Synergistic biodegradation of a compostable film by a marine microbial community

Background

The need for replacing conventional plastics has led to an increase of the use biodegradable plastics. Most biodegradable plastic materials are certified for compostability, and their degradation mechanisms by marine bacterial communities, are still largely unknown.

Methods

Bacterial communities that degrade a poly (butylene adipate-co-terephthalate)-based biodegradable film (PF) were enriched from marine samples. DNA, RNA and proteins were extracted simultaneously from the biofilm and free-living bacteria. Genes of hydrolases similar to the ones involved in polyethylene terephthalate (PET) and monoester mono-2-hydroxyethyl terephthalate (MHET) degradation (PETase and MHETases, respectively) were detected. A MHETase-like gene (Mle046) was then recombinantly expressed. The activity of Mle046 was tested against the end product of PET and PF degradation: MHET and 4-(4-hydroxybutoxycarbonyl) benzoic acid (Bte), respectively. The optimal incubation temperature and pH of Mle046 activity was determined.

Results

PETase-like (Ples) and MHETase-like (Mles) hydrolases and other enzymes needed for PF degradation were expressed within the microbial community. Within the biofilm, Ples were abundant and upregulated while Mles and terephthalate dioxygenases were abundant in the free-living fraction. Mle046 was the only Mle produced in this fraction and it was highly expressed. The purified Mle046 could degrade MHET and Bte. The optimum temperature of Mle046 activity was 20°C.

Conclusion

PF degradation is achieved synergistically by labour division among film-attached and free-living bacteria. Understanding the biodegradability of these plastics will facilitate the development of more degradable materials. In addition, the discovery of new PETases- and MHETases-like enzymes will enable their future use in plastic recycling.

Mle046 Is a Marine Mesophilic MHETase-Like Enzyme

Accumulation of plastics in the oceans presents a major threat to diverse ecosystems. The introduction of biodegradable plastics into the market aims to alleviate the ecological burden caused by recalcitrant plastics. Poly (butylene adipate-co-terephthalate) (PBAT) is a biodegradable commercial plastic that can be biodegraded similarly to polyethylene terephthalate (PET) by PETase-like enzymes and MHETases. The role of MHETases is to hydrolyze the intermediate degradation product of PET, mono-2-hydroxyethyl terephthalate (MHET) to its monomers. We recently identified a homolog of the MHETase of the PET-degrading bacterium Ideonella sakaiensis, Mle046, from a marine microbial consortium. In this consortium, Mle046 was highly expressed when a PBAT-based blend film (PF) was supplied as the sole carbon source. In this study, we recombinantly expressed and biochemically characterized Mle046 under different conditions. Mle046 degrades MHET but also 4-(4-hydroxybutoxycarbonyl) benzoic acid (Bte), the intermediate of PF degradation. Mle046 is a mesophilic enzyme adapted to marine conditions, which rapidly degrades MHET to terephthalate and ethylene glycol at temperatures between 20 and 40°C. Mle046 degradation rates were similar for Bte and MHET. Despite its mesophilic tendency, Mle046 retains a considerable amount of activity at temperatures ranging from 10 to 60°C. In addition, Mle046 is active at a range of pH values from 6.5 to 9. These characteristics make Mle046 a promising candidate for biotechnological applications related to plastic recycling.