Plastic Degradation by Extremophilic Bacteria

Abstract

Intensive exploitation, poor recycling, low repeatable use, and unusual resistance of plastics to environmental and microbiological action result in accumulation of huge waste amounts in terrestrial and marine environments, causing enormous hazard for human and animal life. In the last decades, much scientific interest has been focused on plastic biodegradation. Due to the comparatively short evolutionary period of their appearance in nature, sufficiently effective enzymes for their biodegradation are not available. Plastics are designed for use in conditions typical for human activity, and their physicochemical properties roughly change at extreme environmental parameters like low temperatures, salt, or low or high pH that are typical for the life of extremophilic microorganisms and the activity of their enzymes. This review represents a first attempt to summarize the extraordinarily limited information on biodegradation of conventional synthetic plastics by thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria in natural environments and laboratory conditions. Most of the available data was reported in the last several years and concerns moderate extremophiles. Two main questions are highlighted in it: which extremophilic bacteria and their enzymes are reported to be involved in the degradation of different synthetic plastics, and what could be the impact of extremophiles in future technologies for resolving of pollution problems.

Keywords: biofilms; extremophiles; halophilic degraders; psychrophilic degraders; synthetic plastic biodegradation; thermophilic plastic degraders.

Figure 1

Worldwide distribution of plastic producers. Data according to [1].

Figure 2

Demand distribution and use of different plastics (data according to [1]).

Figure 3

Overview of plastic degradation.

Figure 4

Analytical techniques for monitoring the extent and nature of plastic degradation. Abbreviations: SEM-scanning electron microscopy; GPC-gel permeation chromatography; XRD-X-ray diffraction; DTA TG-differential thermal analysis and thermogravimetric analysis; DSC-differential scanning calorimetry; ESCA-electron spectroscopy for chemical analysis; FTIR-Fourier transform infrared spectroscopy; NMR TLC combined application of nuclear magnetic resonance and thin-layer chromatography; GC-MS-gas chromatography/mass spectrometry; MALDI-TOF-matrix-assisted laser desorption/ionization time of flight.

Figure 5

SEM images showing the interaction between bacteria from a Pomorie Salterns (PS) community and plastics after two weeks of co-incubation [74]. (a) Surface relief of a PCL sample incubated in the absence of bacteria; (b,c) Polycaprolactone (PCL) co-incubated with PS community. At lower magnification (b), a deep groove in the plastic (asterisk) is visible filled with filamentous material formed as a result of partial degradation of the plastic. Adherent bacteria are present on the comparatively unaltered part of the surface (arrows). (c) higher magnification shows biofilm bacteria inside the groove on the plastic (arrows). (d) Surface relief of a polyvinyl alcohol (PVA) sample incubated in the absence of bacteria; (e) PVA co-incubated with PS community. The structure of the polymer is loosened, and bacteria adherent to polymer filaments are observed (arrows).