A thousand meters beneath the surface of the South China Sea, researchers have been catching specks of falling carbon — and what they found could upend what scientists thought they knew about who runs the ocean's carbon engine. The unlikely heroes? Cyanobacteria — the same blue-green microbes that give some ponds their alien-looking tint.
For years, ocean scientists figured that bigger is better when it comes to burying carbon. Large phytoplankton — tiny floating plants that live near the ocean surface — were thought to be the champs at pulling carbon down to the deep sea floor, where it can rest for hundreds or even thousands of years. Tiny cyanobacteria, by comparison, seemed like the underdogs. But new research from Jingjing Zhang and colleagues, published in the journal AGU Advances, flips that story.
Here's how it works. A process called the biological carbon pump moves carbon from near the ocean's surface down to the floor. That journey matters, because carbon that stays deep is carbon that stays out of the atmosphere. Scientists had assumed big phytoplankton did most of this heavy lifting. Zhang's team found otherwise.
Using sediment traps placed 1,000 meters (3,280 feet) deep in the South China Sea, along with years of surface observations, the researchers showed that about two-thirds of the carbon sequestered in the deep ocean comes not from the larger eukaryotic microalgae, but from cyanobacteria. So why do the tiny guys win? Mostly because much of the carbon from microalgae gets lost on the way down, while carbon from cyanobacteria survives the trip.
The team caught its evidence using a clever trick: amino acid carbon isotope signatures found deep in the water. These chemical fingerprints let the researchers trace the carbon they collected all the way back to the phytoplankton groups at the surface. They also found something surprising — while surface carbon production shifts dramatically with the seasons and nutrient availability, the efficiency and origin of the carbon arriving deep below barely change at all.
Two linked processes help the cyanobacteria win the race. First, aggregation — their carbon gets bundled into larger clumps. Then ballasting — minerals join those clumps, weighing them down and protecting the carbon from hungry microbes during the long sinking journey. That combination means less carbon gets eaten or dissolved along the way.
This finding could change how scientists build their models of ocean carbon sequestration. It may also explain a puzzle: why oceans at high latitudes, dominated by big phytoplankton, don't always store carbon better than the low-nutrient "gyre" regions of the tropics rich in cyanobacteria.
As the climate changes and ocean conditions shift, understanding who truly drives the carbon pump matters more than ever. Sometimes the smallest players make the biggest difference — even a humble blue-green microbe can quietly move two-thirds of the carbon that gets locked away in the deep.