Cara Broshkevitch and Anne Richards
Effect of Phanerochaete chrysosporium fungus and the bacteria Pseudomonas putida and
Sphingomonas macrogoltabidus on the degradation of pretreated HDPE plastic
Background Research
High-density polyethylene (HDPE) (#2) plastic has applications in products such as food
packaging, plastic bags, plastic bottles, recycled plastic lumber, toiletry and liquid containers,
outdoor furniture, and piping. As the most commonly produced plastic, around 140 million tons
of HDPE are utilized each year. HDPE’s durability and high melting point make it useful, but
also very hard to break down. It can take HDPE plastic over one hundred years to degrade in a
landfill. Although some HDPE plastic is recycled, recycled HDPE represents only 5% out of 1
trillion plastic bags produced annually in the United States. These discarded plastics are filling
up landfills and, as the population of our country grows simultaneously, we are running out of
places to put this plastic waste (Sivan, 2011).
Two bacterial strains, Pseudomonas and Sphingomonas, have been discovered to digest
HDPE; their metabolisms reduce it into heat, H2O, CO2, and biomass (the bacteria/fungi). This
may provide a solution to the plastic problem. Past research has isolated Pseudomonas putida
from sludge in industrial waste and determined that it used o-chloronitrobenzene (o-CNB) as its
only carbon, nitrogen, and energy source. Most importantly, the highest degradation of o-CNB
(85%) by P. putida was found to be at 32°C and a pH of 8.0. Although o-chloronitrobenzene is
not plastic, this research gives a general idea of ideal growing conditions for P. putida, and
shows that it is capable of using one source as its only carbon, nitrogen, and energy source (He,
Liang, Wang, Wei, & Wu, 2009).
HDPE plastic is made up of polymers with C-C single bonds (-CH2-CH2-)n. When a
plastic degrades, these polymer bonds break up. Past research has investigated the use of
Cara Broshkevitch and Anne Richards
bacteria and fungi to facilitate this degradation process by breaking up the plastic polymers for
energy. This is a two-step process: first, the plastic reacts with oxygen from the air, and second,
biodegradation of products from this oxidation occurs (Ammala, et al., 2011). In one study,
samples of polythene and plastic wastes were gathered from pollution sites such as petroleum
refilling centers, industrial and construction sites, and around automotive businesses.
Pseudomonas bacteria were found degrading this plastic waste. Moreover, this strain was found
to have a high thermal and salt tolerance: its durability making it easy to culture and maintain.
Pseudomonas’s ability to degrade plastic waste suggests that it would be able to degrade the
actual plastic source (Chowdhury, Ghosh, & Gupta, 2010).
A similar study targeted the question of Pseudomonas’s ability to directly degrade
plastic. Samples of Pseudomonas bacteria were collected from a domestic waste disposal site,
from soil from a drainage site, and from soil dumped with sewage sludge. Overall, all three
samples of Pseudomonas produced enzymes which metabolized and decreased the mass of
polyethylene plastic bags. Although it was found that natural polyethylene degraded faster than
synthetic polyethylene, current research is still investigating the effect of the 6% vegetable starch
contained by these natural polyethylene bags. Although starch is a common ingredient of
biodegradable plastics, researchers are not positive of its effects on the rate of Pseudomonas
degradation and on non-biodegradable plastics (Abraham, Nanda, & Sahu, 2010). Strains of
Pseudomonas have also been reported as some of the most commonly used organisms for plastic
degradation (Sharma, & Singh, 2008).
Although itself incapable of degrading bisphenol A, Pseudomonas was found to
accelerate the degradation of bisphenol A by the Sphingomonas bacterial strain, suggesting
symbiosis between the two strains. Since bisphenol A is the main ingredient in polycarbonate
Cara Broshkevitch and Anne Richards
plastic, this research suggests that Sphingomonas may also be capable of directly degrading
plastic. When Sphingomonas was given a nutrient supplement, degradation of the bisphenol A
was accelerated to only 20 hours vs. 40 days, further supporting this theory. Current research is
still investigating symbiotic relationships between Pseudomonas and Sphingomonas bacteria
involved in this plastic degradation (Moriyoshi, Ohe, Ohmoto, Sakai, & Yamanaka, 2006). The
bacteria are so similar; however, that there are numerous instances of species of Pseudomonas
being moved to the genus Sphingomonas, suggesting growing compatibility between the two
strains. For example, the genus of Pseudomonas aeruginosa was changed and the strain
appropriately renamed Sphingomonas paucimobilis (Chung, Kim, Park, & Seo, 2008).
