Bleached Acropora millepora. Image: Ray Berkelmans.

A coral community in the southern inshore region of the Great Barrier Reef is showing signs of adjusting to higher sea surface temperature by quickly changing its main algal partners to types that can better cope with the heat.

This provides the first field evidence to support earlier theoretical and laboratory based research at AIMS that had suggested diversity within corals could be their salvation in the event of higher sea surface temperatures causing widespread coral bleaching.

An AIMS field study near Miall Island, part of the Keppel group of 15 islands on the southern Great Barrier Reef off the Queensland coast near Rockhampton, has revealed a remarkable feat of acclimatisation; the only time such an event has been observed in natural conditions on a coral reef.

The work, published in the Proceedings of the Royal Society, has shown that "symbiont shuffling" took place after a bleaching event in 2006 in the Acropora millepora coral population studied.

Corals obtain much of their energy by hosting single-celled algae known as zooxanthellae in their tissues. This partnership of animal and plant is an example of symbiosis; two organisms living together for mutual advantage. This symbiosis becomes unstable at sea water temperatures much above normal when the zooxanthellae are expelled from the coral's tissues in a process known as coral bleaching. There is no one trigger temperature for bleaching because the heat tolerance of the coral-algal symbiosis varies among different strains of zooxanthellae.

The AIMS researchers found that the corals in the Keppel area now have a much higher proportion of two more thermally tolerant strains of zooxanthellae living in them than they did before the 2006 bleaching event, and therefore they are better able to cope with higher sea surface temperatures.

While this was good news for the Keppel Island reefs, it has yet to be detected on a broader scale. The strains of zooxanthellae in this study are common on inshore reefs such as the Keppels they are rare so far in the same corals elsewhere on the GBR.