THE key to better understanding coral bleaching has surfaced in a common red alga that produces defensive compounds against the phenomena affecting reef environments worldwide.

University of New South Wales PhD candidate Alexandra Campbell told the recent Ecological Society of Australia conference that the seaweed Delisea pulchra, found around southern Australia including WA, produced defensive chemicals called furanones.

These compounds protect the seaweed from being fouled by other organisms growing on it or shading it.

Ms Campbell said the bleaching effects observed in tropical corals since the mid-1990s also affected D. pulchra.

"The disease does not kill the seaweed, but makes it much less healthy. They grow less, photosynthesise less and reproduce less.

"The pathogens have an impact on the plant's health and productivity, rather than killing them."

The UNSW team observed a ‘bleaching’ phenomenon in natural populations of the seaweed in which affected individuals lost pigment on localised areas of their body.

Bleaching is prevalent during summer when water temperatures are high. Bleached individuals have significantly low furanone levels.

Ms Campbell said researchers believed there was a bacterial pathogen behind D. pulchra's infections, the bacterium Ruegeria strain R11 identified in bleached seaweed.

At higher temperatures, the bacterium became virulent. In field experiments, bleaching was induced in plants damaged by simulated herbivore and disease attack.

However, D. pulchra is able to defend itself against colonisation and invasion by bacterial pathogens by producing the furanones that prevent them invading the plant surface.

Ms Campbell, with colleagues funded under an Australian Research Council grant, said the algae, as with coral reefs, also provided an important habitat for a variety of marine species.