Alexei Volkov
Plastic is a material that is extensively used in agriculture, construction, health, and consumer goods. The excessive use of plastics has led to their accumulation on land and in the sea, posing a serious threat to the ecosystem and human life. While incineration is currently the only permanent way to dispose of plastic, it contributes to the climate crisis by releasing carbon and noxious chemicals into the atmosphere. As a result, there is a growing interest in developing sustainable methods to degrade plastics, with a particular focus on the potential of bacteria and other microorganisms to break down these polymers.
| Characteristics | Values |
|---|---|
| Biodegradation of plastics | A potential solution to the plastic crisis |
| Plastic-degrading organisms | Bacteria, fungi, insects, and other microorganisms |
| Plastic types | PE, PET, PP, HDPE, LDPE, PUR, PCL, PS, and more |
| Biodegradation methods | Enzymes, gut bacteria, genetic engineering, natural polymers |
| Challenges | Slow process, environmental conditions, plastic characteristics |
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What You'll Learn
The role of enzymes in plastic degradation
The accumulation of synthetic plastics is a major concern for the environment and human health. Some of the most common synthetic polymers, such as polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET), are extremely resistant to natural biodegradation. This has led to an ever-growing burden on the environment, with plastic pollution causing detrimental effects on both ecosystems and human life.
To address this issue, scientists have turned to the study of plastic-degrading enzymes produced by microorganisms. These enzymes have the capacity to break down synthetic polymers, and thus play a pivotal role in mitigating plastic pollution. The process of enzymatic degradation typically occurs in two stages: the adsorption of enzymes onto the polymer surface, followed by hydro-peroxidation or hydrolysis of the bonds. Enzymatic catalysis of plastic waste offers an innovative and sustainable approach to achieving environmental goals.
A variety of enzymes, including cutinases, esterases, lipases, laccases, peroxidases, proteases, and ureases, have been shown to effectively degrade PE, PET, and PP. For example, the enzyme PETase, produced by the bacterium Ideonella sakaiensis, is capable of breaking down PET plastics. In recent years, scientists have also designed I. sakaiensis strain enzymes with improved degradation efficiency, allowing bacteria to degrade a significant percentage of plastic products within a short time.
The sources of plastic-degrading enzymes can be found in microorganisms from various environments, including the digestive intestines of some invertebrates. For instance, extreme environmental microorganisms, such as halophiles and psychrophiles, exhibit plastic degradation potential in extreme conditions. Additionally, the blending of plastics with natural polymers like starch has been shown to enhance biodegradation by increasing the susceptibility of polymers to both biotic and abiotic degradation.
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Insect gut bacteria and fungi
The accumulation of plastics in the environment has spurred interest in finding ways to degrade these polymers. The excessive use of plastics poses a serious threat to the ecosystem and human life. Plastics are used in packaging for food, pharmaceuticals, detergents, and cosmetics, and are used extensively in agriculture, building, construction, and consumer goods.
The insect gut microbiome has been found to secrete emulsifying factors that enhance respiration on polystyrene (PS). In addition to the gut bacteria, fungi present in insect guts can also degrade plastics. For example, Zhang et al. isolated a PE-degrading fungus, Aspergillus flavus, from the intestine of a Wax moth larva (Galleria mellonella). This fungus can degrade HDPE MP to low molecular weight MP after 28 days of culture.
The role of insect gut bacteria and fungi in plastic biodegradation is an important area of research, as it may offer a safe and efficient treatment for plastic waste.
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Plastic-degrading extremophiles
The accumulation of plastics in the environment poses a serious threat to the ecosystem and human life. Plastics are designed for use in conditions typical for human activity, and their properties change at extreme environmental parameters like low or high temperatures, salt, or low or high pH. These are the conditions in which extremophilic microorganisms thrive.
Extremophiles are microorganisms that can survive and even grow in conditions that would be lethal to humans. Most currently known extremophiles are either Eubacteria or Archaea. Thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria in multiple extreme environments are potentially capable of degrading synthetic plastics. For example, in mangrove swamps on the coastlines of Vietnam and Thailand, scientists are hunting for PET-eating microbes. The hope is that bacteria capable of degrading the mangrove roots will also be able to degrade plastic.
The search for new plastic-degrading enzymes and microorganisms has resulted in the discovery of several extremophiles with plastic-degrading potential. For example, the bacterium Ideonella sakaiensis, discovered in a rubbish dump, produces the enzyme PETase to break down PET plastics. Another example is Chelatococcus sp. E1, a thermophilic strain that increases the biodegradability of HDPE and LDPE.
