the Indian Ocean and Caribbean.

But now there is evidence suggesting that the symbiotic algae may harbour a
virus that makes the corals vulnerable to changing sea temperatures. If so, it
may explain why only some corals fall victim to bleaching.

Willie Wilson and his team at the Marine Biological Association in Plymouth
placed a relative of coral, Anemonia viridis, in water heated to 32
掳C and kept it there for 24 hours. The anemone usually lives in temperate
waters at 15 掳C. At the higher temperature, a virus began to rapidly
multiply in the symbiotic algae, 鈥渂usting open the cells鈥�, says Wilson. He
thinks the virus normally lies dormant in the algae鈥檚 DNA. Other types of
environmental stress, such as ultraviolet light, also woke up the virus.

Wilson says the finding raises the possibility that a similar virus is
involved in coral bleaching. Anemones and corals are more closely related than
they appear鈥攂oth are cnidarians. More importantly, they can share the same
strains of symbiotic algae.

James Porter, a coral reef expert at the University of Georgia, is excited by
the suggestion that viruses might cause coral bleaching in warmer temperatures.
The study shows the dire consequences鈥攑otentially for an entire
ecosystem鈥攐f stressing an organism, he says.

But Robert Buddemeier at the University of Kansas says we need further
evidence to make the jump from a viral disease in a temperate anemone species to
tropical corals. Large temperature hikes may affect temperate species more
strongly, he says.