It’s a routine just about every person in the First World is familiar with. Grab a plastic bottle, use its contents until empty, and then throw it in the recycling bin. But what about the little plastic sauce container from the last time you ate take-out or the plastic wrapping on that pound of beef from the supermarket? What about the Styrofoam tray the meat came on? Those almost certainly ended up in the trash, and that means they ended up in a landfill.
Plastics are well-known to be incredibly resistant to decay. A prevalent dystopian image is the surface of the Earth millennia from now, practically smothered with plastic waste that no one ever figured out what to do with – the sole remnants of a wasteful civilization long outlived by its refuse. But what if there was another option, one that’s been under our noses – or rather, under our rotting logs – the whole time?
Plastics have played a tremendous role in human civilization since the very first manmade plastic invented by Alexander Parkes was introduced as “Parkesine” at the 1862 International Exhibition in London. Though compostable plastic is now starting to make an appearance in food packaging and storage, and recycling programs have made a significant dent in waste management, the vast majority of plastics still end up in landfills. In fact, according to a study from 2007, approximately 2.3 million of the 3 million metric tons of polystyrene produced yearly in the US are not recycled and end up at the dump (Goff 286). In the year 2000, only 1% of post-consumer polystyrene was recycled in the US (286). This means that the time, energy and resources used to make that plastic that ends up in the landfill are chalked up to a total loss. However, scientists are discovering that certain species of fungi are capable of safely decomposing otherwise permanent plastics, and these fungi could turn out to be part of the solution to the plastics problem.
Fungi are spectacularly useful organisms for humans and the world at large, whether we eat them, use them to fight infections, use them to play the psychonaut, or utilize them for any of their other uses. Some edible mushrooms, such as morels and truffles, can even fetch princely sums on the open market. Fungi also play a vital role in the life cycle on Earth, being responsible for much of the decomposition of cellulose, the veritable “main ingredient” of wood. It’s this decomposition functionality that offers so much promise to environmental scientists. In the past twenty years, an exciting new use for fungi has begun to emerge in the scientific community as a few mycelia have been making a name for themselves in the expanding field of bioremediation. Bioremediation means the filtration of environmental toxins by way of natural processes.
One of the heavy hitters of mycoremediation – a term given to the type of bioremediation performed by fungi – is known as white rot fungi. White rot is a variety of fungus that decomposes wood, but that’s no special trick. Many types of fungi feed on wood. More specifically, white rot fungi are able to digest the resinous chemical lignin (Milstein 3225). White rot possesses in its arsenal a variety of enzyme known as laccase. Laccase gives white rot fungi its amazing ability to degrade lignin. It’s estimated that “about 50 x 106 (50 million) tons of lignin are released annually by the pulping industry” (3225). It has been known that white rot could degrade wood for decades, though only recently was it discovered that the same enzyme-based processes it used to degrade lignin could be used to degrade polystyrene as well. Both of these products are very much wasted in their landfill prisons. Lignin in particular would yield a very valuable biomass if a method of degrading it into reusable constituents was widely available and affordable. Lignin surrounds the cellulose molecules in woody plants and protects the energy-rich cellulose from bacteria, insects and damage from the elements. By this same functionality, lignin also prevents a lot of the cellulose from biomass intended for biofuel from releasing its valuable energy, making the ligneous plant matter a lot less efficient for ethanol production than it could be. It’s this idea of un-making otherwise impermeable polymers like lignin, and particularly plastics, which is showing the most promise in many studies across the globe.
