Researchers from the Netherlands at the Royal Netherlands Institute for Sea Research (NIOZ) undertook a search of so-called "missing plastic", and found a possible answer in laboratory experiments: UV light from the sun breaks down the waste into tiny particles, which in turn are ingested and then metabolised by bacteria. With these research findings, the scientists provide evidence that bacteria do indeed digest plastic into CO2 and other molecules.
 

 

According to WWF estimates, 4.8 to 12.7 million tonnes of plastic enter the oceans every year. However, only part of this is ever found during investigations. Researchers are still puzzling over where the rest ends up. A group of scientists from the Royal Netherlands Institute for Sea Research (NIOZ) investigated the question and have now come to the conclusion: bacteria take up the plastic and digest it - at least to a small extent. The research team led by doctoral student Maaike Goudriaan investigated the degradation of plastic by a bacterium called "Rhodococcus ruber" in laboratory experiments. This bacterium, which occurs naturally in seawater, is already known from research to settle as a biofilm on plastics and then cause them to "disappear". In their experiments, the researchers brought the bacterium together in simulated ocean conditions with a polyethylene that had previously been treated with UV light. The aim was to simulate the decomposition of plastics by sunlight, a process known from nature, which breaks down the material into a size suitable for the bacteria to digest. The assumption is that small plastic particles floating in the water are broken down into even smaller particles and into compounds that can then be completely degraded by bacteria. Carbon dioxide levels, as well as other gases produced when the bacteria ingest and digest the plastic, were measured in the model experiment. For their experiments, Goudriaan and her team used a specially manufactured polyethylene in which the carbon is present as the isotope C-13. The results showed that the carbon from the measured carbon dioxide actually came from the plastic and not from another process in the reaction vessel. The researchers calculated from the measurement results that the bacteria are able to break down about one percent of the added plastic into carbon dioxide and other harmless substances per year. However, it could possibly be even more, according to Goudrian, because only the amount of carbon present as the isotope C-13 was measured.
 
"However, the research results do not mean that the bacteria are the solution to the plastic problem in the oceans," she said. "But they are another part of the answer to the question of where all the 'missing plastic' in the oceans has gone." Further research will show whether the bacteria metabolise as much plastic under natural conditions as they do in the laboratory. Results from pilot tests with real seawater and some sediments from the Wattenmeer (North Sea) are promising and indicate that the degradation of plastics by the bacterium Rhodococcus ruber also occurs in nature, Goudrian said.
 
Sources:

• Plastic photodegradation under simulated marine conditions, Annalisa Delre, Maaike Goudriaan, Victor Hernando Morales, Annika Vaksmaa, Rachel Tintswalo Ndhlovu, Marianne Baas, Edwin Keijzer, Tim de Groot, Emna Zeghal, Matthias Egger, Thomas Röckmann, Helge Niemann, Marine Pollution Bulletin, 187, February 2023
• Photo: unsplash.com, Naja Bertolt Jensen