Microbial Bioplastic Degradation
Plastic pollution is a multifaceted problem when it comes to the health of our planet and its inhabitants. Traditional petroleum-based plastics utilize fossil fuels for production and transportation, thus contributing to greenhouse gas emissions. Furthermore, plastic production typically requires the addition of colorants and plasticizers to the base polymer, and although the effects of these additives on health and the environment have not been thoroughly studied, they likely pose another threat to the planet. Petro-plastics are not easily broken down on relevant timescales, taking decades to centuries to naturally decompose, and plastic pollution impacts feeding and reproduction of higher trophic level organisms, ranging from dinoflagellates to marine mammals. Additionally, plastic waste is thought to aid in pathogen dispersal by providing a colonizable raft for organisms to adhere to, especially in aquatic systems; and because pollutants, like heavy metals and antibiotics, also adhere to plastic surfaces, the spread of antibiotic resistance is a concern. The close proximity of microorganisms on plastic waste, compared to the surrounding environment, contributes to horizontal gene transfer of resistance genes.
Bioplastics Broadly Defined
With all of this in mind, scientists and engineers are trying to replace petro-plastics with bioplastics. Bioplastic is a broad term that may refer to bio-based plastics (those derived from biological sources), biodegradable plastics (those that can be broken down by biological entities - especially microorganisms) or plastics that are both bio-based and biodegradable. Notably, biodegradable plastics are not always bio-based and vice versa, and different countries categorize plastics as biodegradable according to various guidelines, with some specifying industrial composting methods (i.e. reliant on high temperatures) and others using home compost techniques. Generally, a plastic is considered biodegradable if it can be broken down in a home compost within 12-24 months. For the purpose of this article, the term ‘bioplastics’ will be used to refer to bio-based, biodegradable plastics.
Structure and Composition of Bioplastics
Biomatter for biobased plastics are typically derived from fermented plant biomass or produced by microorganisms. This biomatter is primarily starch or cellulose, but some bioplastics are derived from proteins. Polylactic acid (PLA), a starch-based polymer, is one of the most widely used bioplastics. It is found in single use packaging and utensils, as well as 3D printing filaments. As of 2020, bioplastics accounted for about 1% of global plastic production.
Biodegradable plastics are structurally more available for breakdown by microorganisms. While traditional plastics are typically composed of hydrocarbon chains with zero-to-few functional groups, including aromatic rings, methyl groups and chlorine atoms, biodegradable plastics usually have ester groups (RCOOR) that are susceptible to enzymatic attack. In fact, one of the most easily degraded petro-plastics, polyethylene terephthalate (PET), includes esters and aromatic rings along its carbon backbone. It is estimated that it takes microorganisms 20-50 years to degrade PET in the environment.
Chemical Structure of Plastics
The chemical structure of 4 plastics are shown here. The plastics on the right are petroleum-based, while the 2 plastics on the left are bio-based and biodegradable. The ester groups (circled in red) are the site of enzymatic activity.
Physical Properties of Bioplastics
The physical attributes of a plastic product, such as flexibility and thermal resistance, can be modulated by changing the polymer backbone, producing blends or adding other molecules, including plasticizers, like phthalates, into the polymer. Chemical properties, however, are not the only factor dictating the rate of polymer degradation. For example, PLA degrades more slowly than poly(3-hydroxybutyrate) (PHB), despite nearly identical glass transition and melt temperatures. Interestingly, PHB is a microbial-derived polymer used by some Gram-negative bacteria to sequester carbon, and part of its improved biodegradability may have resulted from a longer history in the environment, which gave enzymes time to evolve.
When developers design bioplastics, they balance the necessity that the product is sturdy enough to handle its intended use with the end goal of timely degradation. For example, some specialty crops are grown with plastic films (mulches) covering the soil. When farmers employ biodegradable plastics for this purpose, they need the plastic to not only make it through the growing season, but also be completely broken down in the soil before the next year’s crops are ready to be planted. Currently, the majority of bioplastics are utilized for packaging, but other sectors include agriculture, textiles and medical product.
Microbial Enzymes: The Key to Bioplastic Degradation
Microorganisms from diverse taxa, including Firmicutes, Proteobacteria, Ascomycetes and Basidiomycetes, can degrade bioplastics. These microbes are distributed throughout many ecosystems, including terrestrial and marine soil, compost facilities and even insect guts. This highlights the incredible diversity of microbial metabolisms. Many of the identified organisms that are capable of biodegradation typically exist in a growth state that is more similar to stationary phase in the lab setting. In this growth state, cells produce proteins that help scavenge and overcome nutrient stress (e.g. proteinases that provide cells with amino acids to support growth).
Bioplastic-degrading enzymes are often members of the proteinase, cutinase or esterase families. These enzymes have some level of promiscuity, which enables them to break down bioplastics that are not their natural substrates. However, large polymers are often too big to transport into the cells and must be broken into monomers and smaller polymers first. Therefore, bioplastic-degrading enzymes are typically secreted. While most known enzymes work individually, Aspergillus oryzae utilizes a 2-protein system, in which a hydrophobin acts as an anchor for cutinase to degrade the bioplastic poly(butylene succinate-co-butylene adipate). Bioplastic degradation can thus provide organisms with a carbon source. In fact, many studies enrich for bioplastic degrading organisms by using plastic as the sole carbon source.
Environmental conditions also play an important role in the rate of biodegradation. Increased temperatures and moisture both encourage degradation, while colder, drier regions tend to retain plastics longer. Perhaps unsurprisingly, this is due, in part, to increased microbial growth in warmer, more humid conditions. Furthermore, nutrient quality and availability influences the rate of biodegradation, and stimulation of protein production, as well as supplementation with rate-limiting nutrients, can encourage biodegradation.
The Future of Bioplastics
Work in this field is ongoing, with interest in improving biodegradation efficiency, designing new plastics or plastic blends and identifying new organisms capable of degrading more recalcitrant plastics, such as polyethylene. There is also increased interest in developing a circular economy, especially a biologically mediated circular economy, where plastic monomers are turned into plastic, which is used and degraded back to monomers to be recovered for a new purpose. As bioplastics become more prominent, their microbial degraders and producers are likely to gain attention. Overall, plastics and other large molecules, remind us that the diverse metabolisms employed by stressed cells really do work to our advantage.