And Their Pollution-Reducing Enzymes
In the 21st century, plastic is literally becoming part of everything as it gradually breaks down and builds up in the environment. This is a growing problem for life on Earth. Right now there is about 9 billion tons of plastic in the world, with 300 million tons of new plastic being produced every year. As part of this, one million new water bottles are produced every minute, and that’s just one of many different products. As if that wasn’t bad enough, less than 10% of all plastic winds up being recycled. Thus, every year, at least 8 million metric tons of discarded plastic end up in our oceans. So, at this rate, by 2050 there will be more plastic in the oceans than fish, and that’s assuming that the volume of waste won’t increase, but it most certainly will. As part of this, small plastic pellets are the result of industry and domestic plastic waste which has spread across the oceans. Some of the plastic pellets have even fragmented to microscopic particles. This is alarming because plastics are incredibly harmful chemical contaminants that pollute our soil, our food, our water, and our bodies. Plastic can and does disrupt ecosystems and create more and more areas where nothing can survive. Plastic debris kills millions of birds and fish annually. The sinister, and some might say signature, synthetic material has even been found in human stool, so there might be some sitting in your gut right now. The trouble is that there are a number of different health risks connected to the ingestion of microplastics and more specifically nanoplastics. The smallest of which can penetrate our cells and move into our tissues and organs, damaging our bodies in ways that doctors won’t fully understand for decades to come.
To make matters worse, plastics are complex polymers, which are repeating chains of molecules that don’t dissolve in water. So, the strength of these chains makes plastic substances very durable and it takes a really long time to disintegrate. It can take anywhere from about 500 to 5,000 years for some plastic products to fully break down. Just as one example among many, it takes 200 years for a plastic straw to fully decompose. Think about how many plastic bottles, bags, and packages you have already thrown away in your lifetime. Now think about how many more there will be. For all of these reasons and more, the production of plastic needs to be halted immediately and ultimately banned altogether. This is why more people need to pressure politicians to counteract corporate lobbyists, so companies like Walmart will stop littering the planet with shopping bags. The reality is that people will undoubtedly continue to use plastic, especially polyester, long into the future, but in this day and age, there is simply no need to produce virgin plastic from petrochemicals when it’s possible to remake pellets of every grade and type. To hell with concerns about cost-effectiveness, this is about moving toward zero waste for the sake of people, not profits. More to the point, since plastic pollution is so prevalent, in addition to reducing, reusing, and recycling, humanity needs to enlist the help of plastic-eating bacteria. Furthermore, because the problem of plastic pollution is so big, this needs to be done on an industrial scale.
The best plastic-eating bacteria is Ideonella sakaiensis. They are aerobic, rod-shaped motile cells, with single flagella that can specifically digest the plastic used to make single-use drink bottles, polyethylene terephthalate (PET). These extremely useful bacteria possess two enzymes, PETase and MHETase, which are able to digest PET plastic polymers in particular. Of these, PETase breaks down plastic into smaller parts, primarily MHET. Then, MHETase splits this into the two basic building blocks of PET, terephthalic acid and ethylene glycol, as shown in the scheme below. Once formed, these compounds can be further biodegraded into carbon dioxide by I. sakaiensis or other microbes, or they can be purified and used to manufacture new PET. Plastic can be broken down into smaller, soluble chemical units, and those building blocks can be harvested and recycled to form new plastics in a closed-loop system. This is important because PET is the most widely produced plastic in the world. PET is already used in everything from soda bottles to 3D printing filament, and new uses will inevitably be found in the future. More to the point, in a natural setting, once PET has been initially degraded and assimilated by the wild-type of I. sakaiensis, a consortium of microorganisms typically works together in the sediment to mineralize about 75% of degraded plastic into carbon dioxide. As part of this, objects made of lower-crystallinity PET plastics are easier for the bacteria to break down than that of higher-crystallinity plastics, with many household products being higher rather than lower. This makes the process that much more difficult.
Although there are other bacterial enzymes that can slowly digest PET, the enzymes in I. sakaiensis evolved specifically for that job. This is why the unique bacteria has such tremendous potential for use in the polymer biodegradation technologies of the future. Biocatalysis could someday make up a healthy share of plastic recycling. Of course, any prospective applications of the I. sakaiensis PETase enzyme in local and global bio-recycling programs will need to be preceded by genetic optimization through mutation. Scientists desperately need to overtake natural selection by engineering better forms of PETase. As part of this, the MHETase enzyme could also be optimized and used in recycling or bioremediation applications in combination with the PETase enzyme. As biological and technological advancements are made more and more will be done to increase the efficiency of everything, including this. For instance, to further maximize their metabolic power, X-rays can be used to make the bacteria 20% better at eating plastic by creating more effective enzymes. So, it’s possible to mutate the microorganisms and boost their plastic-eating planet-saving power in this way and surely many more. As yet another example of something that could be done to increase efficiency, the overall process could also include a UV pretreatment to destabilize the PET to make it easier for I. sakaiensis to digest polyester. Thus, in the not-too-distant future, the perfect microbial community could be housed in industrial-scale bio-reacters where they can survive and thrive on our plastic waste. The generations to come will just need to keep a lid on this and also hope that not too many more bacteria begin eating plastic in the wild. Otherwise, products that are designed to be durable may not be all that dependable. Still, with the possibility of bacterial bio-reactors, one of the many horrible crises currently facing Homo sapiens could at least be slightly averted.