Barbara Calderon, UMass Lowell
Plastic pollution is one of the most critical problems our society faces. The consequences of plastic pollution can be observed in every corner of the world. Ocean garbage patches, twice the size of Texas, are currently floating in the Pacific Ocean. Furthermore, we consume at least 7,000 microplastics per million from the dust floating in the air, which settles on our food. The future of plastic waste does not forecast a solution to this growing plastic pollution problem. According to the Ellen MacArthur Foundation, scientists estimate that there will be more plastic in the ocean than fish by 2050.
One of the primary reasons plastics waste is such an issue is because of the extremely slow rate of natural decomposition. It is estimated that commercial polymers take over 1,000 years to fully break down into small molecules. Some nations have adopted incineration technologies to fight this issue. However, this practice is just as controversial and harmful as plastic pollution due to the air pollutants released in this process, which degrade the environment and human health. An ideal solution to this problem is a method that can speed up the natural degradation process, does not produce any toxic byproducts, and does not require large quantities of energy. The solution: organisms eating plastics! Since the 1950's, many researchers have investigated the plastic-eating capacity of insects and plants and their ability to degrade packaging materials. There are many opinion articles addressing this matter, claiming the discovery of mushrooms and worms capable of ingesting and later degrading different commodity plastics. Many articles report the potential advantages of this method over recycling or burying plastic into landfills yet, they fail to elaborate on the practicality, efficiency, and scientific basis of this alternative.
To shed some light on this matter I searched for journal articles that provided a scientific take on this issue and decided to study their basis and analysis process. Here is what I found:
Let’s start with mushrooms. A team of researchers showed that a kind of mushroom called Aspergillus tubingensis is capable of decomposing Polyurethane, in a matter of weeks rather than years. Environmental Pollution journal published the work made by Khan about experimentation carried out using Aspergillus tubingensis to degrade Polyurethane (PU) waste. 3 different culture methods were tested for the fungus: SDA culture plate, liquid MSM and soil burial. The PU films were exposed to the different culture and the growth and degradation rate was analyzed by microscopy and spectroscopy techniques. It is important to state that the only practical method addressed in the study that could be implement in large scale is the soil burial culture. The other methods required the addition of other sources or preprocessing methods (such as mixing the film with the liquid medium at a certain temperature at 150 rpm stirring speed).
When using the soil method, the team found that the surface of the PU films were cracked and eroded and from FTIR observed that there was incompletely H bonded urea carbonyls and formation of C-H bonds. The author attributes these changes to biodegradation, however, it is not clear how these is related to chemical decomposition. Also, the growth and enzyme secretion that is the responsible of the degradation process is highly dependent on pH, temperature and carbon source (medium), which means that a careful control of the environment shall be used if we really want this to work out.
The researchers also found that the samples immersed in liquid medium for two months broke down into smaller pieces. These are promising results, however, a change in geometry does not mean a higher degradation rate. A mass loss analysis should be conducted to corroborate if the film has actually been consumed by the fungus. Regardless, the alleged deterioration of the films in agar plates and liquid medium proved to be much higher than the soil burial culture.
While this work provides insight on the role of Aspergillus tubingensis to the mitigation of PU waste it lacks practically and actual evidence of chemical decomposition.
Another interesting alternative of plastic eating organism are worms capable of degrading Polyethylene and Polystyrene. Current Biology published an article that outlines the study conducted on caterpillars of the max moth Galleria mellonella. Bombelli et al. placed waxworms on PE bags and tested them with microscopy analysis, FTIR and mass loss techniques at different periods of time. They reported to see the appearance of many holes and changes in surface roughness of the bags within 40 min of worm exposure. Also, there was a mass loss of 92mg in 12 hours, meaning that the worms were, in fact, consuming the plastic and not only masticating and separating it. Moreover, FTIR analysis suggested the production of ethylene glycol and carbonyl bonds which they take as products of PE degradation.
