Discovery and adaptation of microbes that degrade oxidized low-density polyethylene films

Abstract

There is a growing interest in developing a methodology for effectively cleaving carbon-carbon (C-C) bonds in polymer backbones through bioconversion processes that utilize microorganisms and their enzymes. This upsurge of interest is driven by the goal of achieving a circular economy. Polyolefin post-consumer plastics are a substantial source of carbon, but the recycling potential is severely limited. Upcycling routes are needed for converting polyolefin post-consumer plastics into value-added products while concurrently mitigating adverse environmental effects. These materials contain carbon-based chemicals that can, in principle, serve as the feedstock for microbial metabolism. Some microbes have been reported to grow on polyolefin plastics, but the rate of biodegradation is insufficient for industrial processes. In this study, low-density polyethylene (LDPE) films were subjected to two mild ozone-based oxidation treatments, which facilitated biodegradation. The degree of oxidation was determined by Fourier transform infrared spectroscopy via analysis of the carbonyl index (1,710/1,460 cm-1), which ranged from 0.3 to 2.0, and also via analysis of the carboxylic acid content. Following oxidation of the films, studies were conducted to investigate the ability of a panel of polyvinyl alcohol-degrading microbes to degrade the oxidized films. A defined minimal medium was used to cultivate and assess microbial growth on the oxidized films. Following 45 days of cultivation, the most effective strains were further cultivated up to three additional generations on the oxidized film substrates to improve their ability to degrade the oxidized LDPE films. After these enrichments, we identified a strain from the third generation of Pseudomonas sp. Rh926 that exhibited significant cell growth and reduced the oxidized LDPE film mass by 25% in 30 days, demonstrating an enhanced capacity for degrading the oxidized LDPE films.

One-sentence summary: Discovery and adaptation techniques were used to enhance the metabolic capability of microorganisms for increased biodegradation of ozone-oxidized LDPE films as a step toward a future upcycling process.

Keywords: Adaptation; Biodegradation; Discovery; LDPE; Microbes; Oxidation.

Graphical Abstract

Fig. 1.

Fourier transform infrared spectroscopy (FTIR) of ozone-oxidized polyethylene films. (a) Representative FTIR spectrum of a compared pristine low-density polyethylene (LDPE) film and an ozone/O₂-oxidized film at 80°C, 0.4 L/min, and 20 hr. (b) Expanded FTIR spectrum of “a” highlighting the presence of a carbonyl peak in the oxidized film. (c) Carbonyl index for a UV/ozone (UVO)-treated film as a function of UVO exposure time as determined by the peak height ratio between 1,710 and 1,460 cm⁻¹. (D) Carbonyl index for ozone/O₂-treated films for differing ozone/O₂ conditions.

Fig. 2.

Strains 10, 15, and 18 were inoculated into a minimal medium (MM) at pH 7 with polyvinyl alcohol (PVA) and ozone/O₂-oxidized low-density polyethylene film that was treated at 80°C for 20 hr and a flow rate of 0.4 L/min. The control (C) was not innoculated. The initial ratio of MM to PVA was 8:2, with the medium being changed after 15 days to a ratio of 9:1. This procedure was repeated after another 15 days. The utilization rate was determined by measuring the stable weight change of each film after 45 days of incubation.

Fig. 3.

Various generations of strains 10 and 15 were inoculated and tested for their ability to: (a) grow in the presence of minimal medium (MM) containing PVA, as well as in MM with ozone/O₂-oxidized LDPE film and PVA and (b) utilize the ozone/O₂-oxidized LDPE film in the presence of MM containing PVA. The utilization rate was determined by measuring the stable weight change of the films after 30 days of incubation.

Fig. 4.

Strains 10G2 and 15G3 were inoculated and tested for their ability to utilize the ozone/O₂-oxidized film in the minimal medium (MM) in the presence or absence of polyvinyl alcohol (PVA) at pH 7. For the samples containing PVA, the ratio of MM to PVA was 9.5:0.5. The utilization rate was determined by measuring the stable weight change of the films after 21 days of incubation.

Fig. 5.

Strain 15G3 was inoculated at pH 7 and tested for the growth in minimal medium (MM) (blue), MM with low-density polyethylene (orange), MM with UV/ozone-oxidized film (gray), and MM with ozone/O₂-oxidized film (gold) at pH 7.

Fig. 6.

Strain 15G3 was inoculated in 100 mL of MM with 2 g of low-density polyethylene (LDPE) film (control) (gray) and minimal medium (MM) with 2 g of UV/ozone LDPE film (green) at pH 7. (a) OD was measured at 600 nm. (b) The utilization rate was determined by measuring the stable weight change of the films after 30 days of incubation.