Above: Image © istockphoto.com/mstay
Did You Know? Horses can eat hay because bacteria in their intestines help them break down its cell walls. These bacteria do not live in human intestines, so we cannot digest hay.From pickles to pancakes, human beings eat a wide variety of foods. You can digest many foods entirely on your own, but you rely on bacteria in your intestines to help you break down some starches and sugars into smaller molecules that your body can use. Even with the help of bacteria, however, you can’t digest some things, such as plastic.
Until recently, scientists thought that plastic could not be broken down by any organism. After all, plastic molecules are huge and contain extremely long chains of atoms. Many are so large that bacteria can’t easily envelop them (take them in) in order to break them down. Also, most atoms in a molecule of plastic are held together with strong bonds between carbon and hydrogen atoms. These bonds take a lot of energy to break.
Because it is so strong and difficult to destroy, plastic is both a great material for storage containers and a huge environmental problem. Canadians alone throw out almost three million tonnes of plastic every year. Plastic fills landfills, gathers in oceans, and chokes wildlife. Because it decays so slowly, once it is discarded plastic often stays where it is for hundreds of years.
What is Plastic?
The most common type of plastic is a chain of thousands of carbon and hydrogen atoms called polyethylene:
Other types of plastic include polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, and polypropylene. Some plastics contain other chemicals that give them specific qualities, such as flexibility or fire resistance.
But what if there were an organism that could digest plastic? In the past few years, scientist have actually begun to discover species of bacteria that seem to do just that. For example, Daniel Burd, a Canadian high-school student, found bacteria in soil that can degrade plastic bags. Marine microbiologist Tracy Mincer discovered that certain ocean bacteria are also able to degrade plastic film.
More recently, environmental engineer Jun Yang discovered that bacteria in the larvae of some moths can use polyethylene, a common type of plastic, as food. He had noticed moths in and around a plastic bag of millet in his kitchen and found that the larvae of the moths had chewed their way through the bag. Yang wondered if the larvae—with help from bacteria in their intestines—actually digested the plastic molecules as they made the holes.
To find out, he and his research team isolated the different species of bacteria that lived in the larvae’s intestines and attempted to grow each species on a thin plastic film. The larvae had access to no nutrients other than the plastic film. Yet, over time, the populations of two species of bacteria grew, and the mass of the plastic film decreased.
To be sure that the bacteria were actually breaking down plastic molecules and not just removing whole molecules from the the film, Yang carried out a second investigation: using special imaging techniques, he examined the plastic’s molecular structure. The ends of the molecules had been snipped—evidence that the plastic had actually been broken down!
Challenges and questions
Imagine how helpful it would be if bacteria could break down all the plastic waste that people produce! Unfortunately, they’re not quite ready to do that. The bacteria in Yang’s study digest plastic very slowly, and can only digest the surface layer. They reduced the mass of a thin film by just 20% in six weeks, and it would take them much, much longer to digest a thicker piece of plastic.
Did You Know? When animals in the wild mistake discarded plastic for food, chemicals in the plastic can interfere with their endocrine systems, resulting in birth defects and other abnormalities.Also, plastics often contain additives to give them specific characteristics, but these additives can also harm organisms that eat them. For example, scientists don’t yet know what would happen if bacteria digested plastics containing bisphenol-A, formaldehyde, or benzene. Would the bacteria digest the additives as well, or would these potentially harmful chemicals be released into the environment?
You can bet studies are already underway to learn more. In the meantime, dealing with plastic waste means making some difficult decisions. Cloth shopping bags and reusable water bottles may reduce the amount of plastic sent to landfills, but they are less convenient than disposable plastic bags and bottles. Biodegradable plastics made from corn starch are now available, but they are expensive to produce and use corn that could otherwise be used for food. What do you think are the best ways of dealing with plastic waste?
Plastics Waste Denominator Study For Canada (2012)
Kelleher Environmental, Canadian Plastics Industry Association
Detailed report on the quantity of plastic waste produced in Canada by the residential and non-residential sectors.