Two particular types of white rot fungi tested by a team at the University of Göttingen, Germany back in 1992 hold real promise in this area: T. versicolorand P. chrysosporium. These varieties demonstrated an ability to degrade copolymers consisting of lignin and polystyrene, no matter what ratio of the two chemicals was included in the copolymer. White rot actually showed the most efficiency when degrading compounds made of 50.1% lignin and 32.2% lignin (Milstein 3229). In a fine example of the pleasantly simple math often found in nature,these results indicate lignin and polystyrene can likely be combined in easy ratios (1:1 and 1:2) in order to achieve the most efficient breakdown. In the results section of the study, it is stated that “It can be assumed that white rot fungi in liquid media express their degradation potentials toward incubated plastics less than do the same fungi cultivated in the solid state” (3229). Not needing to incubate the fungi in liquid media means that open air is more beneficial to the process. This fact is a tremendous boon to the idea of this using this process as a real-world solution, where the mycoremediation can take place at the site of the waste rather than requiring the waste to be removed and placed in specific conditions. The fungi show increased efficiency in biodegradation in open air because despite their physical similarity to plants, fungi require oxygen for their respiration and “exhale” carbon dioxide, just like animals. Plants do just the opposite, absorbing carbon dioxide and exuding oxygen. Concerns about the extra carbon dioxide produced by the biodegradation of plastics via fungal strains could potentially be alleviated by growing carbon dioxide-hungry plants in the vicinity of fungi inoculation sites. These plants would not only add to the greenery of the landscape but provide a valuable service in completing the oxygen-carbon dioxide cycle in this area. With the right planning, it might even be possible to make a completely carbon neutral ligno-plastics decomposition facility.
This test also included another type of white rot that performed somewhat less efficiently, and a single variety of brown rot fungi, which is another type of wood-decay fungi. While white rot eats all varieties of plant cell walls including cellulose and lignin, brown rot eats only the cellulose. It was no big surprise, then, that the brown rot fungi tested, G. trabeum, provided no actual deterioration of the polystyrene or the lignin. However, brown rot did show some interesting side effects. When inoculated on the ligno-polystyrene copolymer, the brown rot may not have done any damage on its own, but it did alter the structure of the lignin to allow for other micro-organisms to more effectively degrade the copolymer themselves (3229). It’s not impossible to imagine that inoculating brown rot along with white rot could possibly allow the brown to act as a catalyst to the white and make it even more effective. White rot is without doubt one of the best chances for mycoremediation existing today, and its unique ability to degrade polystyrene and lignin could one day easily make it one of the most important microorganisms on Earth.
Another very promising group of strains of fungi are the Aspergillus. Aspergillus niger and Aspergillus oryzae have both demonstrated some remarkable and potentially important abilities. Aspergillus n.has been found capable of adherence to HDPE, or high-density polyethylene, which is used in everything from bottle caps to plastic surgery. HDPE accounts for 17.4% of the total plastic distributed around the world, and is estimated to accumulate at 25 million tons a year (Mathur 69). Polyethylene as a whole is widely known as the most common plastic in the world, and so its degradation is of extra concern to the environmentalists and conscientious scientists of the world.
A multi-university study conducted in India in 2011 took some regularly attainable strips of HDPE, heated them to 70 degrees Celsius for 10 days, and then sterilized them thoroughly before applying a collection of different fungal strains that had been isolated from an existing plastic waste dump. After 30 days of incubation, Aspergillus niger was successfully able to reduce the mass of the plastic by 3.44% and reduce the tensile strength by 61% (Mathur 72). Though the plastic sample was not fully disintegrated by any means, the damage done did help to enable faster and more complete degradation of the sample.
Aspergillus oryzae already has deep roots in human history. The people of Japan have used Aspergillus oryzae for over 1,000 years to make sake, soy sauce, and miso, a type of soybean paste (Maeda 779). Because of its well documented safety via generations of worry-free usage, the FDA and the World Health Organization have classified it as “generally regarded as safe” (779). Using multiple plastic film strips made of already biodegradable plastics known as PBS and PBSA, a study based out of Tohoku University in Japan found that an enzyme known as a cutinase found in Aspergillus oryzae could digest both PBS and PBSA. PBSA was found to facilitate more enzyme creation and subsequently greater levels of degradation. “80 μg/mL cutinase completely dissolved a 50-μm thick PBSA film in only 6 hours” (785). While these plastic samples are very small, six hours is still a short amount of time for any biological process.
