56-million-year-old algae could shed light on future climate change

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By Marisa Sloan, Dec. 17, 2020 –

Faced with a challenge as mammoth as climate change, scientists are turning to some very tiny organisms for insight — coccolithophores, the single-celled algae that are smaller than a grain of sand.

In order to grow, algae use sunlight to convert carbon dioxide and water into energy in a process called photosynthesis that all plants employ. The algae prefer to take up the lightest of two different forms of carbon, but sometimes resort to the heavier form when carbon dioxide levels in the surrounding water are low.

Louis Claxton, a Ph.D. student at the University of Oxford, uses the ratio of light to heavy carbon trapped in the algae’s fossilized shell to approximate the amount of carbon dioxide in the ocean when it lived. He presented his research at the Comer Climate Conference, an annual conference of global climate scientists that was held virtually this October.

“These things are gorgeous to look at under the microscope,” Claxton said. “And something so small, something that no one really pays much attention to, can hold so much information about our past.”

Right now he examines algae from a period in Earth’s history called the Eocene, which lasted from approximately 34 to 56 million years ago. It was a time marked by hot temperatures, rapid changes in climate and a lack of permanent ice sheets. This was the last such extreme heat spell Earth experienced and scientists believe the period could hold important implications for where today’s warming climate is heading.

“If we can understand how biology responded to changes in carbon dioxide over this period, we may be in a better place to understand how it may change in the future,” Claxton said.

And it is changing.

According to the Intergovernmental Panel on Climate Change, the current amount of atmospheric carbon dioxide exceeds measurements from at least the past 800,000 years. That increase, at a rate unprecedented in modern history, has largely been caused by the burning of fossil fuels and led to surging global temperatures, rising sea levels and severe weather patterns.

“In dealing with these big issues, the history of climate is absolutely essential,” Dr. Richard Alley, a geoscientist at Pennsylvania State University, said during the conference. “History gives us a way to test models.”

By growing algae in his lab and subjecting them to various levels of carbon dioxide, Claxton can create models that predict how ancient algae reacted to similar changes in carbon dioxide levels.

“They are nightmare pets because they are very sensitive to temperature and all sorts of things,” he said. “But I’ve been looking after them for about three or four years, and I’ve gotten quite attached to them.”

Sophie Gill, another Ph.D. student within Claxton’s research group, knows all about that.

“I work on slightly different species because some of the species I work on weren’t evolved during the time that he works on,” she said. “But we’re able to see a common ground [in that] some coccolithophores are not able to cope with very high levels of dissolved [carbon dioxide].”

Gill is currently trying to find the perfect oceanic conditions that will allow the algae to take up more carbon dioxide through photosynthesis than they produce via their calcium carbonate shells. She hopes the algae may someday be used as a more effective carbon sink, absorbing large amounts of carbon dioxide from the ocean’s surface and trapping it within their fossils when they die.

The concept seems ideal on the surface, but Claxton is digging into the past — literally — for any unintended consequences it could have.

“The samples come from about 1,500 meters deep off the coast of Namibia,” he said. “If you were to hold some of the sediment that I was working with in your hand, it looks like… slimy mud that someone’s dug up from the bottom of a puddle.”

Hidden within that mud, however, are the fossilized remains of algae that lived tens of millions of years ago. When Claxton compares their carbon composition to his models, he is able to estimate the carbon dioxide levels in the ocean, and by extent the atmosphere, at the time the algae lived.

So far, his calculations correspond well with existing data from various other methods. Now that Claxton has proven the concept works, he’s ready to jump into the unknown: time periods in the even farther past for which there are no well-constrained data.

“The method that I’m working on to reconstruct carbon dioxide could potentially go back about 200 million years,” he said, referring to how long ago this particular type of algae evolved.

In comparison, ice core records extend to only about 800,000 years.

“We may be missing analogs in Earth’s history that are almost identical to today,” Claxton said. “By identifying those periods, we can perhaps understand what the future may hold.”

Marisa Sloan is a health, environment and science reporter at Medill. You can follow her on Twitter at @sloan_marisa.

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