Mutant Enzyme Recycles Plastic in Hours, Could Revolutionize Recycling Industry
A garbage yard in Lucknow, India where plastic bottles are dumped before being sent to recycling. Abhimanyu Kumar Sharma / Moment / Getty Images
Scientists have engineered a mutant enzyme that converts 90 percent of plastic bottles back to pristine starting materials that can then be used to produce new high-quality bottles in just hours. The discovery could revolutionize the recycling industry, which currently saves about 30 percent of PET plastics from landfills, reported Science Magazine.
Poly(ethylene terephthalate) (PET) is the plastic used in soda bottles, textiles and packaging. With almost 70 million tons manufactured annually worldwide, it is also the most abundant polyester plastic because it is strong and lightweight, explains the study's abstract, which was published in Nature.
Unfortunately, current PET recycling is inefficient. When plastics of different colors are melted down during the recycling process, a gray or black plastic starting material results that few companies want to purchase to package their products, explained Science Magazine. The process results in low-grade plastic fibers only good enough for clothing and carpets, reported The Guardian. These eventually end up in a landfill or incinerated, added Science Magazine.
"It's not really recycling at all," explained professor John McGeehan, the director of the Centre for Enzyme Innovation at the University of Portsmouth, to Science Magazine.
McGeehan, who was not involved in the research, called the new enzyme "a huge step forward," reported Science Magazine. He also noted that Carbios, the French sustainable plastics company behind the breakthrough, is the industry leader in engineering enzymes to break down PET at large scale, reported The Guardian.
For years, scientists at the forefront of the cultural war on plastic waste have searched for microbial enzymes that could break down PET and other plastics. Recently, a group of German scientists discovered a bacterium that breaks down polyurethane plastic and uses it for food to fuel the process, but cautioned it might be a decade before commercial scale could be reached. Similarly, in 2018, researchers led by McGeehan accidentally discovered an enzyme that digests PET, but noted that it doesn't do so very quickly (a few days). Even the base enzyme used in Carbios' research, leaf-branch compost cutinase (LLC) has been studied since 2012, when it was discovered in a compost heap of leaves by researchers at Osaka University, reported Science Magazine.
The team at Carbios screened 100,000 micro-organisms for promising candidates, and eventually began introducing mutations to LLC. Native LLC falls apart after just a few days of working at 65°C, the temperature at which PET begins to soften but not yet melt, explained Science Magazine.
The researchers re-engineered LLC to work faster and at higher temperatures. Alain Marty, Carbios' chief scientific officer, teamed up with Isabelle Andre, an enzyme engineering expert at the University of Toulouse, to isolate an optimized enzyme that is 10,000 times more efficient at breaking down the chemical bonds in PET than native LLC, reported Science Magazine. It also functions at 72°C, close to the temperature at which PET melts, making the process even more efficient.
The mutant enzyme broke down 90% of 200 grams of PET in a small reactor in just 10 hours. Different colors didn't matter because the enzyme can ignore dyes and other plastics in the molten mix. The researchers were able to use the resultant chemical building blocks to produce new PET and food-grade plastic bottles that were just as strong as those made from virgin plastics, reported Science Magazine.
"It's a real breakthrough in the recycling and manufacturing of PET," said Dr. Saleh Jabarin, a member of Carbios' scientific committee in a company statement about the discovery. "Thanks to the innovative technology developed by Carbios, the PET industry will become truly circular, which is the goal for all players in this industry, especially brand-owners, PET producers and our civilization as a whole."
Carbios reports that the enzyme costs just 4% of what virgin plastic made from oil costs, noted The Guardian. The enzyme cannot be added to waste plastic until it is ground up and melted, however, so the recycled PET is still more expensive than virgin plastic. Still, McGeehan noted to Science Magazine that companies may be willing to pay a bit more for a recycled plastic that is as durable and attractive as the virgin material.
Carbios is partnering with major companies, including Pepsi and L'Oréal, to accelerate development and produce the new enzyme at scale, reported The Guardian. It is aiming for industrial-scale recycling within five years. Science Magazine reported that a demonstration plant that can recycle hundreds of tons of PET per year should be online by next year.
"We are the first company to bring this technology on the market," said Martin Stephan, the deputy chief executive at Carbios, to The Guardian. "Our goal is to be up and running by 2024, 2025, at large industrial scale."
McGeehan explained to The Guardian that the discovery could finally make true industrial-scale biological recycling of PET a possibility. The new enzyme is faster, more efficient and more heat-tolerant — qualities that make this such a large advancement for the field, he added.
McGeehan said, "It represents a significant step forward for true circular recycling of PET and has the potential to reduce our reliance on oil, cut carbon emissions and energy use, and incentivize the collection and recycling of waste plastic," reported The Guardian.
