How synthetic biology could help degrade plastic waste

After a year of stag­na­tion due to the pan­dem­ic in 2020, glob­al plastics pro­duc­tion has star­ted to rise again: 390 mil­lion tonnes were pro­duced in 2021, 90% of which was pro­duced from fossil resources¹. Once used, the major­ity is likely to end up as waste, adding to the 5 bil­lion tonnes that had already accu­mu­lated in 2015, rep­res­ent­ing 79% of all plastic waste pro­duced by human­ity². The envir­on­ment­al prob­lems posed by these mater­i­als are numer­ous. Could micro-organ­isms help solve them?

Although plastics are pol­lut­ants for the vast major­ity of liv­ing beings, some bac­teria and fungi have acquired the abil­ity to turn them into an energy source. Found in the soil, the sea or the intest­ines of some anim­als, sev­er­al spe­cies of micro-organ­isms can attack dif­fer­ent types of plastics (PET, PP, PS, PE, PUR, PLA…) thanks to enzymes that alter these long hydro­car­bon chains³. How­ever, this does not mean that simply put­ting them in con­tact with the right plastic will make it disappear.

The pro­cess is most effect­ive when the plastics have already been dam­aged, for example by UV light or chem­ic­al treat­ments, and when the micro-organ­isms are placed under optim­al pH and tem­per­at­ure con­di­tions⁴. How­ever, each spe­cies is only act­ive on cer­tain plastics and being able to attack a mater­i­al does not imply being able to degrade it com­pletely. Even the strains that have attrac­ted the most atten­tion, such as Ideon­ella sakaien­sis, a bac­teri­um that breaks down PET into its con­stitu­ent monomers⁵, have a major lim­it­a­tion: they need a few weeks or even months to degrade lim­ited quant­it­ies of plastics. In oth­er words, these plastic-eat­ing micro-organ­isms are not an ideal solu­tion for man­aging our pol­lu­tion. But they could still be of great help to us!

Pro­gress in genet­ics, in terms of under­stand­ing the mech­an­isms involved, anti­cip­at­ing pos­sible res­ults, and devel­op­ing molecu­lar tools, now makes it pos­sible to vol­un­tar­ily modi­fy gen­omes – in par­tic­u­lar to make organ­isms syn­thes­ise cer­tain types of pro­tein. Many bio­lo­gic­al research pro­jects now involve the pro­duc­tion of cells or even organ­isms that over- or under-express cer­tain genes. The genet­ic modi­fic­a­tions that make it pos­sible to obtain them are almost a craft, with each research­er mak­ing what they need. But this approach can be adap­ted to a com­pletely dif­fer­ent scale!

Since 2000, syn­thet­ic bio­logy has pro­posed apply­ing engin­eer­ing meth­ods to molecu­lar bio­logy. It con­siders genes and oth­er DNA sequences (espe­cially reg­u­lat­ory ones) as build­ing blocks that can be optim­ised and com­bined in a meta­bol­ic engin­eer­ing logic. The new bio­lo­gic­al syn­thes­is path­ways thus devised can then be implanted in cells, gen­er­ally micro-organ­isms, which become small genet­ic­ally mod­i­fied pro­duc­tion factor­ies. This approach has its lim­it­a­tions, the main one being the chaot­ic com­plex­ity of liv­ing organ­isms. A syn­thet­ic path­way that seems optim­al in the­ory does not always work in prac­tice, once con­fron­ted with the real­ity of a cell’s envir­on­ment. And the trans­ition to a lar­ger scale, which is essen­tial for many applic­a­tions, con­sti­tutes an addi­tion­al level of dif­fi­culty: liv­ing sys­tems often remain sens­it­ive and unpredictable.

Nev­er­the­less, in the last twenty years, syn­thet­ic bio­logy has become much more than a the­or­et­ic­al vis­ion. Many com­pan­ies are invest­ing in this approach, which has already led to the com­mer­cial­isa­tion of vari­ous products in fields as diverse as medi­cine, food, and mater­i­als⁷. Fuelled by bio­tech­no­lo­gic­al advances such as DNA syn­thes­is, high-through­put sequen­cing, and new gene-edit­ing tech­niques, sup­por­ted by increas­ingly power­ful com­puter tools and integ­rat­ing new know­ledge struc­tured in increas­ingly rich data­bases, it seems reas­on­able to expect syn­thet­ic bio­logy to pro­duce break­throughs⁸.

