New perspectives in plastic biodegradation
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
References (41)
Biodegradation of plastics
Curr Opin Biotechnol
(2001)- et al.
Priming effect as determined by adding 14C-glucose to modified controlled composting test
Biodegradation
(2002) - et al.
Biological degradation of plastics: a comprehensive review
Biotechnol Adv
(2008) - et al.
Mechanistic implications in plastic degradation
Polym Degrad Stab
(2008) - et al.
Studies on the biodegradability of polithioester copolimers and homo-polymers by polyhydroxyalkanoate (PHA)–degrading bacteria and PHA depolymerases
Arch Microbiol
(2004) - et al.
Degradation of poly (3-Hydroxybutirate) PHB, by bacteria and purification of a novel PHB depolymerase from Comaonas sp
J Environ Polym Degrad
(1993) - et al.
Biofilm formation and partial biodegradation of polystyrene by the actinomycete Rhodococcus ruber
Biodegradation
(2008) - et al.
adherence of bacteria to hydrocarbons: a simple method for measuring cell surface hydrophobicity
FEMS Microbiol Lett
(1980) - et al.
Regulation of laccase gene tanscription in Trametes versicolor
Appl Environ Microbiol
(1997) - et al.
Accumulation and fragmentation of plastic debris in global environments
Philos Trans R Soc Lond B Biol Sci
(2009)