A Step Forward in the Battle Against Plastic

Plastic is one of the world’s leading causes of pollution. It’s in just about everything—electronics, appliances, packaging, clothing, and more. The Environmental Protection Agency reports that the U.S. has produced almost 40 million tons of plastic waste every year since 2018, with numbers only increasing as time goes on. On average, only 10 percent of this waste is properly recycled. This is a huge problem; not only does high plastic waste disrupt animal life by contaminating natural habitats, but it also directly threatens human health as tiny particles known as microplastics circulate through food chains.

Plastics are synthetic polymers made of combinations of monomers ethylene, propylene, vinyl chloride, styrene, or acrylonitrile. Various types of plastics can be found in products you wouldn’t expect such as tea bags, gum, toothpaste, and sunscreen. However, the one-time use plastics that we see on a daily basis like coffee cups, straws, utensils, and food packaging constitute a specific group of plastics called polyolefins, which are made of propylene and ethylene. Polyolefins are some of the most abundant plastics in the world today, making up 50 percent to 70 percent of all plastic waste, but their chemicals don’t gradually decompose. Instead, they break into tiny pieces called microplastics that won’t be chemically reduced for thousands of years.

When polyolefins are manufactured, the monomers are welded together by chemical catalysts under extreme heat. This creates abnormally strong carbon-carbon bonds that aren’t typically found in nature. The vast majority of microorganisms today aren’t equipped with the biological tools needed to break these bonds. However, water vapor and acidic chemical catalysts can break plastic down into its first intermediate when operating at high temperatures and pressures, creating depolymerized oil (DP oil) out of the plastic. DP oil is essentially liquidized plastic; the chemical bonds that kept the plastic solid are broken. In order to consume DP oil, microorganisms need to use the majority of their proteins. When proteins in a cell are diverted toward the breakdown of DP oil, they can’t be used to help the cell grow and the microorganism cannot survive.

Recently, however, scientists have discovered that yeast Yarrowia lipolytica has unique metabolic pathways that let it process DP oils for energy, therefore reducing the amount of time that it takes microplastics to decompose in landfills and other polluted sites. A 2023 mySystems study by a group of scientists from the University of Tennessee details the experiments that revealed how Y. lipolytica breaks down polyolefins into biodegradable resources.

To begin, scientists let the yeast Y. lipolytica adjust their diets to only the DP oils found in polyolefins. The yeast began to repurpose more proteins towards the breakdown of the oil in order to access the hydrocarbons that it contained. Various organelles in the cells of the yeast took up these oil parts and digested them, creating acetyl-CoA—a critical component of cellular respiration in the mitochondria. Once the yeast’s metabolic systems were primed for further experimentation, they were given plastics that hadn’t been fully reduced to DP oil in order to test whether or not they’d be able to consume larger plastic components.

Scientists observed that the adapted yeast could both survive and continue cell growth (to a greater extent than the unadapted yeast) when living off of larger plastics, proving themselves capable of effectively decomposing plastics in three to four days that few other microorganisms can consume. In breaking down plastics, the yeast also produces lipids and acids that can be used by engineers to create more environmentally friendly products and renewable fuel sources. In the past, for example, Y. lipolytica has been engineered to create a biodegradable alternative to petroleum-based plastic, which is just one example of how Y. lipolytica’s bioconversion is an asset in the fight for our planet.

The discovery of this yeast is an incredibly important advancement as microplastics continue to threaten wildlife and human health. When they accumulate in organisms due to continued consumption of food, water, or even the air, microplastics become virtually impossible to remove and can cause malnutrition, fertility problems, cancer, and even death. However, scientists predict that the biological processes Y. lipolytica uses to break down plastics can be isolated and industrialized by copying the yeast DNA into lab-grown cells for widespread use. If done correctly, this new substance could be quickly employed to improve plastic waste management and to rescue the areas that are most threatened by plastic pollution. While microplastics and the role they play in the environment are still being studied and no concrete plans have been made public, scientists are hopeful that Y. lipolytica and similar microorganisms can serve as a way to begin detoxifying the environment.