Another Aspergillus strain, known as Aspergillus sp.has also demonstrated the ability to degrade plastics. A study testing it and yet another strain called Aspergillus versicolor revealed that these two can fully degrade LDPE (low density polyethylene) into carbon dioxide with no other chemical by-products (Sindujaa 144). This study claims that therefore, technically, “LDPE can be biodegradable if the right micro-organism is isolated” (144). These results indicate an incredible potential for biodegrading plastic polymers with fungi. LDPE products include the plastic bags, infamous plastic rings around six-packs of beverages, and plastic cling wrap found in homes around the world.
Another fungal strain known as B47-9 has shown impressive results in degradation. B47-9 is a fungal strain taken from the leaves of certain types of grass. Another study using PBS and PBSA strips subjected the plastic to B47-9 on both sterilized and unsterilized soil. After 6 days, the sterilized soil showed a 99.8% reduction of the PBSA film by mass, while the unsterilized soil showed a 91.2% reduction. Interestingly, the sterilized soil seemed to show more of a tendency to allow the B47-9 to flourish, but even the unsterilized soil showed a near complete degradation of the material. If this data was hypothetically extrapolated and applied to the real world, the consequences could be incredible.
Imagine a huge pile of plastic on the ground outside getting a large culture of fungus dumped on it, and a week later that plastic pile is significantly smaller than it used to be – less than 10%, in fact. Now imagine that pile is collected and moved to a place where it can be degraded on sterilized soil. A return trip a week later would reveal the pile is all but gone, with nothing but carbon dioxide in its place. Such a thing is no longer an impossibility, as these fungi are showing themselves to be proficient at degrading the plastics we used to think were practically immortal. Even if we don’t find a variety that can singlehandedly reverse the mounting tonnage of plastic filling the Earth, a few have real potential to change the game and possibility alter the fate of our plastic world in a positive way.
- Bond, Allison. Discover Magazine. Kalmbach Publishing, 21 Aug. 2009. Web. 16 Oct. 2012. <http://blogs.discovermagazine.com/80beats/2009/08/21/plastic-is-more-biodegradable-than-we-thought-thats-bad>.
This piece is written in a more informal tone, but still has plenty of statistics. It talks about how ‘permanent’ plastics like polystyrene and polyethylene are thought to last a very long time in nature, but how ocean water might assist in its breaking down – but not in an environmentally friendly way.
- Goff, M., Ward, P.G., & O’Connor, K. E. (2007, March 15). Improvement of the conversion of polystyrene to polyhydroxyalkanoate through the manipulation of the microbial aspect of the process: A nitrogen feeding strategy for bacterial cells in a stirred tank. Journal of Biotechnology, 283-286. Retrieved October 16, 2012, from <https://www.dropbox.com/s/yrn1wultfblko6m/goff2007.pdf>
This paper details a new method of up-cycling, or converting the non-degradable plastics into safely biodegradable ones through use of microorganisms that can perform this task under specific conditions. It tells of how nitrogen fed to the bacteria helped stimulate it to convert polystyrene to PHA.
- Kenny, S. T., et al. (2008, August 6). Up-Cycling of PET (Polyethylene Terephthalate) to the Biodegradable Plastic PHA (Polyhydroxyalkanoate).Environmental Science Technology, 7696-7701.Retrieved October 16, 2012, from <https://www.dropbox.com/s/dlns3ea1khex5bo/es801010e.pdf>
In a similar vein as the paper by Goff, Ward and O’Connor, this paper discusses the process by which polystyrene is broken down into its simpler components by pyrolysis and the use of fungi.
- Koitabashi, M., et al. (2012, August 2). Degradation of biodegradable plastic mulch films in soil environment by phylloplane fungi isolated from gramineous plants [Electronic version]. AMB Express, 2(40). doi:10.1186/2191-0855-2-40. Retrieved October 16, 2012, from <https://www.dropbox.com/s/jpxvlxnfh57fjgg/2191-0855-2-40.pdf>
A paper which discusses the use of fungi derived from grassy plants to degrade plastic mulch films in a nonsterile soil environment – one that somewhat resembles the real environment. This paper shows the results of testing these fungi and their effectiveness at safely breaking down plastic mulch.
