Diversity of Microbes and Enzymes to act on Plastics is still limited – New Future Challenges
Plastic waste pollution is a global environmental problem, specifically in oceans- where plastic debris kill sea animals and birds. Millions of tons of plastic trash are fouling the world's ocean, most of the trash breaks down into tiny pieces of microplastic less than a quarter-inch in size. It is coated with microbes to make a biofilm “plastisphere” to influence their fate- breaking them into tiny bits. It tastes like food to marine animals, as a result these threaten their lives.
Despite the increased level of recycling activities, most of plastic waste still end up in the landfills or environment. It releases Toxic bisphenol A and PS Oligomer in to water during decomposition. Under sunlight it breaks down into small pieces through photodegradation.
Generally speaking, plastic is incredibly resistant to break down. The larvae of a common insect, Galleria mellonella is able to biodegrade one of the toughest, most resilient plastic: polyethylene into ethylene glycol which degrades rapidly- discovered by Federica Bertocchini of the Institute of Biomedicine and Biotechnology of Cantabria, Spain. Wax worms can create holes in plastic shopping bags, which convert polyethylene into ethylene glycol (antifreeze agent). The larvae of Plodia interpunctella wax worm also have shown to digest plastic.
Average one worm gets through about 2 mgs of plastic a day. You’d need billions of caterpillars eating constantly all year round to deal with that!!
We might also think twice to breed bees in plastic processing centres. As the bee population is already under stress from pesticides. The idea is if bees disappear can create trouble in pollination and crop cultivation.
The Indian meal moth house the bacteria in digestive tract that can breakdown plastic, discovered in 2014. The gut microbes in mealworm, caterpillars can eat through it and even the larvae can chew to plastic. It’ll take some time to establish the process of recycling by using microbes and worms.
A rare fungus Pestalotiopsis microspora in the Amazonian rain forests of Ecuador have found to consume polyurethane. Fungi Mutarium is a prototype project to generate edible fungus biomass mushroom cups, uses pod seaweed agar filled with plastic. It is tastes sweet, licorice like. The entire process takes several months from start to finish.
It not very clear that this project of fungus Mutarium is whether a prototype, devouring plastic waste or perhaps one day it’ll be a food as well.
A team of Japanese scientists identified a bacterium existing in the wild can feed on another common plastic, polyethylene terephthalate used for making soft drinks and water bottles.
Enzymes like putative polyurethanases, alkane hydroxylase, lipase, esterase, phenol oxidase etc. help in degradation of plastic, these are secreted by microbes. When plastic is heated anaerobically it breaks down in to terephthalic acid, oil and gas. Pseudomonas bacterium easily digest the terephthalic acid, which is mostly used in various medical supplies. Plastic items take 10-1000 years to decompose generally. Polymers degrade under the influence of environmental factors such as heat, light and chemicals.
Advanced research says that bacteria can degrade the plastic.
There were small number of bacteria and fungi used to degrade PET plastic before the discovery of Ideonella sakaiensis family Comamonadaceae, originally isolated from the plastic bottle recycling facility, which has a potential to degrade plastic. Ideonella first discovered by Kohei Oda of Kyoto Institute of Technology.
Jun Yang of Beihang University conducted an isotopic labelling study for the consumption of polystyrene. Wei-Min Wu at Stanford University suggests that the organisms convert half of the polystyrene carbon they ingest to CO2 rather than to styrene monomers. Raman Narayan, a specialist in biodegradable polymer system, is still interested to have some reproducibility in results. Yang’s team used mass measurements, gel permeation chromatography and infrared and nuclear magnetic resonance spectroscopy instead of isotopic labelling study. The Yang has also sequenced the genome of bacteria to identify the relevant enzymes and scaling up their production.
The methods for determining the degradation level are indirect, not enough to show that organisms are breaking down the polymers. These demonstrate the incomplete degradation and there’s a chance that leftover plastic can go back to the environment and contribute to the pollution in oceans
“It would only be natural that microbes have and will continue to evolve to consume plastics, since they are increasingly abundant in nature,” says Richard Gross, Rensselaer Polytech Institute.
