Saturday 19 November 2016

On The Origin Of Coral Symbiosis

The beautiful colours of corals come from dinoflagellate
algae living within their tissues in a symbiotic relationship
Ever since the advent of cameras capable of filming underwater, there have been a surfeit of nature documentaries full of beautiful, vibrant images of coral reefs. Not only are corals morphologically diverse, they often display an extraordinary range of colours. This results from algae which live within the coral tissue and form symbiotic relationships with the animals. Ecological studies have shown that these symbioses have proved critical to the success of corals in recent geological history, hence why the phenomenon of coral bleaching - the loss of the algae under stressful conditions - is a worrying prospect for future biodiversity. It has been less clear, however, when this relationship first arose and how it may have contributed to the success of corals in the past.

The algae, specifically dinoflagellates, have little in the way of a fossil record, let alone one which can be conclusively linked to corals. Instead an international team of researchers looked at the ratios of isotopes of nitrogen, carbon and oxygen, as dinoflagellates have a significant impact on ammonium uptake and carbonate deposition in the coral tissue. Studying modern corals, they found that symbiosis correlated with a low ratio of 14N to 15N. Polished sections of corals from 210 million year old rocks from Antayla, Turkey, were then examined in the same fashion.

'Although algae were not present in the fossils, they left behind chemical signatures,' said Xingchen Wang from Princeton University. 'We found strong evidence that the fossilized coral were symbiotic and that they lived in a nutrient-poor environment. We were able to link the environmental conditions from 200 million years ago to the evolution of corals.'

Polished sections of 210 million year old, well
preserved corals from Antayla, Turkey
Corals are generally assumed to indicate shallow, sunlit marine conditions, although deep water, darker forms are known. This study helps confirm that the Antayla corals did hail from such an environment, and provides a methodological framework for investigations of dinoflagellate symbiosis in other coral fossils.

Corals suffered a catastrophic decline during the PT mass extinction 250 million years ago, along with virtually all other marine clades. In the wake of this event, modern scleractinian corals evolved. This clade expanded rapidly 205 million years ago, suggesting that symbiosis may have driven their rise. This highlights the importance of preventing coral bleaching in order to help preserve the biodiversity of not just corals, but the reef habitats they form, habitats which are increasingly under threat from human activity.