- Maeda, H.(2005, January 27). Purification and characterization of a biodegradable plastic-degrading enzyme from Aspergillus oryzae Received:.Applied Microbiology and Biotechnology, 778-788. doi:10.1007/s00253-004-18536. Retrieved October 16, 2012, from <https://www.dropbox.com/s/to1hkc8os0c0w43/maeda05.pdf>
This paper also details the potential of Aspergillus enzymes to biodegrade plastics, and it fills out more information about this particular strain and its abilities.
- Mathur, G., Mathur, A., & Prasad, R. (2011). Colonization and degradation of thermally oxidized high- density polyethylene by Aspergillus niger (ITCC No. 6052) isolated from plastic waste dumpsite. Bioremediation Journal, 15(2), 69-76. doi:10.1080/10889868.2011.570281. Retrieved October 16, 2012, from <https://www.dropbox.com/s/7t26ne1w69r0wk8/60827875.pdf>
This article discusses another type of fungi which demonstrates a lot of potential to biodegrade plastics, especially polyethylene. It shows that when thermally oxidized, the plastic can break down under the influence of Aspergillus n.,a fungi found already adhering to polyethylene in plastic waste sites.
- Milstein, O, R Gersonde, AHuttermann, M Chen, and J Meister. “Fungal Biodegradation of Lignopolystyrene Graft Copolymers.”Applied and Environmental Microbiology Oct.(1992): 3225-32. EBSCO.Web.16 Oct. 2012. <https://www.dropbox.com/s/v9znn7bzv6prwcx/aem00051-0025.pdf>
This article is one of many detailing the amazing potential of the white rot fungi to biodegrade many types of plastic and otherwise non-degradable materials that would otherwise end up in landfills. This particular article deals with the breakdown of lignin, a non-degradable resin usually found in wood products and related polymers.
- Sindujaa, P, M Padmapriya, R Pramila, and K V. Ramesh.”Bio-Degradation of Low Density Polyethylene (LDPE) by Fungi Isolated from Marine Water.” Research Journal of Biological Sciences 6.4 (2011): 141-45. EBSCO.Web.16 Oct. 2012. <https://www.dropbox.com/s/1fl7z7tauh1fksv/79683802.pdf>
This paper shows the results of applying fungi isolated from marine water to low density plastics and the efficiency and chemical results of such things. It shows how the fungi actually used the plastic as a source of carbon and breaks it down into its simpler, safer components.
- Shah, Aamer A. Role of Microorganisms in Biodegradation of Plastics.Islamabad: Quaid-i-Azam University, 2007. 8-89. EBSCO. Web.16 Oct. 2012. <https://www.dropbox.com/s/cjedrjiype9vkxq/2216.pdf>
This is an enormous paper that details a great deal of microorganisms and information pertaining to their biodegradation of both normally biodegradable and normally non-biodegradable plastic.
- Shrivasatva, N.(2011, January 7). Biodegradation of plastic polymer by selected fungal strains : A feasibility study. Journal of Pharmacy Research, 4(9), 3056-3059.Retrieved October 16, 2012, from <https://www.dropbox.com/s/416rhgwiawmm1ym/74266194.pdf>
This study is more of an overview on the feasibility of using fungi to biodegrade plastic polymers. It shows promising results for a number of fungal strains.
- Teffeau, Marc. “To biodegrade or not to biodegrade, that is the question.” American Nurseryman May 2009: 46-47. Web.16 Oct. 2012. <https://www.dropbox.com/s/lsvd13guwvycqz5/38593573.pdf>
An informal article, this is an informative piece detailing the layman’s view on biodegradation in terms of tree and plant nurseries and the materials used in that field. It talks about the clarification of the difference between compostable and biodegradable products, and possible controversies surrounding the use of such polymers.
- Zheng, Ying, Ernest K. Yanful, and Amarjeet S. Bassi.”A Review of Plastic Waste Biodegradation.” Critical Reviews in Biotechnology 25 (2005): 243-50. EBSCO.Web.16 Oct. 2012. <https://www.dropbox.com/s/kkte8qz209a2pox/18970542.pdf>
This article gives a good general overview on the biodegradation potential of many kinds of plastics. It’s well written and full of basic statistics on various plastics. Like many of my sources, it is intended for a more academic audience.