The vast amounts of plastic waste that our society produces each year is a huge problem. Waste makes its way into the rivers and oceans and clogs up landfills in developing countries, causing a ticking ecological timebomb. This is a huge challenge for governments and scientists that must be overcome.
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The Scale of the Plastic Problem
The scale of the problem cannot be understated. It is estimated that there are as many as 5.25 trillion pieces of plastic waste in the oceans. There are so many that the Great Garbage Patch in the Pacific Ocean has grown to three times the size of France. Marine ecosystems are being damaged almost beyond repair.
Plastic can take centuries to degrade. Most plastic is designed as single-use, thrown away after being used only once. A total of 5 trillion plastic bags are used worldwide each year, and researchers estimate that since the early 1950s, more than 8.3 billion metric tons of plastic have been manufactured. That is as heavy as one billion elephants, or 822,000 Eiffel Towers.
A total of 60% of all plastic produced ends up in landfills and the ocean. The current way to manage plastic waste is recycling, but not all plastics can be recycled by existing methods. We only recycle around 9% of all plastic waste, with 79% ending up in landfills or the natural environment, and 12% incinerated. Many developed nations, lacking the infrastructure to recycle all their plastic, ship it overseas where it piles up into mountains of waste that damage local communities.
Why 99% of ocean plastic pollution is "missing"
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New Ways of Thinking Vital to Meet the Challenges
If the challenge of plastic waste is to be met, new ways of thinking must be implemented. Scientists working in several fields are stepping up to the plate to provide solutions to the problem. Companies working in the field of robotics are providing solutions to waste cleanup, new biodegradable materials are being used in manufacture, and public tastes are shifting away from single-use plastic to more sustainable options.
Alternative Methods to Recycling
Commonly used recycling methods turn plastic into another product that contains the same one. However, quality can degrade over time. Alternatives include chemical recycling, where a polymer is depolymerized into its constituent monomers, allowing for reconstruction of the same or a different polymer. Another alternative is upcycling the polymer into a different value-added chemical. However, all these methods are closed-looped.
Another method is to use bio-sourced and biodegradable substances. Nature handles natural polymers such as lignin and cellulose differently than synthetic polymers. The problem with this approach is that they still degrade slowly, and the materials take time to grow. If these are being used for products with a short life span, there is a disparity between recycling and production that means the plastic is not as “green” as it could be.
No matter how you look at it, a sustainability issue exists for all current recycling methods. Research has been published looking at a plastic recycling alternative using proteins. This new approach has been termed nature-inspired circular-economy recycling (NaCRe.)
How Nature-Inspired Circular-Economy Recycling Works
Plastics are synthetic polymer-based molecules. Nature produces many more types of polymers, but these are sustainable. Proteins are a class of sequence-defined natural polymers that can be easily digested and broken down by organisms. They can then be reassembled into new proteins from the constituent amino acids, which are then utilized by the organism for multiple purposes. Proteins have reversibly cleavable bonds in their backbones that link amino acids together.
Natural systems and processes are inherently sustainable. The circular nature of protein recycling is impressive when one considers that there are only 20 naturally occurring amino acids that make up their structure. This is possible because proteins are sequence-defined polymers whose diversity is based upon the sequences of amino acids they are made up of, not their chemical diversity. A protein can be more complex than its “parent protein”, which breaks the recycling paradigm that says materials are recycled into lower versions.
In biological organisms, a staggering number of proteins can be synthesized by ribosomes from a random mixture of amino acids. Polymers with significant differences to the parent polymer can be made by this cell machinery. NaCRe works on this principle.
The Research
The team, led by Simone Giaveri, has presented this new approach in a paper published in Advanced Materials. In the study, fluorescent and bioactive proteins were synthesized extracellularly from a mix of amino acids from depolymerized proteins and/or peptides. An amino-acid-free, cell-free transcription-translation (TX-TL) system was utilized. The system used was PUREfrex, which is based on the PURE system and is commercially available.
Three peptides (glucagon, somatostatin 28, and magainin II) were first digested using the enzymes thermolysin (a thermostable metalloproteinase) and leucine aminoacidase. The method successfully recycled commercially viable proteins (β-lactoglobulin films, which are used in water filtration) into biotechnologically relevant proteins (catechol 2,3-dioxygenase.) Catechol 2,3-dioxygenase catalyzes the degradation of monoaromatic hydrocarbons.
The team performed control experiments to ensure that the TX-TL system was amino acid-free, showing that there was a lack of detectable protein expression. Initially, the research was focused on short peptides to develop a robust depolymerization method.
To better establish the commercial relevance of NaCRe, technologically relevant materials were successfully recycled. Incubating silk fibroin with thermolysin and LAP recovered free amino acids that were then used to express a green fluorescent protein (GFP.) The relevant yield was ≈95%, demonstrating NaCRe is capable of recycling polymers with high molecular weight structures. The method is in its infancy but shows interesting potential.
What Does This Mean for Future Sustainability?
By 2050, the world population is predicted to top 10 billion people. With this growth, the demand for plastics will increase, meaning more waste will be generated. This research demonstrates a new way to recycle polymers into other commercially viable materials, providing the means for a circular economy to flourish.
One of the main challenges that face the widespread adoption of this technology is scaling, which will require optimizing the process. However, upscaling the NaCRe process will allow for the identification of suitable proteins and therefore give the method true technological opportunities.
References and Further Reading
Giaveri, S et al. (2021) Nature-Inspired Circular-Economy Recycling for Proteins: Proof of Concept Advanced Materials, https://onlinelibrary.wiley.com/doi/10.1002/adma.202104581
Phys.org (Website) 8.3 billion metric tons: Scientists calculate total amount of plastics ever produced. Available at: https://phys.org/news/2017-07-billion-metric-tons-scientists-total.html
United Nations Environment Programme (Website) Our planet is drowning in plastic pollution – it's time for change! Available at: https://www.unep.org/interactive/beat-plastic-pollution/
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