Plastic is known to be difficult to break down. Bacteria, fungi and archaea in the rumen of a cow could be crucial helpers, as researchers have found out. According to this, the community of microbes in the bovine stomach is particularly effective at breaking down synthetic polyesters such as the PET used in plastic bottles or packaging. The microorganisms could thus offer an environmentally friendly way of reducing plastic waste in the future.
Billions of different bacteria and fungi live in the rumen, the first and largest of the four beef stomachs. Among other things, these process urea, which enters the rumen from the bloodstream, for protein biosynthesis. Microorganisms that use enzymes to break down the long-chain molecules of the plant fibers that are otherwise indigestible for the cattle are also particularly important for the animals. Without this help the cattle could hardly use their food.
Plastic food for rumen bacteria
The ability to break down the long-chain plant molecules could also make the bacteria in the bovine rumen interesting for the breakdown of plastics. Scientists working with Felice Quartinello from the University of Natural Resources and Life Sciences in Vienna have therefore investigated whether these bacteria could possibly also break down plastic fibers that are difficult to break down. “A huge microbial community lives in the rumen reticulum, which is responsible for the digestion of food in the animals, so we suspected that some biological activities could also be used for polyester hydrolysis,” says Quartinello’s colleague Doris Ribitsch.
To test their hypothesis, the researchers examined the rumen fluid of a slaughtered cattle for polyester-degrading bacteria and their enzymes. To do this, they added three types of polyesters to the liquid: the synthetic polymer polyethylene terephthalate (PET), which is used in plastic bottles and packaging, among other things, the biodegradable plastic polybutylene adipate terephthalate (PBAT), which is mostly found in compostable plastic bags, and the bio-based material Polyethylene furanoate (PEF), which is used for food packaging. With all three plastics, Quartinello and her team tested how effectively they are broken down by the rumen microorganisms in both powder and film form.
All three plastics degraded
The result: all three plastics could be broken down by the microbes from the beef stomachs. The scientists noticed that the polyesters degraded more quickly if their polymer chains were flexible, as they made it easier for the bacterial enzymes to gain access. In addition, all three plastic powders dissolved faster than the foils. “As expected, the hydrolysis of powders was faster compared to foils due to the higher surface-to-volume ratio,” the researchers explain.
For example, within 72 hours, 0.25 millimoles of hydrolysis products were released from the PET film and even 0.6 millimoles from the PET powder. The amount of the PBAT powder, 0.75 millimoles, was about 25 percent higher than that of the film. The largest amount of degradation products released in PEF powder, around five millimoles, was even more than five times as much as the products in PEF film. In addition, the research team found that the plastic breakdown of the entire microbial community in the rumen fluid was more effective than previous studies, in which only individual microorganisms were examined for their ability to break down plastic.
Only effective in cooperation
The results thus indicate that not only a specific enzyme is responsible for the degradation of the polyester, but that the enzyme cocktail released by the microbial community in the cattle rumen is responsible for the effective degradation of plastic. According to the scientists, the very common species of the genus Pseudomonas could be involved in the breakdown of the plastic in the rumen fluid. “These bacteria are known to be able to perform various hydrolytic activities,” the researchers said. The same applies to the bacterial species of the genus Acinetobacter as well as to Aspergillus or Penicillium fungal species and unicellular organisms such as Methanocorpusculum labreanum.
Further, larger-scale research will follow in the future to learn more about microbial communities as potentially environmentally friendly plastic decomposers, said Quartinello and her team. “Due to the large amount of rumen that accumulates in slaughterhouses every day, upscaling would be easy to imagine,” estimates Ribitsch. In the future, this will enable the types of microbes involved in the breakdown of plastic to be identified more precisely and cultivated in the laboratory.
Source: Frontiers, Article: Frontiers in Bioengineering and Biotechnology, doi: 10.3389 / fbioe.2021.684459