And, one more thing about plastics: They are durable. Perhaps too durable.
Plastics just won’t go away. We throw them away of course, but they cascade out of our landfills, onto our seashores, and into our oceans (2). Unfortunately approximately one-third of all plastic produced is single-use and only 15% of the plastic produced across the globe is recycled (3). This long life, single-use production, and lack of recycling combined with an increase in plastics production from 1.5 million tons in 1950 to 230 million tons in 2010 (4), makes plastic pollution a monumental problem.
Scientists have been working on creative ways to attack the plastics pollution problem. Bioremediation has been an area of active research for quite a long time. The idea is to look to the natural environment, to microorganisms that can breakdown plastics. Many microorganisms, both bacteria and fungi, have been isolated that can metabolize a variety of plastic polymers (5). The Pacific Garbage Patch, which consists of a ocean current of circulating waste, much of it very tiny particles of plastic, has even become a “working laboratory” of sorts for scientists around the world seeking to understand how best to deal with plastic waste (2). Some scientists have also taken the approach of using microorganisms to stick the tiny plastic particles floating in the ocean together, into larger particles that can be physically gathered and then recycled (6).
Recently I came across a paper published in Applied and Environmental Microbiology in 2011 by Scott Strobel and colleagues (7) who are looking at the bioremediation problem from a slightly different perspective. They are looking for microorganisms that are naturally highly adept at metabolizing complex polymers. Such an organism might be able to use a plastic polymer as a sole carbon source, and it might metabolize synthetic polymers using a novel enzyme or mechanism.
So they set out to look at endophytes. Endophytes are organisms live within the inner tissues of plants. These organisms play important roles in plant decomposition, and many of them are capable of degrading the complex polymer lignocellulose. Because every plant species on earth potentially hosts several species of endophyte, they are incredibly diverse, and the opportunity to discover new species not beyond reason.
In this study Strobel and his colleagues, which included undergraduate students participating in a discovery-based research program, isolated endophytes from plant stems collected in the Ecuadorian rainforest. Some of the endophytes isolated were evaluated for their ability to degrade polyester polyurethane (PUR), a plastic used in textiles and textile coatings.
Endophytes were isolated from the surface-sterilized stems and then grown in pure culture for sequencing and phylogenetic analysis. Isolated endophytes were first tested in a PUR clearance screen in which researchers look for a zone of clearance within a milky white suspension of Impranil DLN, a polyester polyurethane. Fifty nine isolates were screened; 18 of them produced a clearance zone on the medium. Several of these fell into the fungal genus Pestalotiopsis, with at least three isolates exhibiting high activity. Isolates testing positive in this initial test, were retested in two other PUR clearance assays, and compared to known organisms that also degrade PUR. Relative rates of PUR clearance were also measured in a liquid assay.
The five most active organisms from these clearance assays were tested for their ability to use PUR as the sole carbon source. Two isolates of Pestalotiopsis were able to use PUR as a sole carbon source under both aerobic and anaerobic conditions.
The researchers next investigated the mechanism of degradation of PUR by these organisms. IR analysis indicted the ester bond was the target of the activity, and the enzyme produced appears to be a serine hydrolase that is secreted and diffusible; they were also able to show degradation of PUR after purification of the enzyme.The fact that the enzyme can work to degrade PUR outside of the cell is intriguing and has potential implications for bioremediation technology. Also the endophytes are a numerous and diverse set of organisms that are fairly easily isolated, indicating that they may be a rich source of material for looking at bioremediation of more difficult to degrade synthetic polymers.
Meanwhile, if you have the opportunity to stroll through the Ecuadorian rain forest, wear cotton.
- Plastics Industry Trade Association. Fast Facts. http://www.plasticsindustry.org/AboutPlastics/content.cfm?ItemNumber=787&navItemNumber=1280 (accessed 9/13/2012)
- Ocean Conservancy. The Pacific Garbage Patch: Myths and Realities. http://www.oceanconservancy.org/our-work/marine-debris/the-pacific-garbage-patch.html (accessed 9/13/2012)
- Plastic Disclosure Project Website http://www.plasticdisclosure.org/about/why-pdp.html (accessed 9/13/2012)
- PlasticsEurope (2008) The compelling facts about plastics, 2010 http://www.plasticsrecyclers.eu/uploads/media/20101028135906-final_plasticsthefacts_26102010_lr.pdf (accessed 9/13/2012)
- Russell, J.R., Huang, J., Anand, P., Kucera, K., Sandoval, A.G., Dantzler, K.W., Hickman, D., Jee, J., Kimovec, F.M., Koppstein, D. & (2011). Biodegradation of Polyester Polyurethane by Endophytic Fungi, Applied and Environmental Microbiology, 77 (17) 6084. DOI: 10.1128/AEM.00521-11
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