Plastics have become an integral part of our day-to-day lives. From our household to our workplaces, they are everywhere. Plastics persist for a long time (>50 years) and do huge damage to the environment making them absolutely unsustainable. Their accumulation in our environment has become a cause of concern all over the world. A major portion of the plastic products fail to reach recycling centers because most of such products are thrown away and they end up landing in landfills, ocean, and shores. The amount of this scattered waste is so high that even if all the plastic waste is managed properly starting from today, it would take about hundred years to clean it up. Also toxic gases such as hydrogen cyanide and hydrogen chloride are emitted when plastic is burned making burning another undesirable option. Hence, need of the hour is to research methods of degradation which do not have any further deleterious effects on the environment. Microorganisms can be employed in a number of degradation processes involving a range of compounds. Among the vast spectra, it is possible to find microorganisms with the potential of degrading these high molecular weight molecules. Biodegradation of plastics with the help of microorganisms can provide an answer to this global issue. The present review focuses on various aspects of biodegradation of plastic waste.
Keywords: Plastic, Biodegradation, Microorganisms
Introduction:
Plastics, or synthetic polymers, are being widely used all over the world for packaging, storage and number of other applications ranging from small households to large industries. Being a durable and cheap option available, they are preferred over other naturally occurring resources. Every person around us almost always carries a plastic bag along with. Most of the shopping contents are delivered in plastic bags; let it be fruits and vegetables, grocery, clothes or anything else. Not only our houses and kitchens but our roads, garbage dumps, drains and even rivers are flooded with plastics. They are almost everywhere. Their durability and excessive usage has caused serious damages to our environment. They are primarily synthesized from petroleum by-products and the process co-synthesizes various other harmful chemicals which are quite toxic. As they are chemically synthesized, they are essentially non-biodegradable. Hence, most of the plastic waste lays around untreated and the stock keeps on piling up. When ingested by stray animals, they choke their internal system leading to suffocation and death. Plastics enter the drainage systems and clog the pathways leading to logging of sewage water which further leads to numerous health problems. However, various agencies are involved in the recycling and reuse of these plastic materials but still most of the waste eventually finds its way to the dump yard. Despite all the efforts the quantity of plastic waste is on a rise and the efforts for managing such huge waste are proving insufficient.
But, what if this waste could be degraded naturally? It seems unlikely but nature has bestowed us with some remarkable creatures, microorganisms. Microorganisms are present everywhere around us just like air and soil. They possess some marvellous biosynthetic and bio-degradative properties. Biodegradation is the ability of microorganisms to decompose certain materials naturally. This ability of microorganisms is being increasingly used now-a-days for decomposition of a number of toxins and chemicals which is otherwise a daunting task. In the huge consortia of microorganisms it may be possible to find some strains blessed with the potential to degrade this notorious material.
Biodegradation:
Bioremediation is a process of biodegradation where the site of contamination is treated using microorganisms. Most of these processes are performed in-situ, i.e. carried out at the site itself and make use of the ability of microorganisms to degrade off a number of toxic chemicals (Jain et al., 2005). The process focuses on exploiting naturally occurring indigenous microbial flora with supplementation of nutrients to encourage their growth and enhance the degradation rates. With the diverse population of microorganisms present in our ecosystems, it is possible to treat various types of contaminants naturally and the affected sites can be restored (Jain et al., 2005). These contaminants are converted to non- toxic and harmless substances after microbial digestion and degradation such as carbon dioxide and water. When considering polymeric compounds, degradation aims at breaking down the long chains into smaller fragments.
Method of polymer biodegradation:
Biodegradation of synthetic polymers involves a series of steps, the first one being the attachment of the microorganisms to the surface of the polymer. Microbial attachment is favoured on hydrophilic surfaces. Hence, initial reactions at the surface aim to enhance the hydrophilic nature of the material to promote microbial attachment (Arutchelvi et al., 2008). After attachment, the degradation is mainly accomplished with the help of extracellular enzymes secreted by the microorganisms. A number of bacteria and fungi have been reported to secrete such enzymes (Pometto et al., 1992; Iiyoshi et al., 1998). The microorganisms consume the polymer as a course of carbon and degrade the high molecular weight polymer into smaller fragments of lower molecular mass which are then further consumed (Vasile, 1993).
The biodegradation process may be either aerobic or anaerobic. As is already known, aerobic processes take place when oxygen acts as final electron acceptor in the reaction, while anaerobic processes involve electron acceptors other than oxygen such as nitrogen, sulphur or any other molecule. When following the aerobic path, the polymer is eventually converted into carbon dioxide and water and subsequently biomass is also generated. On the other hand, anaerobic process leads to the formation of methane gas along with carbon dioxide and other gases depending on the electron acceptor available.
