Biodegradation of Synthetic Plastic in the Marine Habitat
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The marine environment is quite different from the land ecosystem in terms of temperature, oxygen concentration and light intensity, among other factors. Biodegradation of plastic waste on land can proceed over decades. The question then is what happens to the plastics in the marine environment? The estimated 5.25 trillion plastic pieces floating on the ocean surface1 have been identified as a serious global health issue. The accumulating data on marine plastics give us a better appreciation of the challenges and opportunities with biodegradation of marine plastics by marine microbes and their enzymes and its potential to be employed in marine plastics treatment.
The “missing” puzzle of plastic litter in the ocean
Approximately 1.5 to 4.1% of the plastics produced globally enter the oceans both intentionally and un-intentionally; thus, a total of about 117 to 320 million tons of plastics are present in the ocean.2 This amount of plastic litter would be sufficient to wrap up a medium sized country in Europe.
The plastic wastes on land are typically collected and treated in various ways, including dumping in landfills, incineration, granulation and pyrolysis. However, plastics finding their way into the oceans have an entirely different fate, with humans practically unable to manipulate marine plastics on a large scale currently.
Although there is a lack of technologies to detect, monitor and quantify plastics floating on ocean surfaces or settling on the ocean floor, scientists have enough evidence to suggest that a large bulk of the plastics in oceans are “missing”.3 So, where do or did they disappear to?
As attested to by the hundreds of distressing pictures depicting an unusual intermingling of marine animals and synthetic plastics, some of the plastics in the oceans are ingested by or become intertwined with marine species. The other explanation for this puzzle is degradation.
Biodegradation of marine plastics
Plastics are degraded in the ocean through three pathways:
- mechanical degradation (break-down of large plastic pieces into smaller plastic fragments by mechanical action)
- photodegradation (via free radical chain reactions initiated by solar UV radiation)
- biological degradation or biodegradation (by microorganisms in the marine habitat)
Data on the degradation extent and the degradation products from photooxidation and biodegradation of plastics in the oceans are relatively limited.
Model studies on plastic photodegradation suggest that it could take several years to degrade into microplastics or nanoplastics and it could take hundreds of years for plastics to disappear completely (for example, into hydrocarbon gases).
Biodegradation of plastics relies on the marine microbes. Since the first introduction of plastics into oceans in the 1950s, some marine microorganisms, namely bacteria and fungi, have shown a capacity to adapt and evolve to utilize plastics as a source of carbon and/or chemical energy. The microbial communities colonizing marine plastic surfaces, which are also referred to as plastispheres are unique and different to those from the surroundings. Microbial populations on plastispheres favor plastic degradation4 and thus, plastispheres have been a focal point to discover microbes able to degrade plastics in the marine environment.
Microbial degradation of plastics requires enzymes capable of breaking down plastic materials into oligomers, dimers and monomers, as presented in Figure 1, below. One such famous plastic-degrading enzyme, polyethylene terephthalate (PET) enzyme (PETase), was first discovered from Ideonella sakaiensis collected in a PET bottle recycling sludge in Japan in 2016.5 Recent finding suggested that the increasingly available plastics in ocean have driven a rapid evolution of oceanic PETases globally.6 This enzyme catalyzes the cleavage of PET into monomeric mono-2-hydroxyethyl terephthalate (MHET) which can be further degraded into non-hazardous monomers by another enzyme, MHETase. PETase can shorten the degradation of PET from decades to days. Other enzymes found with capacity to degrade plastics include amidases, oxygenases, laccases, peroxidases, lipases, esterases, cutinases and serine hydrolases. Therefore, plastic monomers could also provide a carbon source for marine microbial communities, traversing the cell membrane and subsequently being oxidized during microbial metabolism, resulting in the release of CO2, N2, CH4, and H2O.
Figure 1: Biodegradation of marine plastics by marine microbes and their enzymes. Credit: Yi Zhang, PhD.
Although knowledge about plastic biodegradation in the ocean in terms of the rate, process and mechanisms is still scarce, the information known and lessons learnt from
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