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.