Based on a model study with plastic in artificial seawater in the lab, Goudriaan calculated that bacteria can break down about 1% of the fed plastic per year into CO₂ and other harmless substances. "But," Goudriaan emphasizes, "this is certainly not a solution to the problem of the plastic soup in our oceans. It is, however, another part of the answer to the question of where all the 'missing plastic' in the oceans has gone."

Special plastic

Goudriaan had a special plastic manufactured especially for these experiments with a distinct form of carbon (¹³C) in it. When she fed that plastic to bacteria after pretreatment with "sunlight"—a UV lamp—in a bottle of simulated seawater, she saw that special version of carbon appear as CO₂ above the water. "The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria," the researcher explains.

"This is the first time we have proven in this way that bacteria actually digest plastic into CO₂ and other molecules," Goudriaan states. It was already known that the bacterium Rhodococcus ruber can form a so-called biofilm on plastic in nature. It had also been measured that plastic disappears under that biofilm. "But now we have really demonstrated that the bacteria actually digest the plastic."

When Goudriaan calculates the total breakdown of plastic into CO₂, she estimates that the bacteria can break down about 1% of the available plastic per year. "That's probably an underestimate," she adds. "We only measured the amount of carbon-13 in CO₂, so not in the other breakdown products of the plastic. There will certainly be ¹³C in several other molecules, but it's hard to say what part of that was broken down by the UV light and what part was digested by the bacteria."