Just last year wildfires generated over 2.1 billion metric tons of carbon dioxide emissions around the globe. That’s the equivalent of driving 500 million gas-powered cars around for a year, according to the EPA. With the wildfire season burning its way through this summer, several research groups are now working to demonstrate one small plant species’ ability to offset some of those pollutants.

In a satellite view of the planet, pockets of the ocean appear a bit murkier than the blue waters around them. Those spirals are full of microscopic plant life known as phytoplankton that produce much of the oxygen we breathe. 

Tiny phytoplankton thrive on the surface of oceans, estuaries and rivers across the globe. They’re first on the menu for zooplankton and small fish. But aside from supporting the food chain, these nearly invisible organisms also take on a major mission: carbon dioxide sequestration that boosts the oceanic carbon sink effect. Their behavior serves as a buffer against the effects of natural and human-driven climate change, reducing the dangerous levels of carbon emissions building up in the atmosphere. 

Phytoplankton interact with an aerosol called black carbon, a dark and very fine particulate commonly known as soot. Black carbon is a pollutant released by burning fossil fuels, biomass and wood. It’s associated with increased risk of asthma and a range of respiratory diseases, said Will Barrett, senior director of nationwide clean air advocacy with the American Lung Association.

But black carbon does have one saving grace: It’s rich in iron and nitrogen, of which certain phytoplankton species are in desperate need.

“Those are nutrients that they require, and often they don’t have enough of them in the ocean,” said David Hutchins, a professor of marine and environmental biology whose lab focuses on phytoplankton behavior. His team recently published a paper in the journal Nature Geoscience that lays new groundwork for how global warming affects different phytoplankton populations. 

Large forest fires can emit anywhere from 40 to 250 million metric tons of black carbon a year, said Rodrigo Riera, an associate professor of marine sciences and author of a separate paper examining wildfire ecology. These emissions can take days or weeks to reach a nearby ocean. But the consequences of such fires can affect local ecosystems for months, as they did with the massive Australian wildfires in 2019 and 2020 that burned through 59 million acres of land. 

It’s situations like these where phytoplankton thrive. Researchers studying the wildfires that covered the northern portion of the Indo-China peninsula in March of 2019 recently found that the fires released 430,000 metric tons of carbon. Of that amount, 64 metric tons were black carbon aerosols that traveled eastward in a matter of days, settled into the Pacific Ocean and turned into fodder for hungry phytoplankton. 

With enough nutrients from black carbon, phytoplankton colonies grew and started capturing more of the other carbon particulates that reached the ocean. The study predicted that of all the carbon dioxide emissions released from those March wildfires, phytoplankton helped the ocean absorb and tuck away over half that amount by turning it into the solid carbon they need to survive.

That storage step is crucial. When phytoplankton die off, they and their carbon sink to the bottom of the ocean. 

“That’s a process we call the biological pump,” said Hutchins, who is unaffiliated with the Indo-China study. It’s one of many ways the oceanic carbon sink functions. 

Both Hutchins and Riera—who study marine microbial species independent of one another—also saw phytoplankton communities that lacked iron prior to wildfires were thriving once black carbon came into the picture. As the trend of wildfires ramps up, their work suggests phytoplankton will offset some of the pollution as they latch onto soot’s nutrients. 

It’s a promising outcome and a signal that the Earth has some natural feedback systems acting as barriers against emissions-driven warming.

But phytoplankton alone can’t stave off the full effects of a fire. They don’t take up all the carbon dioxide that falls into their waters, let alone other harmful pollutants pumped out by these disasters. 

“All that CO2 that’s being released is destroying the climate,” Hutchins said. He added that while “that pollution has a minor positive effect on storing carbon in the ocean,” what phytoplankton communities are able to store simply isn’t enough to offset all the damage a fire causes elsewhere. 

The amount of carbon that phytoplankton can hold also varies depending on external factors like ocean currents and water temperature. James Cloern, a scientist emeritus at the U.S. Geological Survey, said that while some populations may thrive in warmer sections of the ocean, others suffer. Phytoplankton productivity can even decrease in especially hot waters. 

“Some areas of the ocean are approaching the upper temperature limits of some phytoplankton, phytoplankton that have really important roles in the food chain and in carbon storage,” Hutchins added. 

Once those upper limits are reached, the phytoplankton communities may die off, leaving gaps in the biological carbon sequestration cycle. 

Too many nutrients can be harmful as well. Hutchins said that some experts advocate for deliberately sprinkling iron into the ocean in hopes of boosting phytoplankton activity. However, that method runs the risk of fostering toxic algal blooms that kill off fish and seagrass, or permanently altering marine ecosystems. 

Cloern also said that some phytoplankton growth isn’t attributable to warming or wildfires. Human activity can dump pollutants into the waters they border. Phytoplankton activity oscillates depending on the season as well. 

“Whatever the responses are that phytoplankton are having to global warming, they’re not universal across world oceans,” Cloern said.