Researchers from the University of Oviedo offer new data on the relation between the greenhouse effect and an increase of the temperatures in a study published by Nature and titled "Late Miocene threshold response of marine algae to carbon dioxide limitation". The study offers evidences of a very close link between the decrease of atmospheric CO2 and the cooling and glaciations during the geological past, in a period that was not fully clear for scientists: between 10 and 2 million years ago. This discovery is the first to prove that the greenhouse effect brought about an increase in the temperatures during this period, which was warmer than today's climate and had the range of concentrations of CO2 expected for the end of the century. The research was conducted by analyzing the history of the adaptation of marine algae to the increasing levels of CO2, which take place much faster than what was previously thought. The study, authored by researchers Clara Bolton and Heather Stoll, of the Department of Geology of the University of Oviedo, has been funded by the European Research Council.

The amount of carbon dioxide in the atmosphere is central to climate, because it regulates the greenhouse effect. It is also central to all the earth´s plants – on land and in the sea – because it is the key ingredient used for photosynthesis. When CO2 levels are low, photosynthesis may not proceed as rapidly, so plants have developed coping mechanisms. Many marine algae use and transport supplementary "fuels", other more abundant forms of carbon in the ocean like bicarbonate (the form of carbon in baking soda) for photosynthesis. But this approach takes extra energy and nutrients. So algae might be expected to stop this extra accumulation when CO2 increases.

The researchers developed a new indicator to track when algae forego using one of these extra sources of carbon. The result sheds light on both the adaptation of algae to CO2 as well as the history of CO2 in the atmosphere. Because some algae make microscopic shells that accumulate on the sea floor, just like clams make shells that accumulate on the shore, it is possible to use these fossil shells to understand how ancient algae coped with CO2 levels when they were alive.

The preview, published in Nature, has only been possible after discovering the history of the adaptation of marine algae to different concentrations of CO2.

A new model of how algal cells transport carbon shows there is a change in the chemical makeup of the shells when the cell needs to use supplementary "fuels" like bicarbonate to grow. By measuring the chemistry of fossil shells which grew in the ocean at different times over the past 60 million years, the authors show that algae started to rely heavily on these supplementary carbon sources relatively recently, between 7 and 5 million years ago.

The emergence of the adaptation during that period is surprising. Up until the Industrial Revolution, the earth´s climate had been slowly cooling for tens of millions of years, with ice caps emerging first on Antarctica 33 million years ago and then later on Greenland about 2.5 million years ago. The cooling has been broadly linked to a gradual weakening of the greenhouse effect as the atmosphere´s CO2 was slowly removed by natural processes. There is evidence for strong CO2 decreases 33 million years ago, coincident with the start of Antarctic glaciation.

However, the history of atmospheric CO2 over the last 10 million years is controversial, with many studies suggesting that CO2 was low and constant despite long-term climate cooling over this time. "The results of the new study suggest that CO2 was declining and crossed a critical threshold between 7 and 8 million years, a result consistent with the evidence for ocean cooling," notes Heather Stoll. Until now, the only direct measurements of CO2 in the past were limited to the past 800,000 years and demonstrated a close link between temperature and CO2, but in periods colder than present.

The study, conducted using sediments obtained from the Caribbean and South Atlantic Oceans, also suggests that algae adapt at levels of CO2 around 500 parts per million. Clara Bolton explained, "these levels will most likely be attained again later in this century due to fossil fuel use, and this adaptation may have consequences for the ecosystem in the surface ocean in the future."

American researcher Heather Stoll arrived at the University of Oviedo in 2005, with a contract from the Ramón y Cajal program, and currently holds the title of Tenured Professor. This project, which was the highest-funded one at the time of its inception and which will end in November 2014, has brought to the University four PhD and pre-PhD researchers, including Clara Bolton, to work here for five years. The project, named PACE (Precedents for Algal Adaptation to Atmospheric CO2: New Indicators for Eukaryotic Algal Response to the Last 60 Million Years of CO2 Variation), is conducted in collaboration with researchers from the Departments of Geology, Ecology, Analytical Chemistry, and Physics.