Microbial biodegradation of plastics:
Microorganisms have been employed into a number of processes dealing with human welfare. They are used not only for generation and isolation of newer molecules but are also equally useful in degradation studies. Considering the natural environments, they adapt themselves according to the growth conditions available and according to their food supplies. Hence, it is imperative to find wood degrading microorganisms at sites with deteriorating wood or to find lactose degrading microorganisms at sites where milk and milk products are disposed off. Similarly, going by this judgment, it is also possible to find microorganisms that are capable of degrading plastics naturally at the dump sites for plastics. Sangale et al. (2012) have reviewed polythene biodegradation and have stated four sites as rich source of polythene degrading microorganisms, namely rhizospheric soil of mangroves, polythene buried in the soil, plastic and soil at the dumping sites and marine water.
Bioremediation studies have already led to discovery of some microorganisms with the potential of degradation of synthetic polymers. Swift (1993) has been reported the production of polyester degrading enzymes by a number of microorganism, namely Staphylococcus, Streptococcus, Micrococcus, Moraxella and Pseudomonas among bacteria and Aspergillus glaucus and Aspergillus niger among fungi along with Actinomycetes sp. and Saccharomonospora genus. These strains form major population of microorganisms screened for biodegradation of synthetic polymers. Kathiresan (2003) have isolated a similar consortium of polythene and plastics degrading microbes from the mangrove soil samples. Bacterial isolates other than those mentioned above included Bacillus and Diplococcus. Various fungal strains belonging to the genus Aspergillus were also isolated. The bacterial percent degradation observed for plastics ranged between 0.56 to 8.16%, with Pseudomonas and Moraxella being the most active strains. While fungal degradation rates were found to be highest for Aspergillus glaucus and were about 7.26% in one month (Kathiresan, 2003).
Table 1: Synthetic Polymer Degrading Microorganisms (Reproduced from Sangale et al., 2012)
Microorganism identified
Polymer degraded
Reference
Staphylococcus, Streptococcus, Micrococcus, Moraxella, Pseudomonas, Aspergillus
Polyester
Swift, 1993
Comamonas acidovorans
Polyurethane
Nakajima-Kambe et al., 1995
Phanerochaete chrysosporium ME-446, Trametes versicolor IFO 7043, and IZU-15413
High-molecular-weight polyethylene
Iiyoshi et al., 1998
Penicillium pinophilum and Aspergillus niger
Powdered LDPE
Volke-Sepulveda et al., 2002
Rhodococcus ruber
Polythene
Sivan et al., 2006
Serretia marscence
Polythene
Aswale and Ade, 2009
Streptomyces, Pseudomonas, Bacillus, Staphylococcus, Aspergillus
Polythene
Usha et al., 2011
Factors affecting biodegradation:
A number of factors affect the biodegradation potential of microorganisms, the first one being the type of polymer being degraded and its characteristics. The molecular weight of the polymer, the extent of substitution and the functional groups associated play a major role in determining its biodegradation rates along with pre-treatments given (Gu et al., 2000; Artham and Doble, 2008). The absorption and consumption of synthetic polymers increases with decreasing molecular weight (Tokiwa et al., 1976) and decreasing number of side chains associated with the main backbone chain (Koutny et al., 2006). Other factors are listed below as reviewed by Boopathy (2000).
Table 2: Factors Affecting Biodegradation (Source: Boopathy, 2000)
Microbial
Microorganisms present
Growth rate
Genetic adaptation
Enzyme induction
Enrichment of the capable microbial populations
Environmental
Availability of nutrients
Environmental conditions
Substrate
Concentration of contaminants
Chemical structure of contaminants
Toxicity of contaminants
Solubility of contaminants
Biological aerobic v/s anaerobic process
Oxidation/reduction potential
Availability of electron acceptors
Growth substrate v/s co-metabolism
Alternate carbon source present
Microbial interaction (competition, succession, and predation)
Mass transfer limitations
Oxygen diffusion and solubility
Diffusion of nutrients
Solubility/miscibility in/with water
Conclusion:
The main cause of concern governing the excessive use of plastics is the increasing rate of their accumulation in the biosphere and the basic reason for their accumulation is that they are non-biodegradable in nature. These light weight polymers are proving to be a heavy burden on the environment. Microorganisms can help in the biodegradation of these polymers without the formation of any toxic intermediates or end products. Such naturally occurring processes, if enhanced appropriately, can help overcome the problems associated with the enormous production and usage of synthetic polymers.
