Marine bacteria can digest plastic

A marine bacterium can ingest and digest plastic. A Dutch-German research team proved this in a laboratory test. The group led by Maaike Goudriaan from the Royal Netherlands Institute of Sea Research on the North Sea island of Texel brought the bacterium Rhodococcus ruber together with a special polyethylene and measured the carbon dioxide that was produced. Extrapolated over a year, the bacterium metabolizes slightly more than one percent of the plastic into CO2. Other metabolites are not considered.

It was known that Rhodococcus ruber can form a biofilm on plastic in nature. In addition, it has already been measured that plastic disappears under this biofilm. "But now we have shown that the bacteria actually digest the plastic," says Goudriaan. The researcher evaluates the results as an answer to the question of where a small part of the plastic disappears into the sea. But she emphasizes: "This is certainly not a solution to the problem of plastic soup in our oceans."

Goudriaan and colleagues used a specially manufactured polyethylene for their experiments: the carbon in it is in the form of the isotope C-13, of which only 1.1 percent occurs in nature. With the C-13 polyethylene, the scientists were able to prove that the carbon from the measured CO2 actually comes from the plastic and not from another process in the reaction vessel. However, other reaction products such as methane, sugar or proteins cannot be measured with this method. Therefore, the extent of the metabolism is greater than the approximately one percent CO2 per year indicates.

The scientists also looked at whether it makes a difference when plastic floats on the surface or is completely submerged in the water. The result: Slightly more CO2 was produced in the vessels with the plastic on the surface than in the vessels with the submerged polyethylene. However, both were significantly more CO2 than in the control vessels without the bacteria. The researchers now want to find out whether this process also occurs in nature. "The first results indicate that plastic also degrades in nature," reports Goudriaan.

Perhaps one day we will be able to determine exactly how much plastic is broken down by bacteria. But even then, the researcher emphasizes, it is much better to take precautionary measures against the introduction of plastic than to clean up the sea afterwards. According to Goudriaan's group, between 1950 and 2015 an estimated 117 to 320 million tons of plastic ended up in the oceans.

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