BIOLOGICAL SOLUTIONS TO PLASTIC POLLUTION
Plastic pollution has become one of the most severe environmental problems, affecting our oceans and marine life, as well as human health. Does Biology have the solutions for it?
by-
Paarangi Chawla, B-Tech (Biosciences & Bioengineering), 1st year
IIT Roorkee
Remember the last time you ate some spicy momos on a plastic plate from the roadside vendor and got food poisoning? Well, that day, we all hated bacteria. But not all are bad!! Some actually work towards degrading that very plastic plate you ate in.
In recent years, reports on the biodegradation of synthetic plastics by microorganisms or enzymes have sprung up, and these offer a possibility to develop biological treatment technology for plastic wastes.
In 2016, a Japanese group led by Kohei Oda from Kyoto Institute of Technology found that a bacterium Ideonella sakaiensis can grow with Polyethylene terephthalate (PET), a common polymer of plastics, without other carbon sources, indicating its ability to degrade PET and break it down to small carbon units for cellular metabolisms.
A group led by Gregg T. Beckham from National Bioenergy Center subsequently found that the bacterium secretes two enzymes, PETase (PET-digesting enzyme) and MHETase (MHET-digesting enzyme), to sequentially break down the PET. They introduced mutation to see if it could change the PETase substrate’s specificity. One of the mutants showed increased PET degrading efficiency and exhibited the ability to degrade another plastic substitute, polyethylene-2,5-furandicarboxylate.
Meanwhile, a French research group, jointly run by the Toulouse Biotechnology Institute and a private company, has managed to discover another family of bacteria capable of secreting an enzyme, a cutinase, which is very efficient in breaking down PET, ten thousand times faster than the microorganism discovered by the Japanese. This could be a decisive forward step, but only time will tell.
Many other plastics have also been researched. One of the most common is Poly-Ethylene (PE). PE comprises a linear backbone of carbon atoms, which is resistant to degradation. Although PE is believed not to be susceptible to bio-degradation, a few attempts have been made, as PE is the most common packaging plastic. Slow (that occur over weeks/months) PE biodegradation has been observed, given appropriate conditions. For example, modest degradation of PE was observed after nitric acid treatment and incubation for three months in a liquid culture of the fungus Penicillium simplicissimum. Slow PE degradation was also recorded after 4 to 7 months of exposure to the bacterium Nocardia asteroides. In both cases, Fourier transforms infrared spectroscopy (FTIR) analysis of treated samples showed the formation of an absorbance peak around 3,300 cm-1, a signature for ethylene glycol, confirming PE degradation.
Scientists have also reported fast biodegradation of PE by the wax worm, the caterpillar larva of the wax moth Galleria mellonella of the snout moth (Pyralidae) family Lepidoptera. When a PE film was left in direct contact with wax worms during experimentation, holes started to appear after 40 minutes, with an estimated 2.2±1.2 holes per worm per hour. Gravimetric analysis of the treated samples confirmed a significant mass loss of 13% PE over 14 hours of treatment. This corresponds to an average degradation rate of 0.23 mg cm-2 h-1, markedly higher than the rate of PET biodegradation by bacterium Ideonella sakaiensis.
Scientists still doubt what allows the wax worm to degrade a chemical bond not generally susceptible to bio-degradation. The answer may lie in the ecology of the wax worm itself. They feed on beeswax, and their natural niche is the honeycomb; the moth lays its eggs inside the beehive, where the worms grow to their pupa stage, eating beeswax.
Beeswax comprises a highly diverse mixture of lipid compounds, including alkanes, alkenes, fatty acids, and esters. The most frequent hydrocarbon bond is the CH2–CH2, as in PE. Although the molecular details of wax biodegradation require further investigation, it seems likely that the C–C single bond of these aliphatic compounds is one of the targets of digestion. The appearance of holes when PE films are left in direct contact with wax worms, and the FTIR analysis of degraded PE, indicate a chemical breakdown of the PE, including breakage of C–C bonds. It is not clear whether the hydrocarbon-digesting activity of G. mellonella derives from the organism itself or the enzymatic activities of its intestinal flora. Further investigation is also required to determine if related species have the capacity for PE degradation. So, there is something that the waxworms enjoy more than honey, huh! PLASTIC!
Other plastics like PUR are also under the radar of many scientists. For example, in March 2020, German scientists discovered strains of bacteria capable of degrading Polyurethane plastic after collecting soil from a brittle plastic waste site in Leipzig.
Polyurethane (PUR) is a polymer derived from the condensation of polyisocyanate and polyol, and it is widely used as a base material in various industries. PUR, in particular polyester PUR, is known to be vulnerable to microbial attack. Recently, environmental pollution by plastic wastes has become a serious issue, and polyester PUR had attracted attention because of its biodegradability. There are many reports on the degradation of polyester PUR by microorganisms, especially by fungi. Microbial degradation of polyester PUR is mainly due to the hydrolysis of ester bonds by esterases. Recently, polyester-PUR-degrading enzymes have been purified, and their characteristics reported. Among them, a solid-polyester-PUR-degrading enzyme (PUR esterase) derived from Comamonas acidovorans TB-35 had unique features. This enzyme has a hydrophobic PUR-surface-binding domain and a catalytic domain, and the surface-binding domain was considered essential for PUR degradation. This hydrophobic surface-binding domain is also observed in other solid-polyester-degrading enzymes such as poly(hydroxy alkanoates) (PHA) depolymerases.
However, there was no significant homology between the amino acid sequence of PUR esterase and that of PHA depolymerases, except in the hydrophobic surface-binding region. Thus, PUR esterase and PHA depolymerase are probably different in terms of their evolutionary origin. Still, PUR esterases may come to be classified as a new solid-polyester-degrading enzyme family.
But to make any of these naturally occurring bacteria and fungi useful, they must be bioengineered to degrade plastic hundreds or thousands of times faster. And this is where the beloved synthetic biology makes its entry. In 2018 scientists in the U.K. and U.S. modified bacteria so that they could begin breaking down plastic in a matter of days. In October 2020, the process was improved by combining the two different plastic-eating enzymes that the bacteria produced into one “super enzyme.”
The first large-scale commercial applications are still years away, but thanks to the much research work done in the past few decades, it is still within sight.
Though biological solutions appear the most promising solutions to this plastic menace, there remain some question marks.
Even if these new technologies are one day deployed at scale, they would still face significant limitations and could even be dangerous.
All the breakthroughs have only discussed plastic-eating enzymes that digest plastics like PET and PE. However, other plastics, such as HDPE, used to make more rigid materials such as shampoo bottles or pipes, could prove more challenging to biodegrade using bacteria.
Even if one day it becomes possible to mass-produce bacteria that can be sprayed onto piles of plastic waste, such an approach could be dangerous — biodegrading the polymers that comprise plastic risks releasing chemical additives that are normally stored safely inside the un-degraded plastic.
Moreover, there are potential unknown side-effects of releasing genetically engineered microorganisms into nature. Since most likely genetically engineered microorganisms would be needed, they cannot be released uncontrolled into the environment.
Thus, we come back to the traditional solutions told by our school teachers every year on World Environment Day, SAY NO TO PLASTICS, SAVE THE MOTHER NATURE!! But despite any solution technology gifts us with, it will come with its pros and cons. Hence prevention right now is better than the cure.
“It is the worst of times but still the best of times because we still have a chance.”
