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 (NIOZ) on the North Sea island of Texel brought the bacterium Rhodococcus ruber together with a special polyethylene (PE) and measured the carbon dioxide (CO₂) produced. Extrapolated over a year, the bacterium metabolizes a little more than one percent of the plastic into CO₂ – other metabolites are not taken into account.
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 really shown that the bacteria actually digest the plastic,” Goudriaan is quoted as saying in a statement from her institute. The researcher evaluates the results as an answer to the question of where a small part of the plastic disappears in the sea. But she emphasizes: “This is certainly not a solution to the problem of plastic soup in our oceans.”
Bite-sized chunks by sunlight
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.
In the reaction vessels, the group recreated the conditions in the sea: the water was slightly salty and the vessel irradiated them with ultraviolet light, like that found in sunlight. “The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria,” explains Goudriaan. A recent study by researchers from the same institute found that about 2 percent of the visible plastic in the ocean disappears each year due to degradation from sunlight.
Maaike Goudriaan and research leader Helge Niemann in the laboratory.
© Source: Maaike Goudriaan, NIOZ
Precautionary measures against pollution are essential
The scientists also looked at whether it makes a difference when plastic floats on the surface or is completely submerged in the water. As a result, slightly more CO₂ was produced in the vessels with the plastic on the surface – 1.24 percent per year – than in the vessels with the submerged polyethylene – 1.04 percent per year. However, both were significantly more CO₂ than was produced in the control vessels without Rhodococcus ruber bacteria, presumably as a result of the UV radiation, as the team writes in the specialist journal “Marine Pollution Bulletin”.
The researchers now want to investigate whether this process also occurs in nature and have already carried out experiments with silt from the Wadden Sea. “The first results of these experiments 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 that it is far 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.