By Helen Briggs
BBC News Online environment correspondent
The Great Barrier Reef, one of the natural wonders of the world, could be under threat from a new form of algae.
The Great Barrier Reef: A haven to sea creatures
Australian scientists say they are concerned that the blanket of golden algae could smother and kill the coral.
Seaweed-like fronds of the blooms have been seen for the past three summers on reefs north of the Queensland city of Cairns, which is popular with tourists.
Some areas were badly affected, said Dr Britta Schaffelke of the CRC Reef Research Centre in Townsville.
When the algae were removed by hand, the underlying coral was noticeably bleached.
This suggests some of the coral had died, either before or after it was colonised.
Further research was needed to assess how big a threat to the Great Barrier Reef it might be, said Dr Schaffelke.
She told BBC News Online: "Currently, there is hardly anything known about the biology and ecology of this alga, hence, we don't have any evidence on impacts on corals or other organisms.
"We have some indication that it may pose a risk but further research needs to confirm these concerns."
The species, Chrysocystis fragilis, has been dubbed the "golden noodle" alga because of its colour and shape.
It was first found on reefs around the island of Guam in the North Pacific Ocean in 1995.
A similar type of alga was spotted on the Great Barrier Reef in the 1980s but disappeared before it could be classified.
Scientists believe the microscopic photosynthetic organisms may have been newly introduced into the area.
Alternatively, they may have remerged after chronic disturbance of the reefs. Corals have been hit by disease, damage from starfish and bleaching.
The Great Barrier Reef is one of the natural wonders of the world.
It consists of more than 3,000 individual reefs and covers an area of about 300,000 square kilometres off Australia's east coast.
Reefs are formed from thousands of tiny creatures known as coral polyps crowded together to form colonies.
Each polyp lives inside a hard shell, which we recognise as coral.