Microbiology: Natural plastic eaters | nd-aktuell.de

PET bottles, cigarettes, plastic bags – if you walk 100 meters along a German country road, you will find several kilos of plastic waste. There is also plastic waste lying around on agricultural green spaces – which is then shredded when the grass is chopped. Cattle on pastures also ingest plastic with their mouths, as animal owners report. At first the animals stop eating food and lose weight. If they are not treated successfully, the only option is often to euthanize them.

Visually, plastic parts cannot be distinguished from the rest of the food, says Christian Gerspach, head of the livestock department at the Zurich Animal Hospital. Plastic cannot be seen on x-rays or ultrasound. Small chopped pieces are usually excreted. However, large pieces can lead to intestinal blockages or block the passages from one stomach to the other, preventing the rumen from emptying. When the food is regurgitated, the stomach contracts so much that pointed and sharp-edged parts can pierce the stomach wall. The stomach contents flow into the abdominal cavity, causing infections.

Rumen bacteria break down polyester

Scientists from the Austrian Center of Industrial Biotechnology and the University of Natural Resources and Life Sciences (Boku) Vienna also found amazing abilities of cow stomachs. Bacteria living in it are able to break down some polyesters, which are used to make textiles, packaging and compostable plastic bags, among other things. The scientists come to this conclusion in their paper published in 2021 Study.

According to this, cattle feed contains natural plant polyesters that are broken down and digested by the rumen bacteria. A huge microbial community lives in the rumen reticulum, a section of the cow’s stomach, which is involved in the digestion processes of food. Several enzymes isolated from the cattle rumen work together. They break down the plastic into its original building blocks – monomers and oligomers. The scientists examined three types of polyester. These were a synthetic polymer (PET) used in textiles and packaging, a degradable plastic used in compostable plastic bags, and a bio-based material made from renewable resources.

Plastic is difficult to break down, but its structure is in many ways similar to that of natural polyesters such as cutin, one of the main components of the plant cell, which is broken down by natural enzymes in the compost heap, for example, says Doris Ribitsch from the Institute for Environmental Biotechnology at Boku Vienna. Some biological activities could also be used for the hydrolysis of some types of plastic, that is, for the breakdown through the addition of water molecules. In this way, either new polymers or completely different molecules could be produced.

Marine bacteria digest polyethylene

The marine bacterium Rhodococcus ruber can also form a biofilm on plastic in nature. In addition, it has already been measured that plastic disappears under this biofilm.

The bacterium is also able to incorporate plastic and digest it. Extrapolated over a year, it metabolizes just over one percent of the plastic into carbon dioxide, according to a Dutch-German research team published in January 2023 in the journal “Marine Pollution Bulletin”. Study found out. The team led by Maaike Goudriaan from the Royal Netherlands Institute of Sea Research on the North Sea island of Texel brought the bacterium together with a specially manufactured polyethylene (PE) and measured the resulting carbon dioxide.

The carbon it contains is in the form of the isotope C-13, which only occurs in nature at 1.1 percent. Using this C-13 polyethylene, the researchers demonstrated that the carbon in the measured carbon dioxide actually comes from the plastic. Other reaction products such as methane, sugar or proteins cannot be measured using this method. The extent of the metabolism is therefore greater than the approximately one percent carbon dioxide per year indicated, the authors write.

It was already known that sunlight breaks down plastic into bite-sized chunks for bacteria. According to a recent study, about two percent of visible plastic in the ocean disappears every year because it is broken down by sunlight.

To do this, the scientists recreated the conditions in the sea in the reaction vessels: salty water was irradiated with ultraviolet light, as occurs in sunlight.

In addition, slightly more carbon dioxide was produced in the containers with the plastic on the surface – 1.24 percent per year – than in the containers with the submerged polyethylene – 1.04 percent per year. However, there was significantly more carbon dioxide in both than in the control vessels without Rhodococcus ruber bacteria. The researchers suspect this is a result of UV radiation. They now want to investigate whether this process also occurs in nature – for example in mud from the Wadden Sea.

Enzymes from the recycling plant

Around 80 of the biocatalysts mentioned can be polyethylene terephthalate (PET). In 2016, Japanese biochemists discovered bacteria with a plastic-destroying enzyme in a recycling plant. Accordingly, the bacterium Ideonella sakaiensis can completely metabolize PET and use it to cover its energy needs, albeit very slowly. Two years later, a team of researchers from the USA and Great Britain improved the enzyme by making a “small molecular change”. This means it can be a fraction of the plastic decompose even faster, as they say.

Plastic, which has only been around for a few decades, can even be broken down by enzymes from ancient bacteria. Recently, researchers discovered the PET-degrading enzyme PET46 in ancient bacteria (archaea) that occur in the deep sea. Using complex analytical methods, the team led by Pablo Pérez-Garcia from the University of Hamburg searched a database with samples from around the world totaling 10,000 enzymes. In the end, ten enzymes were examined in the laboratory. The aim is to find new enzymes that can ultimately recycle PET industrially, explains the microbiologist.

Scandinavian lakes are polluted with plastic. For his work published in the journal “Nature Communications”. Study A British-German team examined the water of a total of 29 very different lakes in Scandinavia. To do this, the scientists filled bottles with cut-up plastic bags from the supermarket and distilled water and exposed the mixture to the light of a UV lamp. After a week, they added a small amount of the “plastic water” to samples from all the lakes. The result: The biomass of the bacteria increased almost 2.3 times after adding the plastic water. It seems as if plastic pollution stimulates the bacteria’s appetite, explains study author Andrew Tanentzap in a press release. Bacteria in lakes metabolize plastic even more easily than dead plant material. More bacteria means more food for larger organisms like ducks or fish. Although plastic increases bacterial growth, exactly which components the bacteria metabolize is unknown.

Looking at bacterial communities

It is better to test entire bacterial communities for their abilities and breed them in the laboratory than to look for individual species of bacteria that produce plastic-decomposing enzymes.

Scientists from the Geomar Helmholtz Center for Ocean Research and the University of Kiel came to this conclusion four years ago. If a single species of bacteria is found that is able to break down a type of plastic using a special enzyme, the genetic information of the bacterium is only searched for the code for this enzyme. There is a risk that you will only find information that you already know and overlook other important skills.

According to the German Nature Conservation Association, plastic waste costs the lives of around 130,000 marine mammals every year. According to projections, around 20 million tons of plastic end up in the world’s waterways every year – that’s around two truckloads per minute. Between 1950 and 2015 alone, an estimated 117 to 320 million tons ended up in the sea. How many billions of bacteria or enzymes are necessary to decompose these masses of plastic? Even if we knew how much plastic is broken down by bacteria, precautionary measures would make more sense than cleaning up the sea afterwards, sums up plastic researcher Maaike Goudriaan. Avoiding plastic remains the most important strategy in the fight against global mountains of waste.