The burgeoning field of plastic biorecycling was given a boost in 2016 when a paper in Science described a new bacterial species, Ideonella sakaiensis 201-F62. This microbe, found growing on bottles in a Japanese plastic recycling facility, contained the first described PETase enzyme, which depolymerizes PET into mono(2-hydroxyethyl) terephthalate (MHET). A second enzyme in the same bacterium then cleaves MHET into its constituent monomers: terephthalic acid and ethylene glycol. These are, conveniently, the key ingredients to make more PET.
Bacteria evolve rapidly and so will find a way to adapt to their environment and to eat whatever carbon sources are available, including plastic, explains Ronan McCarthy, an engineer at Brunel University in the UK. It’s likely that I. sakaiensis 201-F6 evolved its plastic-eating enzymes from ferulic acid esterases, which are useful for digesting through plant walls.
“There’s a reason microorganisms inhabit almost every site on the planet,” says McCarthy admiringly, who adds that bacteria are powerful tools “to tackle some of the more challenging environmental pollutants in a very sustainable and eco-friendly way.”
Ideonella sakaiensis 201-F6, as the first bacterium found to use plastic as a food source, sparked a wave of interest in bioremediation. Until then, scientists’ focus had been on cutinases—lipolytic/esterolytic enzymes that hydrolyze cutin (a component of the plant cuticle). Cutinases allow fungi to digest their way through the plant skin, allowing spore adhesion and release, but some can also digest plastic, albeit slowly. Plastic degradation represents an “enzymatic kind of promiscuity” for these species, says McCarthy, as naturally occurring cutin has carbon–carbon bonds similar to those of manufactured plastic.
For Carbios, the secret sauce is plastic-eating microbial enzymes, which it supercharges using genetic engineering to speed up the naturally slow process. To find the best plastic-eating enzyme for its biorecycling plant, the biotech screened five fungal and bacterial enzymes (including from I. sakaiensis 201-F6) to see which was fastest at depolymerizing PET. The winner was the unimaginatively named ‘leaf-branch compost cutinase’—discovered by another Japanese team in 2012 (ref. 3) and so called because it was found during a metagenomic screen of leaf and branch compost collected from a Japanese park.
“This enzyme was the best,” says Alain Marty, CSO of Carbios, “but was not very good,” and so he and his team set out to improve it through genetic engineering4. Marty directed the team to target 11 amino acids in active sites for mutagenesis, generating 209 variants. They then used microfluidic screening to test the mutants, searching for those with increased depolymerization activity or improved thermal stability. They hit the jackpot, with the best of the mutants able to depolymerize 90% of PET in under 10 h. Crucially for Carbios’ business model, the products of this reaction could then be used for plastic manufacturing of new bottle-grade PET.
The company announced this year that it will churn out biorecycled plastic for two ‘grandes dames’ of French cosmetics, L’Oréal and L’Occitane, as well as for clothing manufacturers, who call PET by its common name: polyester. The plant’s location in the French commune of Longlaville is conveniently close to the borders of Germany, Luxembourg and Belgium, and it will soon be biorecycling 50,000 tonnes of plastic waste per year (much of it coming across the nearby borders), equivalent to 2 billion bottles. “It is a good location to find waste,” says Marty.
Carbios was founded by VC fund Truffle Capital (Truffle CEO Philippe Pouletty is chair of the Carbios board) and has received grants of €54 million from French national and regional governments, as well as investment from L’Oréal’s BOLD (Business Opportunities for L’Oreal Development) fund, L’Occitane and tire company Michelin. The company raised €114 from the issuance of 3 million new shares in May 2021 to help cover the €230 million cost of the new plant. As well as selling the raw recycled ingredients for plastic, Carbios aims to sell its enzymes and license the technology to other companies, says Marty.
Government interest in plastic biorecycling is high. French president Emmanuel Macron praised Carbios’ new plant, which is due to open by the end of 2025, in a statement for the ground-breaking ceremony in April: “At a time when governments are negotiating an international treaty against plastic pollution in Ottawa, the groundbreaking of Carbios’ biorecycling plant is particularly significant.” In August, Carbios announced that it would partner with Northampton-based FCC Environment, a recycling and waste management company, to build a similar plant in the UK.
