- Each year, humans generate millions of tons of persistent waste from synthetic polyesters and other plastics.
- Scientists believe the digestive microbiomes of cattle can break down certain synthetic polyesters.
- Bovine stomach fluids sourced from slaughterhouses could provide a low-cost means of reducing plastic pollution.
According to the Environmental Protection Agency (EPA), United States landfills alone receive tens of millions of tons of plastic waste each year.
While synthetic plastics are inexpensive and useful in a variety of applications, they are also difficult to break down. That means safe disposal and reuse are challenging, making plastic waste a global environmental problem.
To help mitigate the damage from synthetic waste products, scientists are looking to nature. A study that appears in Frontiers in Bioengineering and Biotechnology presents new evidence that the natural microbiome in bovine digestive systems is effective in decomposing certain synthetic polyesters.
The research was a joint effort between researchers from three Austrian institutions: the University of Natural Resources and Life Sciences in Vienna, the Austrian Centre of Industrial Biotechnology, and the University of Innsbruck.
Beyond their results, the authors discuss possible implications, limitations of scale, and directions for future research.
The research began with the observation that some animals already consume natural plant polyesters while grazing. These animals, called ruminants, have specialized organ systems for processing food otherwise difficult to digest.
In extracting nutrition from grass and similar plant matter, ruminants follow an elaborate feeding process. For example, cattle start by fermenting ingested food in their foregut. They then regurgitate the fermented bolus as cud for more thorough chewing, known as rumination.
In all, the process involves four distinct stomach chambers. The largest chamber, called the rumen, houses a microbiome that processes plant matter in multiple stages.
“A huge microbial community lives in the rumen reticulum and is responsible for the digestion of food in the animals,” says study co-author Dr. Doris Ribitsch, “so we suspected that some biological activities could also be used for polyester hydrolysis.”
Scientists suspect that during one phase of this digestive process, key enzymes in the rumen interact with water to separate the chemical bonds of natural polyesters that cows and bulls consume as they graze. This process, called hydrolysis, also appears to be effective at processing other polyesters — including those in plastic waste that humans produce.
Scientists have already examined ruminant digestion for application in synthetic waste processing and recycling. But most of that research has centered on digestive processes in isolation. In these studies, scientists analyzed individual enzymes and microorganisms separated from the rest of the digestive microbiome.
The new study takes a more holistic approach, starting with intact rumen liquid obtained from a slaughterhouse in Austria. The researchers incubated this rumen liquid with three different polyesters used in manufacturing:
- Poly(butylene-adipate-co-terephthalate) (PBAT): Common in the manufacturing of compostable plastic bags, PBAT is also seen in consumer product packaging and other applications.
- Poly(ethylene terephthalate) (PET): A near-ubiquitous polymer, PET is frequently used to produce single-use beverage bottles and synthetic fabrics.
- Poly(ethylene furanoate) (PEF): A biobased polyester, PEF is a chemical analog of PET with a superior renewability profile.
Their experiments showed improved polyester decomposition when compared with previous studies focusing on isolated enzymes and microorganisms. The authors believe this improvement could be due to synergistic interactions between the rumen’s diverse elements. The authors write:
“Compared to published data for pure enzymes, supernatants of single microorganisms, or both, the polyester-hydrolyzing activity of rumen fluid was relatively high. Apparently, not only one type of enzyme is present in the rumen mixture, but rather a synergistic action of different esterases, lipases, or cutinases may occur.”
According to the authors, this synergistic activity is not entirely surprising. In nature, there are situations where multiple enzymes are required to break down a single product, such as cellulose.
“Despite the fact that rumen fluid could be a cheap source for polymer degrading enzymes,” the authors continue, “future studies should aim at identification and cultivation of the microbes and enzymes involved in synergistic hydrolysis of polyesters as well as on possible community changes during incubation with polyesters.”
However, the researchers note the need for more studies before this method is effective on an industrial scale. Still, they believe their results signal untapped potential. “Due to the large amount of rumen that accumulates every day in slaughterhouses, upscaling would be easy to imagine,” notes Dr. Ribitsch.