A unique 'natural laboratory' in the Mediterranean Sea is revealing the effects of rising carbon dioxide levels on life in the oceans. The results show a bleak future for marine life as ocean acidity rises, and suggest that similar lowering of ocean pH levels may have been responsible for massive extinctions in the past.

The scientists, from the University of Plymouth and the University of Santa Catarina, Brazil, studied a single celled organisms called Foraminifera around volcanic carbon dioxide vents off Naples in Italy. The study, published in the September issue of the Journal of the Geological Society, found that increasing CO 2 levels caused foram diversity to fall from 24 species to only 4.

'Previous studies have shown a reduction in diversity of 30%, but this is even bigger for forams', said Dr Jason Hall-Spencer, one of the study's co-authors. 'A tipping point occurs at mean pH 7.8. This is the pH level predicted for the end of this century'.

Rising carbon dioxide levels acidify the ocean, which has a particularly devastating effect on organisms that have calcium carbonate shells, like Foraminifera.

'Forams are well preserved in the fossil record, which is why we chose to study them', says Dr Hall-Spencer. 'We knew the results were likely to show a decline in foram diversity but we weren't expecting such a seismic shift'.

Forams record past events in the geological record -- in particular, the effect of the Palaeocene-Eocene Thermal Maximum (PETM), a period of massive carbon release and rapid warming, 55 million years ago, accompanied by extinctions in marine life. It is also thought to have seen a period of ocean acidification.

'That was a period when massive changes in marine ecology happened' says Dr Hall-Spencer. 'Our natural laboratory provides a glimpse into the future of our oceans'.

'These are the first CO 2 vents to be used to study ocean acidification. They allow us to observe how ecosystems react to changes in ocean acidity. We can see for our own eyes what increasing CO 2 levels do to marine communities'.

'At a mean pH level of 7.8, calcified organisms begin to disappear, and non calcifying ones take over. We are headed towards that being the case in this century. The big concern for me is that unless we curb carbon emissions we risk mass extinctions, degrading coastal waters and encouraging outbreaks of toxic jellyfish and algae.'

Co-author Professor Malcolm Hart are presenting the research to the 2010 FORAMS meeting in Bonn on Friday 6 September. This weekend, Deborah Wall-Palmer will present the work being done at Plymouth on the last 250,000 years of ocean acidification at the International Palaeoceanography Conference in San Diego, being held on 29 August -- 3 September.