New perspectives in plastic biodegradation

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During the past 50 years new plastic materials, in various applications, have gradually replaced the traditional metal, wood, leather materials. Ironically, the most preferred property of plastics – durability – exerts also the major environmental threat. Recycling has practically failed to provide a safe solution for disposal of plastic waste (only 5% out of 1 trillion plastic bags, annually produced in the US alone, are being recycled). Since the most utilized plastic is polyethylene (PE; ca. 140 million tons/year), any reduction in the accumulation of PE waste alone would have a major impact on the overall reduction of the plastic waste in the environment.

Since PE is considered to be practically inert, efforts were made to isolate unique microorganisms capable of utilizing synthetic polymers. Recent data showed that biodegradation of plastic waste with selected microbial strains became a viable solution.

Research highlights

► The main hazard of plastic pollution is its accumulation in the food chain. ► Extracellular laccase exerts biodegradation of polyethylene. ► Cell surface hydrophobicity correlates with biofilm formation and biodegradation.

Introduction

The discovery of the chemical process for manufacturing synthetic polymers (plastics) from crude oil was a breakthrough, in Chemistry and in Material Sciences, and paved the way to the production of one of the most versatile group of materials ever produced. These new materials combined features exhibiting strength, flexibility, light-weight, easy and low-cost production. However, these materials were found to be extremely durable and were considered among the most non-biodegradable synthetic materials. These traits facilitated the application of plastics to almost any industrial, agricultural or domestic market. For example, current soil mulching with PE in Agriculture is a common practice (Figure 1).

The most consumed synthetic polymer is PE with a current global production of ca. 140 million tons per year. In the absence of efficient methods for safe disposal of plastic waste these synthetic polymers accumulate in the environment posing an ever increasing ecological threat to terrestrial and marine wild life [1, 2•].

Until a few years ago the environmental pollution by plastic wastes had been considered merely as an aesthetic interference demonstrated by the plastics dispersion in the wind and thereby pollute large terrestrial and marine environments. However, when plastic debris is exposed to u.v. irradiation from sunlight it undergoes photo oxidation. Consequently, the plastic deteriorates, lose its tensile strength, becomes brittle and crumbles to small fragments and particles called microplastics. This physical fragmentation of the polymer exhibits real degradation in terms of molecular weight. One of the most ubiquitous and long-lasting changes to the environment is the accumulation polyethylene (PE), mulation of fragmented plastics. Within just a few decades, since mass production of plastic products has initiated, plastic debris has accumulated in terrestrial and marine environments. These microplastics can be ingested by various marine animals that, by mistake, identify the microplastics as plankton. Thus, the ingested plastic debris is likely to penetrate and accumulate in the food chain, exerting multiple hazards that their outcome still have to be elucidated [3, 4••].

Section snippets

Abiotic degradation of plastics may affect biodegradation

Abiotic degradation includes the physical and/or chemical processes that exerts intramolecular modifications in the polymer. Biodegradable polymers are comprised of two types: a) Polymers that are intrinsically biodegradable; whose chemical structure enables direct enzymatic degradation (e.g. starch, cellulose, chitin, etc.) and b) Polymers that undergo photo oxidation or thermo oxidation upon exposure to u.v. or heat, respectively. Often, the synthetic polymers will contain pro-oxidant (a

Microbial degradation of plastics

During the past two years two comprehensive reports on the degradation of plastic had been published [11••, 12••]. These studies reported on plastic biodegradation (i.e. exerted by the aid of microorganisms) and on biodegradability (i.e. the potential of a synthetic polymer to be degraded by microorganisms). The degradation process of PE, provided as the sole source of carbon and energy in soil microorganisms specifically, showed that small fragments were consumed faster than larger ones [13].

Direct biodegradation of polyethylene by extracellular enzymes

In search for depolymerases that could serve as candidates for oxidizing durable synthetic polymers we have recently isolated a putative laccase produced by the actinomycete R. ruber that is involved in polyethylene biodegradation. Laccases are best known in lignin-biodegrading fungi, where they catalyse the oxidation of aromatic compounds. However, there is evidence of laccase activity on non-aromatic substrates [33]. Since laccase is a copper-binding enzyme with 4 binding sites that may

Conclusions

Safe disposal of plastic waste via biodegradation should focus on the most consumed polymers (i.e. polyethylene, polypropylene and polystyrene). Unfortunately, these polymers are also the most durable plastics. In view of these obstacles, several tasks should be addressed in order to obtain safe waste disposal. These include a) photo and/or thermo oxidation applied before exposure to the biotic environment b) selection and isolation of a strain (or a consortium) that produce high levels of

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

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