In the previous article, bioremediation processes involved in treatment and removal of oil hydrocarbon pollutants was discussed. In the present article, the problem of plastic pollution and associated bioremediation solutions are reviewed. Plastic waste or debris are one of the most hazardous pollutant entering the seas and oceans, after oil spills and sewage discharge. First of all, plastic waste contributed 60-80% of the marine litter and in 2012, global plastic production reached an all-time high of 330 million tons per year. Furthermore, it is estimated that of the total plastic waste produced every year, most of the plastic waste enters landfills. However, approximately 13 million tons enters the oceans every year (1,2).
Most noteworthy, among the plastic debris released into the seas and oceans, a significant proportion becomes floating debris or washed ashore. But, most sinks to the seafloor or gets broken into microplastics and subsequently enters the ecosystem as birds and marine mammals ingest them. With this respect, plastic microfibers is a bigger problem that has been estimated to be found at high levels of 4 billion per sq. km. (3,4). Although numerous projects have been established to improve plastic recycling systems and collecting marine trash, degradation of plastics has been at the forefront of the battle against marine plastic waste.
Among different degradation strategies, Bioremediation is one such tool that uses bacteria or fungi which can use plastics as a source of fuel and degrade them. This offers a highly advantageous solution to the global problem of marine plastic pollution.
Types of plastic and different degradation process
Plastics are high weight polymers, derived from organic monomers. They can be classified into two main groups on the basis of their source;
- Natural (renewable biomass like vegetable oil or microorganism).
- Synthetic (non-renewable biomass like fossil fuel).
These materials are considered highly useful as a result of their stability and durability, as well as resistance to degradation. But in case of dealing with plastic waste, they have been known to degrade through 3 main processes:
- Photo-degradation (UV-light),
- Thermo-degradation (heat & oxygen) and
- Biodegradation (microbes).
Among the three processes, biodegradation is the most environmental friendly process (5). Through biodegradation, bacteria and fungi can convert these highly inert synthetic polymers to simpler compounds. Furthermore, this aids in removal of the contaminants from the environment and prevent them from entering the ecosystem.
|Poly-β-hydroxybutyrate (PHB)||Polyethylene (PE)|
|Polyhydroxyvalyrate (PHV)||Polyethylene terephthalate (PET)|
|Cellulose acetate||Polyvinyl Chloride (PVC)|
|Polyhydroxyhexanoate (PHH)||Polyurethane (PUR)|
|Polylactic Acids (PLAs)||Polypropylene (PP)|
|Polyamide 11 (PA 11)||Polycarbonate (PC)|
Types of Natural and Synthetic Plastics, Source: Shah et. al., (2007) and Thomas et. al., (2013)
Microbial degradation as efficient process for plastic waste
Microbial degradation of plastics has been found to be a sustainable and efficient conversion of the pollutants to simple compounds by several studies in the last decade. This reveals a whole range of both soil and marine microbial species capable of this activity. As seen in the below table, several species of bacteria and fungi have been isolated, showing degradation properties of different types of plastic polymers. Consequently, plastics being resistant to degradation need certain pre-treatment like photo-oxidation or hydrolysis or enzymatic degradation by microorganisms, before the polymers can be metabolized by the organisms. Therefore, microbes are known to initiate the process of degradation on marine plastic through the formation of a biofilm and secretion of extracellular enzymes to aid in breaking down of large plastic polymers (5).
Type of Plastic
Species of Microorganism
|Polyethylene terephthalate||Ideonella sakaiensis (6)|
|Polystyrene||Rhodococcus ruber (7)|
|Polyethylene||Brevibacillus borstelensis (8)|
|Polycaprolactone||Pseudozyma jejuensis (9)|
|Low Density Polyethylene (LDPE)||Aspergillus versicolor (10)|
|Polyester polyurethane||Pestalotiopsis microspore (11)|
Microbial Species degrading different types of plastics
When marine plastic debris are studied for the bacterial biofilm community, a wide range of microbes are found within the biofilm. This includes:
- hydrocarbon degrading heterotrophs,
- autotrophs, symbionts and
- even predatory bacteria (12).
In addition, the most important step in degradation of bacteria is adhesion of microbes to the plastic surface, followed by biofilm formation. Here, the microorganisms have access to polymers (carbon source) for undertaking hydrolytic activities and also as source of energy.
A common property found among most of these microbes are their ability to break down complex polymers to smaller ones using depolymerase enzymes, before they can be absorbed into the cell for metabolism. In case of polymer degrading enzymes employed by bacteria, two main groups exist which are intracellular and extracellular depolymerases (5). While the extracellular enzymes help to breakdown the plastic polymer into shorter water-soluble chains like oligomers, dimers and monomers, these chains enter the microbial cell and are metabolized by the intracellular enzymes. Furthermore, several groups of depolymerases have been isolated from a large number of microbial (both bacteria and fungus) species capable of degrading different types of plastics (Table 3) (13,14).
|Poly (Ethylene Adipate)||Penicillium sp., R. arrizus, R. delemar, Achromobacter sp. and Candida cylindracea||Lipases|
|Penicillium sp., Aspergillus sp., R. arrizus, Clostridium sp.,||Lipases and Esterases|
|Poly (hexamethylene carbonate)||Roseateles depolymerans, Amycolatopsis sp., Candida cylindracea, Pseudomonas sp., Chromobacterium viscosus and R. arrhizus||Cholesterol esterase, Lipase, Lipoprotein Lipase|
|Polyurethanes (PU)||R. delemar, Curvularia senegalensis, Comamonas acidovorans,||Lipase and Esterase|
|Polyethylene (PE)||Acinetobacter sp., R. ruber||Alkane hydroxylase, laccase|
|Poly (3-hydroxybutyrate) (PHB)||R. eutropha, R. rubrum, Pseudomonas, Alcaligenes||Intracellular PHB depolymerase and extrcellular PHA depolymerases (15)|
|Polyethylene terephthalate||Thermobifida fusca||Lipase and Esterase (16)|
Microbial enzymatic degradation of different types of plastics, source: Tokiwa et al., (2009) and Santo et al., (2013).
In conclusion, a vast body of knowledge exists for degradation capabilities by microbes, there is still a lack of technological and real time applications of these microbial processes in the environment. Plastic bioremediation studies suffers from a major limitation, which is, the recalcitrant nature of plastic polymers, which require additional treatment. This treatment could be either chemical or physical methods, that could breakdown the polymer chains and help accelerate the biological processes (17). Such treatments can generally be harsh, and may create hazardous toxic by-products.
In the next article, an equally challenging pollutant,Lead (a heavy metal) is discussed, with respect to the application of microbial remediation techniques for cleaning up. Heavy metals are highly toxic in nature and require complete removal from the polluted environment, to prevent health related hazards to organisms.
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