This planet has a problem with plastic. Not just the big masses of it accumulating in the Pacific, but with the tiny bits that are blowing into pristine mountaintop habitats. The flecks showing up in a range of sea creatures. The specks materializing even in human feces.

Now scientists have exposed a potential new consequence of the plastic menace: The toxins the material leaches into seawater inhibit the growth and photosynthetic efficiency of the bacteria Prochlorococcus, which is responsible for producing an estimated 20 percent of the oxygen we breathe. That means Prochlorococcus is also responsible for 20 percent of carbon capture on this planet (one molecule of carbon goes in, one molecule of oxygen goes out), theoretically spelling trouble for humanity’s quest to keep CO 2 out of the atmosphere. This is early research that comes with several big caveats, and also exposes the challenges of studying a threat as new and omnipresent as plastic pollution.

Prochlorococcus is a kind of cyanobacteria (taking their name from their blue color) that floats in oceans the world over. We’re talking a lot of single-celled organisms, with an estimated global population of 1027. Like a plant, Prochlorococcus uses photosynthesis to manufacture its own food, taking in carbon and spitting out oxygen, making it a lead actor in the carbon cycle that humans have spun so out of control.

Unfortunately, the researchers found that in addition to carbon, the bacteria are taking in plastic toxins leached into the water, known as leachates. They did this in the lab by mixing different amounts of plastic into an artificial seawater base, in which they grew Prochlorococcus. They compared the results with a control of Prochlorococcus grown in untainted artificial seawater.

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The researchers found clear reactions that varied based on the concentration of leachate, which is indicative of a toxicological response. At low concentrations, there was no difference with the control. But as they increased the concentration of the leachate, they saw the bacteria’s physiological response going progressively haywire. “When the plastic leachates increase in concentration, you see that the cells don't grow as well, and in fact at the highest concentrations they are dying,” says microbial oceanographer Lisa Moore of Macquarie University in Australia, a coauthor on the paper.

Moore and her colleagues were also able to measure photosynthetic activity at these different concentrations with an instrument that looks at the intensity of the cells’ fluorescence. “We saw parallels to what we saw with the growth: a decrease in photosynthetic efficiency, and in fact a pretty dramatic decrease with the higher concentrations,” says Moore.

Going further, the researchers looked at the genes of these bacterial populations, whether they were being expressed more or less in the presence of leachate. A large portion of those being expressed less were associated with photosynthesis, “which was absolutely consistent with what we were seeing in terms of the photosynthetic efficiency being decreased, and then the growth being decreased,” says Moore.

At fault could be any number of things in the plastics. Flame retardants, for one, and other additives that give plastic its flexibility. Zinc in particular might be having an outsized effect on the bacteria—it’s used in plastic components ranging from colorants to heat stabilizers.

Now, we’ve got some caveats to this study. The biggest is that the work was done in the lab, not out in the ocean, and necessarily so. To do an experiment like this, the researchers had to carefully control the samples of artificial seawater so they wouldn’t be tainted with other contaminants that could throw off the results.