The development of extremozymes and the growth of extremophiles in severe environments could help solve the problem of plastic waste. However, information about enzymes acting on high-molecular-weight plastics is still scarce, and the specific mechanisms of plastic biodegradation by microorganisms are not yet fully understood. Nevertheless, the potential of extremophiles in future technologies for resolving pollution problems is significant.
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Plastic-degrading microbes in the ocean
Marine bacteria can break down polymers and use them as a primary source of carbon for energy. However, the lack of essential nutrients like nitrogen in plastic means that plastic alone cannot support microbial growth. Nevertheless, certain species of bacteria have evolved to degrade plastics into harmless by-products.
The discovery of bacteria that can feed on plastic has sparked hope that these microbes could help clean the ocean of plastic. However, some experts argue that this viewpoint misses the point, as the rate of degradation by these microbes is low, even in optimized laboratory settings. Furthermore, releasing genetically engineered bacteria into ocean ecosystems could have unintended negative side effects on the ecosystem.
Nevertheless, researchers are working on genetically engineering bacteria to be more efficient at degrading plastic. For example, scientists are working on engineering Ideonella sakaiensis to break down PET plastic at a faster rate. Researchers at the University of Portsmouth have discovered that mixing PETase with a second enzyme called MHETase creates an "enzyme cocktail" that degrades PET plastic six times faster.
The microbial degradation of plastics in the ocean is influenced by various environmental factors, such as humidity, temperature, UV irradiation, pH, wind, and waves. These dynamic conditions create varied growth conditions for bacteria and increase the possibility of diversified plastic degradation metabolisms. Additionally, the presence of pathogenic microorganisms on plastic debris, such as Vibrio, can have significant negative impacts on marine ecosystems.
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The future of plastic-eating bacteria
Plastic is a material that has become integral to our daily lives. It is used in agriculture, construction, health, and consumer goods. However, the excessive use of plastics poses a serious threat to the ecosystem and human life. The current methods of plastic disposal, such as incineration, contribute to climate change by releasing carbon and noxious chemicals into the atmosphere. As a result, there is a growing interest in developing sustainable methods to reduce the environmental impact of plastics.
In 2001, a group of Japanese scientists discovered bacteria that could break down plastic in a rubbish dump. This bacterium, named Ideonella sakaiensis, produces an enzyme called PETase that specifically breaks down PET (polyethylene terephthalate), a common plastic in clothing and packaging. Since then, scientists have been working to understand and enhance the plastic-degrading capabilities of microorganisms.
The discovery of plastic-eating bacteria has opened up new possibilities for recycling and waste management. Researchers are now exploring different environments, such as mangrove swamps and hot springs, to find new plastic-degrading microbes. By studying the natural enzymes that break down plant leaves and coatings, scientists hope to engineer more efficient enzymes that can target specific plastics. This approach, known as "evolving the crap out of an enzyme," involves subjecting enzymes to various mutations to enhance their plastic-degrading abilities.
Additionally, the application of plastic-eating bacteria in industry is already underway. A French company, Carbios, has been using a bacterial enzyme to process PET plastic waste since 2021, bringing us closer to achieving infinitely recyclable plastic materials. This technology not only breaks down plastic into precursor molecules but also allows for the creation of new plastic products.
In conclusion, the future of plastic-eating bacteria holds great potential for addressing the global plastic crisis. With continued research and innovation, we can expect to see more efficient and sustainable methods for plastic recycling and waste management. By harnessing the power of microorganisms and enzymes, we may be able to mitigate the environmental impact of plastics and move towards a more circular economy.
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Frequently asked questions
The sole permanent way of plastic disposal we have found is incineration, which is done to nearly 70 million tonnes of plastic annually. However, this method contributes to the climate crisis by releasing carbon and noxious chemicals into the air.
Bacteria can produce many extracellular enzymes to break down plastic macromolecules. These include lipases, depolymerase, esterase, proteinase K, cutinase, urease, and dehydrase.
Many bacterial and fungal strains can degrade microplastics under laboratory conditions and in the environment. Some examples of plastics that can be degraded include PE, PET, PP, PUR, and PS.
Biodegradation is a slow process, and not all types of plastic are easily broken down by bacteria. For example, nylon has a high resistance to degradation due to its crystalline morphology and strong intermolecular hydrogen bonds. Additionally, the strength of plastic increases with depth in water, making it more difficult to degrade. The spatial proximity of bacteria to the plastic is also a critical factor in the effectiveness of biodegradation.