Even though all these findings seem encouraging, it is necessary to look into the scientific basis in more detail. Weber et al. wrote a correspondence in regards of the Bombelli article. They suggested that, from the results provided, it is not possible to state that there is formation of ethylene glycol in the samples. Bombelli reported only one single infrared absorption band as evidence of the compound but in fact two of its characteristic bands are missing. Weber said that the band could come from protein contamination on the bag surface. Also, Bombelli stated that the roughness of the bag changed and became more coarse after exposure to the worms. However, Weber said this could also be due to material left after treatment with worm homogenate.
It is not possible to rule out that the waxworm is capable of the chemical decomposition of PE however, a more careful study to confirm the breakdown of the PE molecules should be conducted. Nonetheless, the practicality of this method needs further analysis. It is unknown whether the wax worm can consume the whole bag and how long this could take. Moreover, it is unknown if the organism can consume other grades of PE. A question that arises from these experimentations is whether the molecular weight, additives and even colorants have an influence in the degradation rate of the polyolefin. Moreover, the worms should be exposed of other kinds of waste to test the efficiency of this method in real life conditions.
There is another worm that allegedly eats plastic. Actually, the larvae leaving in the yellow mealworms gut (Tenebrio molitor Linnaeus) is supposed to be capable of digesting the PS. The work done by Yang et al. involved variables encountered in real case waste conditions, such as products made out of different types of PS, for instance, expanded PS (EPS), exposure to different source of food and changes in temperature. According to the results obtained, the worms survived over 32 days just by ingesting plastic. However, the worms rather eating the other sources of food than PS. GPC and spectroscopy techniques conducted on the PS residues in the worm frass showed reduction in the polymers molecular weight (meaning that the PS experienced degradation) and changes in the chemical structure due to depolymerization. Also, they found that EPS is consumed at a faster rate than solid PS. This is probably because its low density would make it easier to chew and digest.
Here is my take on the results observed: Even though the worm was capable of reducing the Mw of PS, this is not considered biodegradation in practical term. The sample found of the frass is still ponder a polymer, the worm was not capable of breaking down the polymer into small, nontoxic molecules like CO2 and H2O. In terms of the PS consumption calculated, this number is far from optimum. Only 46% of the PS was consumed after 32 days. Moreover, the frass collected comprised undigested PS, digested PS and undigested skeletons. This means that not all the PS consumed is processed by the worm. The biodegradability of the fecal matter of the PS eating worm is questionable.
In conclusion, there have been many efforts to search and study organisms that can degrade hydrocarbon based polymers. However, these biodegradation methods, if they truly work, are far from been practical and hence, cannot be implemented in the near future. It is necessary to explore these processes in more depth so they can be considered as potential solution for plastic waste. For instance, it is required to understand what enzymes are taking part of the degradation process in these organisms. This is a crucial point to considered if we want to engineer the organisms into more potent and efficient versions of themselves. Even though biodegradation of plastics by organisms may take some time to develop, the discoveries made are highly promising and provide a baseline for work that could lead to high efficiency plastics degrading engineered organisms. We will have to stay tuned to see the advances to come in the following years.
S. Khan, S. Nadir, Z. Shah, A. Shah, S. Karunarathna, J. Xu, A. Khan, S. Munir and F. Hasan, Biodegradation of polyester polyurethane by Aspergillus tubingensis, Env. Poll., 2017, 225, 469.
P. Bombelli1, C. J. Howe1, F. Bertocchini, Polyethylene bio-degradation by caterpillars of the wax moth Galleria mellonella, Curr. Biol., 2017, 27, R292.
C. Weberm, S. Pusch and T. Opatz, Polyethylene bio-degradation by caterpillars? Curr. Biol., 2017, 27, R744.
S. Yang, M. Brandon, J. Andrew Flanagan, J. Yang, D. Ning, S. Cai, H. Fan, Z. Wang, J. Ren, E. Benbow, Biodegradation of polystyrene wastes in yellow mealworms (larvae of Tenebrio molitor Linnaeus): Factors affecting biodegradation rates and the ability of polystyrene-fed larvae to complete their life cycle. Chemosphere 2018, 191, 979.