Research has also determined that Phanerochaete chrysosporium, commercially known
as white-rot fungus, is able to degrade UV and thermal treated bisphenol A polycarbonate.
Results showed approximately a 5.4% weight loss in the polycarbonate and suggested that
pretreatment enhanced the degradation of the plastic (Artham, & Doble, 2009). In another
instance, P. chrysosporium was found to increase natural degradation of polyethylene films in
the soil. This suggests that the fungus is capable of digesting a variety of plastic types (Ammala,
et al., 2011). Fungi continue to be researched and probed for their plastic degradation properties.
Although not directly focused on P. chrysosporium, current research has found that ligninbiodegrading fungi contribute to the degradation of polyethylene. Since P. chrysosporium also
digests lignin, this research provides further support that it will metabolize HDPE plastic (Sivan,
2011).
Currently, research is being done on factors which will maximize bacterial degradation of
plastic. For example, abiotic degradation involves physical and chemical processes which
change the intramolecular structure of the plastic’s polymers. Abiotic degradation is being used
Cara Broshkevitch and Anne Richards
to make biodegradable plastics, but also may be able to accelerate the degradation of normal
plastics. The idea is that abiotic degradation will cause photo or thermo oxidation to the plastic’s
polymers when they are exposed to UV rays or heat, respectively. This process has been
supported by past research which shows increased degradation in low-density polyethylene
(LDPE) after a longer exposure to UV radiation. Since HDPE is even more durable, such
pretreatment with UV light and heat is likely to accelerate degradation (Sivan, 2011). Levels of
radiation harmful to polymers are 295-315 nm for UV-B, 315-400 nm for UV-A, 400-760 nm for
visible sunlight, and 760-2500 nm for infrared radiation (Shah, Hasan, Hameed, & Ahmed,
2008). More specifically, HDPE has been shown to have rapid degeneration under a UV-A lamp
of 340 nm at 50°C (a UVA-340 lamp emits light from 300-400 nm) (Ammala, et al., 2011).
Overall, temperature of radiation is directly related to plastic degradation. The reason for this is
that pretreatment using UV and infrared radiation has been shown to increase biodegradation by
increasing the surface area available for microbial colonization, and by reducing the mass itself
(Sharma, & Singh, 2008).
Finally, current research is investigating the problem of cultivating the plastic degrading
bacteria on the plastic’s surface. A biofilm on the plastic’s surface is the most efficient method
determined thus far (Sivan, 2011). It’s also been found that microbial species secrete a kind of
glue which fills in pores on materials, helping the species stick (Belloy, et al., 2008).
Physical signs that provide evidence of plastic degradation include a rough surface,
cracks in the plastic, and formation of biofilms. Another sign of degradation is the presence of
CO2, a product of biodegradation. Therefore, when bacteria are grown on plastic with minimal
media, CO2 tests are often used to verify this degradation. These tests indicate that the bacteria
are digesting and being sustained by the plastic. Moreover, an observational “clear-zone test”
Cara Broshkevitch and Anne Richards
can be used: when biodegradation begins, a clear halo forms around the colony of
microorganisms (Shah, Hasan, Hameed, & Ahmed, 2008).
While work in the past has identified bacteria found on degrading plastic, focus has not
been placed on attempting to maximize the degradation by a specific species of bacteria.
Although separate research has been done on the ability of Pseudomonas and Sphingomonas
bacteria, P. chrysosporium fungus, UV radiation, and thermal pretreatment to degrade different
types of plastic, the combination of these may result in accelerated decomposition of HDPE.
HDPE plastic will first be pretreated by exposure to UV and thermal radiation. Metabolization
of HDPE plastic by P. putida and S. macrogoltabidus bacteria, and P. chrysosporium fungi, will
then be established. Once the degradation rates of each species are determined, the percentage of
each species will be varied to determine the best combination resulting in the greatest decrease in
HDPE mass. These percentages will be based on relationships between the species and any
discovered patterns of symbiosis, as well as their degradation rates themselves. This
investigation will differ from past research since it will combine bacteria and fungi, examining
their relationships. Moreover, it will focus on HDPE plastic instead of plastic waste,
polycarbonate plastic, LDPE, or bisphenol A.
Cara Broshkevitch and Anne Richards
References
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Chung, I.Y., Kim, E., Park, J.M., & Seo, S.W. (2008). A case of postoperative Sphingomonas
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Cara Broshkevitch and Anne Richards
Moriyoshi, K., Ohe, T., Ohmoto, T., Sakai, K., & Yamanaka, H. (2006). Biodegradation of
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