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By Jacob L. Steenwyk and Antonis Rokas
When Looking Through a Microscope Isn’t Close Enough.<p>For the last few years, <a href="http://www.rokaslab.org/" target="_blank">our team at Vanderbilt University</a>, <a href="https://www.researchgate.net/lab/Gustavo-Goldman-Lab" target="_blank">Gustavo Goldman's team at São Paulo University in Brazil</a> and many other collaborators around the world have been collecting samples of fungi from patients infected with different species of <em>Aspergillus</em> molds. One of the species we are particularly interested in is <a href="https://doi.org/10.1006/rwgn.2001.0082" target="_blank"><em>Aspergillus nidulans</em>, a relatively common and generally harmless fungus</a>. Clinical laboratories typically identify the species of <em>Aspergillus</em> causing the infection by examining cultures of the fungi under the microscope. The problem with this approach is that very closely related species of <em>Aspergillus</em> tend to look very similar in their broad morphology or physical appearance when viewing them through a microscope.</p><p>Interested in examining the varying abilities of different <em>A. nidulans</em> strains to cause disease, we decided to analyze their total genetic content, or genomes. What we saw came as a total surprise. We had not collected <em>A. nidulans</em> but <em>Aspergillus latus</em>, a close relative of <em>A. nidulans</em> and, as we were to soon find out, <a href="https://doi.org/10.1016/j.cub.2020.04.071" target="_blank">a hybrid species that evolved through the fusion of the genomes</a> of two other <em>Aspergillus</em> species: <em>Aspergillus spinulosporus</em> and an unknown close relative of <em>Aspergillus quadrilineatus</em>. Thus, we realized not only that these patients harbored infections from an entirely different species than we thought they were, but also that this species was the first ever <em>Aspergillus</em> hybrid known to cause human infections.</p>
Several Different Fungal Hybrids Cause Human Disease.<p>Hybrid fungi that can cause infections in humans are well known to occur in several different lineages of single-celled fungi known as yeasts. Notable examples include multiple different species of <a href="https://doi.org/10.1002/yea.3242" target="_blank">yeast hybrids</a> that cause the human diseases <a href="https://rarediseases.info.nih.gov/diseases/6218/cryptococcosis" target="_blank">cryptococcosis</a> and <a href="https://www.cdc.gov/fungal/diseases/candidiasis/index.html" target="_blank">candidiasis</a>. Although pathogenic yeast hybrids are well known, our discovery that the <em>A. latus</em> pathogen is a hybrid is a first for molds that cause disease in humans.</p>
The Evolutionary Origin of an Aspergillus Hybrid.<p>Multiple evolutionary paths can lead to the emergence of hybrids. One path is through mating, just as the horse and donkey mate to create a mule. Another path is through the merging or fusion of genetic material from cells of different species.</p><p>It is this second path that appears to have been taken by our fungus. <em>A. latus</em> appears to have two of almost everything compared to its parental species: twice the genome size, twice the total number of genes and so on. But unlike other hybrids, which are often sterile like the mule, we found that <em>A. latus</em> is capable of reproducing both asexually and sexually.</p><p>But how distinct were the parents of <em>A. latus</em>? By comparing the parts contributed by each parent in the <em>A. latus</em> genome, we estimate that its parents are approximately 93% genetically similar, which is about as related as we humans are with lemurs. In other words, <em>A. latus</em>, an agent of infectious disease, is the fungal equivalent of a human-lemur hybrid.</p>
How A. Latus Differs From its Parents.<p>Elucidating the identity of closely related fungal pathogens and how they differ from each other in infection-relevant characteristics is a key step toward reducing the burden of fungal disease. For example, we found that <em>A. latus</em> was three times more resistant than <em>A. nidulans</em>, the species it was originally identified as using microscopy-based methods, to one of the most common antifungal drugs, <a href="https://www.drugbank.ca/drugs/DB00520" target="_blank">caspofungin</a>. This result provides a clear example of the potential importance of accurate identification of the <em>Aspergillus</em> pathogen causing an infection.</p><p>We also examined how <em>A. latus</em> and <em>A. nidulans</em> interact with cells from our immune system. We found that immune cells were less efficient at combating <em>A. latus</em> compared to <em>A. nidulans</em>, suggesting the hybrid fungus may be trickier for our immune systems to identify and destroy.</p><p>In the midst of the COVID-19 pandemic, our quest to understand <em>Aspergillus</em> pathogens is becoming more urgent. Growing evidence suggests that <a href="https://doi.org/10.1111/myc.13096" target="_blank">a fraction of COVID-19 patients are also infected with <em>Aspergillus</em>.</a> More worrying is that these <a href="https://doi.org/10.3201/eid2607.201603" target="_blank">secondary <em>Aspergillus</em> infections</a> can worsen the clinical outcomes for those infected with the novel coronavirus. That being said, we stress that little is known about <em>Aspergillus</em> infections in COVID-19 patients due to a lack of systematic testing, and none of the infections identified so far appear to have been caused by hybrids.</p><p>So, when it comes to hybrids, some are fantastic (the minotaur), some are helpful (the mule) and some are dangerous (<em>Aspergillus latus</em>). Understanding more about the biology of <em>Aspergillus latus</em> may help in our understanding of how microbial pathogens arise and how to best prevent and combat their infections.</p>
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