'Engine of the reef' revealed

A team of coral researchers has taken a major stride towards revealing the workings of the 'engine' that drives Australia's Great Barrier Reef and corals the world over.

The science has critical importance in understanding why coral reefs bleach and die, how they respond to climate change - and how that might affect humanity, researchers say.

Scientists at the ARC Centre of Excellence for Coral Reef Studies, James Cook University and the University of Queensland have compiled what they claim is the world's first detailed gene expression library for Symbiodinium, the microscopic algae that feed the corals - and so provide the primary energy source for the entire reef.

"Symbiodinium uses sunlight to convert CO₂ into carbohydrates for the corals to feed on. At the same time there's evidence the corals control its output, suggesting that they are farming their captive plants," Professor David Yellowlees explains.

"But these microscopic algae are quite weird and unlike any other life form. They have different photosynthetic machinery from all other light harvesting organisms. They have 100 times more DNA than we do and we have no idea why such a small organism needs so much. They really are like no other living creature we know."

Symbiodinium not only rules the fate of the world's coral reefs - it also plays a significant role in soaking up carbon dioxide from the atmosphere, turning it into nourishment for the corals and powering calcification. Its decline would not only kill the reefs but accelerate CO₂ build-up.

Dr Bill Leggat says the team has focused particularly on understanding the biochemical relationship between Symbiodinium and corals when they are stressed by heat, light, increased CO₂ levels and pollutants from land run-off.

These stressful conditions cause corals to 'bleach' by expelling the Symbiodinium and - if they do not recover them within a few days - the corals die.

"Our aim is to identify the genes that make the symbiotic plants susceptible to these stresses and lead to the coral expelling them," Leggat said.

In experiments at Heron Island Research Station they exposed corals to various stresses associated with climate change and then analysed the gene composition in the symbiotic algae. Another team analysed the effects in corals.

Working together and using the micro-array technology, they hope to assemble a picture of the 'chemical conversation' that goes on between the corals and its symbiotic plants that leads to a breakdown in the relationship, a divorce - and the corals starving themselves to death.

"An example of the challenge we face is the gene which is expressed the most when Symbiodinium is stressed. It's obviously important - but at this stage we have no idea what it does. It is even stranger when you consider that this gene was originally acquired from a bacterium," Yellowlees said.

So far the team has identified about 4500 genes in Symbiodinium, compiling them into what claims to be the world's first gene expression library for this symbiotic organism. It is hoped this will have value for understanding other symbiotic relationships in nature.