We need more examples to answer this question. Researchers in Japan in 2016 tested the sludge from a recycling plant, uncovered a microbe that could completely breakdown the PET film to CO2 and H2O. The two enzymes degrade the PET monomers into terephthalic acid and ethylene glycol.
Breakdown tool for plastic recycling by microbes is too slow to implement this technology for immediate applications.
Cutinase enzyme degrade the cutin and some PET like polysters. More work needs to be done as on the following:
- Optimization of the enzyme level works better
- Higher activity of mutated enzymes could erode crystalline PET, as enzymes evolve
- Stability of enzymes at higher temprature to improve the binding affinity for PET & break the bonds
- Process optimization is needed to stop the enzyme aggregation to enhance the performance and strategically putting the carbohydrate groups to prevent aggregation.
Emily Flashman, an enzyme specialist added a biocycling process can help discover the productive way to harvest and reuse monomer products. In monomer isolation they want to make the enzyme which can be stable at higher tempratures.
Optimization process is expensive for biotechnological processes requires more investments. Narayan says it is tough to compete with low price of monmers. Changing attitude and changing demands from consumers could create the balance in favour of recycling methods that use biocatalysts rather than the chemical catalysts if it reaches to the competitive cost.
Could nature get rid of the plastic waste? Scientists have managed to biodegrade plastic, but the degradation level is slow & incomplete. If the polymer biodegradation technologies were implemented, they would make the plastic recycling process cost effective & environmentally friendly.
Carbios, a French firm, has optimized an enzyme that depolymerizes 97% of PET starting material ( 200 Kg) into monomers in 24 hours. Their goal is to produce a new plastic product from recovered monomers and test the resulting product with their cooperate partners. Audebert says, their business model is built on the fact to be competitive with virgin PET (Polyethylene Terephthalate).
Molecular crystallinity in polymer chains, harder for enzymes to breakdown. Polymers are not always 100% crystalline. For example, PET films are reported to breakdown because of this reason.
Future Challenges:
- Diversity of microbes and enzymes acting on polymers is still very limited.
- Implementation of enzymatic process can be a challenge for microbiologists at this stage. The main problem is highly robust polymers and their crystalline structures.
- Development and implementation of enzyme function - based assay is also important. Commercially available polymer substrates, contains additives, plasticizers and biodegradable impurities like phthalate, breaks down easily thus interferes with degradation analysis providing false positive results.
- Diversity of microbes and enzymes is possible by using functional metagenomics and biological dark matters studies, help find out unidentified microorganisms and non - coding RNA sequences which could relate to the evolutionary changes, could be a promising source to identify the enzymes.
- A short - term realistic goal would be related to develop “plastosomes” in microbes to attack the crystalline fibre along with the development of highly active enzymes for textile industries could help reduce the annual plastic pollution.
- Using synthetic biology to create microorganisms that would produce a highly valued compound like monomers and polymers from plastic waste will contribute to the circular use of plastics eg. value added product or biodegradable polymers.
A cocktail of enzymes could handle the problems to fully degrade not only PET but other plastics with different types of chemical bonds.
Plastic biocycling holds promise, but It is hard to state it as planet-saving breakthrough at this point of time. It is not right to say that problem is solved when it hasn’t actually. Otherwise, it can undermine the relationships between science and society.
Plastic biodegradation/ biocycling will be a highly rewarding research task, to generate highly active enzymes and production of true biopolymers, to reduce the plastic pollution significantly.
https://cen.acs.org/environment/sustainability/Plastics-recycling-microbes-worms-further/96/i25
https://www.theguardian.com/commentisfree/2017/apr/25/plastic-eating-bugs-wax-moth-caterpillars-bee
https://www.sciencedaily.com/releases/2017/04/170424141338.htm
https://www.smithsonianmag.com/smart-news/chow-down-plastic-eating-fungus-180958127/
https://aem.asm.org/content/85/19/e01095-19
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