Several common species of seaweed are serial coral killers, scientists have discovered.

The weeds despatch coral with deadly chemicals when the two come into contact on the sea floor.

Killer seaweed is now seen as another threat to coral reefs already suffering from the effects of pollution and climate change.

Over-fishing is allowing the seaweed to proliferate and become an increasing danger to coral, say scientists.

Mark Hay, from the Georgia Institute of Technology in the US, said: "The evolutionary reasons why the seaweeds have these compounds are not known.

"It may be that these compounds protect the seaweeds against microbial infection, or that they help compete with other seaweeds. But it's clear now that they also harm the corals, either by killing them or suppressing their growth."

A team led by Prof. Hay compared the fate of two different types of coral when placed next to common seaweeds from Pacific and Caribbean reefs.

In as little as two days, corals in contact with some seaweed species had bleached and died.

Other seaweeds took up to 20 days to harm the coral or caused no damage. Ultimately 70 per cent of the seaweed species studied had harmful effects when they were in direct contact with coral.

Similar damage was seen when the scientists exposed corals to chemicals extracted from the seaweeds.

"In all cases where coral had been harmed, the chemistry appeared to be responsible for it," said Prof. Hay, whose research appears in the journal Proceedings of the National Academy of Sciences.

The coral species used in the experiments - Porites porites from Panama and Porites cylindrica from Fiji - are among the hardiest known.

The researchers suspect that other more vulnerable species may be even more dramatically affected by seaweed.

In the Caribbean, five of seven seaweeds studied caused coral bleaching while the same damage was inflicted by three of eight Pacific species.

Seaweeds are kept in check by plant-eating fish whose populations are dwindling as a result of over-fishing, the scientists pointed out.

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