Microor­gan­isms cap­able of degrad­ing plastics may not be effi­cient enough to be use­ful on a large scale, but their study provides new ammuni­tion for syn­thet­ic bio­logy! Each enzyme dis­covered enriches the cata­logue of tools avail­able to design and optim­ise meta­bol­ic path­ways. And the modi­fic­a­tion of these pro­teins after study­ing their struc­ture some­times makes it pos­sible to obtain even more effi­cient ver­sions of them, only a few years after their dis­cov­ery⁹¹⁰.

Unlike oth­er recyc­ling meth­ods, which involve a loss of mater­i­al qual­ity, the bio­lo­gic­al degrad­a­tion path­ways allow the con­stitu­ent monomers of the plastics to be returned. These can then be reas­sembled to pro­duce a product equi­val­ent to new, with no con­straints on the col­ours or types of objects that can be man­u­fac­tured. How­ever, there are still prob­lems to over­come. On the one hand, this decon­struc­tion of plastics releases the addit­ives added to these mater­i­als, which must be man­aged on their own. On the oth­er hand, these approaches remain more expens­ive than pro­duc­tion from fossil resources, and incent­ives will be needed to push man­u­fac­tur­ers to imple­ment them. Moreover, as yields are nev­er per­fect, the life cycle of plastics will not be infin­ite. Finally, even if they are no longer purely the­or­et­ic­al, these pro­cesses are still being developed!

One of the pion­eers of plastic degrad­a­tion using syn­thet­ic bio­logy is a French com­pany: Car­bios (https://​www​.car​bios​.com/fr/). It has developed a pro­cess based on an enzyme called LCC, iden­ti­fied in 2012 by Japan­ese research­ers who car­ried out a meta­ge­n­om­ic ana­lys­is (https://​www​.poly​tech​nique​-insights​.com/​t​r​i​b​u​n​e​s​/​s​a​n​t​e​-​e​t​-​b​i​o​t​e​c​h​/​l​a​-​m​e​t​a​g​e​n​o​m​i​q​u​e​-​c​o​m​m​e​n​t​-​e​t​u​d​i​e​r​-​l​a​-​b​i​o​d​i​v​e​r​s​i​t​e​-​m​i​c​r​o​s​c​o​pique) of a com­post¹¹. Com­pared with oth­er pro­teins cap­able of degrad­ing PET, LCC was found to be par­tic­u­larly effect­ive. Muta­tions were used to improve its activ­ity and tem­per­at­ure res­ist­ance to pro­duce monomers that effect­ively reman­u­fac­tured PET to a qual­ity com­par­able to new PET at a reas­on­able cost¹². After set­ting up an indus­tri­al demon­strat­or¹³, the com­pany con­tin­ued its devel­op­ment with the con­struc­tion of its first biore­cyc­ling site¹⁴.

Bey­ond this example, the num­ber of pat­ents linked to the recyc­ling of plastics and the devel­op­ment of altern­at­ive mater­i­als shows the dynam­ism of this sec­tor¹⁵, which is of interest to both industry and fun­da­ment­al research. Alone or in com­bin­a­tion with oth­er approaches¹⁶, syn­thet­ic bio­logy is one of the tools that could make it pos­sible to lim­it the pol­lu­tion linked to the pro­duc­tion and use of plastics. 

How­ever, this should not detract from the many issues raised by its use. Some are very clear, such as the man­age­ment of addit­ives, the improve­ment of yields, the optim­isa­tion of costs, the adapt­a­tion to dif­fer­ent types of plastics and, gen­er­ally, the dif­fi­culties of devel­op­ment and scal­ing up. Oth­ers touch on more del­ic­ate issues. Indeed, the micro-organ­isms pro­duced by syn­thet­ic bio­logy are genet­ic­ally mod­i­fied. This raises ques­tions about the pat­entab­il­ity of liv­ing organ­isms, but also about the risks of release into the nat­ur­al envir­on­ment. While the abil­ity to degrade plastics is use­ful in a waste man­age­ment con­text, it seems import­ant to keep con­trol of it. 

In gen­er­al, there is still a long way to go to achieve a reas­on­able use of plastics. Estab­lish­ing a true cir­cu­lar eco­nomy is a chal­lenge in itself¹⁷, redu­cing our depend­ence on these ubi­quit­ous mater­i­als is anoth­er. While it is rel­ev­ant to con­sider each tool to move in the right dir­ec­tion, no single tool will be suf­fi­cient to solve all the problems.