Antarctic algae that once cooled Earth are now disappearing

For something you can’t see with the naked eye, algae sure know how to shake things up. Around 14,000 years ago, microscopic Antarctic algae helped slow down global warming by pulling huge amounts of carbon dioxide out of the atmosphere.

Scientists just figured out how they did it – and why the same process that once helped cool the planet could play a critical role in shaping the future of our climate.

Antarctic Algae changed the climate

Back then, Earth was gradually warming after the last ice age. However, something unusual occurred in the Southern Hemisphere: the warming paused.

This period, referred to as the Antarctic Cold Reversal, was a time when sea ice expanded quickly in winter and melted quickly in spring. That spring melt converted the Southern Ocean into a super buffet for a particular type of algae, Phaeocystis.

These tiny ocean plants went wild. They multiplied, and in doing so, they pulled vast quantities of carbon dioxide – one of the primary drivers of global warming – out of the atmosphere and stored it away in the sea.

Here’s the twist: until now, we had no idea Phaeocystis even played a role. Scientists couldn’t find any trace of them in the usual fossil records.

Now, thanks to a new method that recovers ancient DNA from the ocean floor, researchers from the Alfred Wegener Institute in Germany have filled in this crucial missing piece of the climate puzzle.

Unlocking Antarctica’s hidden record

The team studied a sediment core taken from nearly 2,000 feet deep in the Bransfield Strait, just off the Antarctic Peninsula. Layer by layer, this core preserves snapshots of ocean history going back 14,000 years.

By analyzing the ancient DNA trapped in that sediment, the scientists found genetic fingerprints of Phaeocystis during the cold reversal period. No one had ever identified these algae in the past using standard geochemical methods.

“Our study shows that these algal blooms contributed to a significant reduction in global atmospheric CO2 levels during a climatically important transition phase characterized by high sea ice extent,” said Josefine Friederike Weiß from the Alfred Wegener Institute, lead author of the study.

They also measured something else: a high ratio of barium to iron in the sediment. That ratio is tied to how much organic material – like dead algae – sinks to the seafloor. In this case, the data pointed to intense algal growth during times of heavy spring melt.

“The further the sea ice expands in winter, the larger the area in spring where nutrient-rich meltwater enters the surface sea – and therefore the zone where Phaeocystis algae find ideal growth conditions. As a result, greater sea ice extent leads directly to a larger region with high algal productivity.”

Why today’s melting ice is a warning

The same Phaeocystis algae that once helped cool the world are now struggling to survive. With Antarctic sea ice declining at a record rate, the conditions that previously drove these algae blooms are vanishing.

That’s not just bad news for the algae. These blooms power entire food webs. They feed tiny animals, which feed fish, which in turn feed everything from squid to seals. If Phaeocystis disappears, the whole system could unravel.

Even worse, these algae are really good at one specific job: getting carbon to sink deep into the ocean. Losing them could mean more carbon sticks around in the atmosphere, heating the planet faster.

And there’s another concern. Phaeocystis releases a gas called dimethyl sulfide (DMS), which helps form clouds. More clouds mean more sunlight bounces back into space. Fewer algae mean fewer clouds. That’s one more way climate change could end up feeding on itself.

Antarctic algae holds future lessons

The study shows why scientists must look beyond conventional methods. By combining geological tools with DNA analysis, researchers are creating a more accurate picture of how the ocean shapes our climate – and how it might respond in the future.

The research also highlights a larger trend in climate science. For years, scientists focused mainly on ice cores and chemical markers.

Now, by analyzing the genetic material of ancient microbes, we can see what species were doing during pivotal climate events – and what those behaviors could mean for us today.

This is important because forecasting future climate change isn’t only about tracking temperature or carbon levels. It’s about understanding the living systems that help regulate them – such as the algae and the seas they inhabit.

The full study was published in the journal Nature Geoscience.

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