Plastics and the microbiome: impacts and solutions

Abstract

Global plastic production has increased exponentially since manufacturing commenced in the 1950's, including polymer types infused with diverse additives and fillers. While the negative impacts of plastics are widely reported, particularly on marine vertebrates, impacts on microbial life remain poorly understood. Plastics impact microbiomes directly, exerting toxic effects, providing supplemental carbon sources and acting as rafts for microbial colonisation and dispersal. Indirect consequences include increased environmental shading, altered compositions of host communities and disruption of host organism or community health, hormone balances and immune responses. The isolation and application of plastic-degrading microbes are of substantial interest yet little evidence supports the microbial biodegradation of most high molecular weight synthetic polymers. Over 400 microbial species have been presumptively identified as capable of plastic degradation, but evidence for the degradation of highly prevalent polymers including polypropylene, nylon, polystyrene and polyvinyl chloride must be treated with caution; most studies fail to differentiate losses caused by the leaching or degradation of polymer monomers, additives or fillers. Even where polymer degradation is demonstrated, such as for polyethylene terephthalate, the ability of microorganisms to degrade more highly crystalline forms of the polymer used in commercial plastics appears limited. Microbiomes frequently work in conjunction with abiotic factors such as heat and light to impact the structural integrity of polymers and accessibility to enzymatic attack. Consequently, there remains much scope for extremophile microbiomes to be explored as a source of plastic-degrading enzymes and microorganisms. We propose a best-practice workflow for isolating and reporting plastic-degrading taxa from diverse environmental microbiomes, which should include multiple lines of evidence supporting changes in polymer structure, mass loss, and detection of presumed degradation products, along with confirmation of microbial strains and enzymes (and their associated genes) responsible for high molecular weight plastic polymer degradation. Such approaches are necessary for enzymatic degraders of high molecular weight plastic polymers to be differentiated from organisms only capable of degrading the more labile carbon within predominantly amorphous plastics, plastic monomers, additives or fillers.

Keywords: Biodegradation; Bioremediation; Community dysbiosis; Microbial community; Microplastics; Plastic additives; Plastic pollution; Plasticiser; Plastisphere; Rafting of pathogens and invasive species; Toxic impact.

Fig. 1

Schematic highlighting the diversity of direct and indirect impacts of plastics for gut and environmental microbiome communities and possible microbial solutions for the remediation of plastic waste

Fig. 2

Evidence for a role for insects, host-associated microbes, or host-independent, free-living microbes in plastic degradation. Degradation of the plastic polymer may be detected by a variety of methods, including: [i] mass loss of plastic such as clear zone development around colonies on plastic-infused/overlaid agar, [ii] altered plastic surface properties (e.g., visible by scanning electron microscopy) and [iii] generation of degradation products (e.g., CO₂, polymer metabolites detected by Fourier-transform infrared spectroscopy or high-performance liquid chromatography)

Fig. 3

Number of putative plastic-degrading organisms reported by Gambarini, et al. [28], classified at the level of phylum level. The number following the phylum name represents the number of species from that specific phylum that are reported as plastic-degraders

Fig. 4

Percentage of studies using evidence for plastic degradation by microbial species based on: (i) changes in polymer structure (blue), (ii) physical loss of plastic mass (red), or (iii) detection of plastic metabolites (green), or these techniques in combination. Data were compiled using the