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Plastic pollution in the ocean. Photo Retrieved from Oceana.com

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The world's top Environmental hazard —— Plastics. Humans produce lots of plastic waste, and Canada alone has 3.3 million tons per year (1). Of all artificially produced plastics, 86% of plastics go to landfill, 8% claim to be recycled, while the rest is incinerated (1). But the alternative to petrochemical plastic, biodegradable plastics made from microbial production became the desired option. Polyhydroxyalkanoate (PHA, PHB) is a commercially viable option for biodegradable plastic.

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If one digs deeper into the endings of 86% of the plastic, it will be discarded in landfill sites across the country. Wastes in the site go through anaerobic decomposition
of the organic matter, releasing a mixture of gas called biogas, or landfill gas. Landfill gas consists of approximately 1:1 CO2 to CH4, which are two greenhouse gasses that are harming the environment.
Landfill gas is responsible for 23% of methane emissions in Canada, and noticeably, methane is 86 times more potential for global warming than carbon dioxide in a 3yr span (2).

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Flowchart of the designed strategy. Retrieved from Chidambarampadmavathy et al., 2017 (3)

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The strategy! Converting landfill gas to PHB compounds

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The strategy! Converting landfill gas to PHB compounds

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The "traditional" plastics made from fossil fuel contribute to carbon emissions because of fossil fuel involvement in the manufacture, and the plastic is disposed into the landfill site posing environmental problems. Degradation of the waste in landfill sites emits biogas that drastically increases the global warming effect as well. By utilizing engineered strains of bacteria to treat landfill gas that captures the single carbon compound and convert them into PHB, the biodegradable plastic, to substitute the petrochemical plastic and solve the problem from its root.

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Illustration of metabolism in C. necator. Chidambarampadmavathy et al., 2017 (3).

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There are a list of considerations when choosing the optimal engineering chassis. For the purpose of this project, I need a chassis that could utilize the single carbon compound and produce PHB effectively. C. necator is a known chassis that has high carbon fixation efficiency because it could use carbon dioxide directly instead of requiring to convert it into HCO3- first, as cyanobacteria would need (3). C. necator has CBB cycle for carbon capture that allows autotrophic pathways. Intriguingly, wildtype C. necator could covert CO2 to PHB via CBB, central metabolism cycle, and through catalyst phA, phB, phC. Last but not least, C. necator is also great because it is a versatile chassis that has multiple pathways that would be commercially desirable as shown in the graph.

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The reductive glycine pathway. Modified from Sánchez-Andrea et al., 2020 (4).

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As mentioned, carbon dioxide could be utilized as a carbon source, while methane does not have a degradation pathway in C. necator. Methane in C. necator, as the same in most bacteria, would be oxidized to formate, which would be further oxidized to carbon dioxide (6). Hence, if one introduces the formate degradation pathway into the engineered C. necator, it will become methanotrophic. There are many approaches to achieving that goal, in which the glycine reductive pathway would be the most efficient and the easiest for C. necator (5). By comparing the existing pathway to the metabolism database, biocyc.com, it turns out that integrating this pathway would require the introduction of ftl, metcd, and the glycine reductase complex to complete the glycine reductive pathway (4) (6).

The rational designs could also include up-regulating some essential genes along the pathway, and directed evolution could play a major role in improving the whole pathway. In-situ-directed evolution refers to subjecting every single organism to the selection process, hence leaving the black-box of the mechanism of optimizing metabolism due to selection pressure to evolution.

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The proposed industrial chain. Images retrieved and modified from CBC news, Chidambarampadmavathy et al., 2017 (3), alibaba.com, respectively.

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The business only operates when there are profits along its manufacturing loop. Starting from the raw material, the cost of landfill gas is essentially negligible, as currently, a portion of landfill gas had to be flared in order to get rid of them. This suggests a demand for solutions for processing biogas. The setup cost would inevitably be a financial burden in the very short term, as it would involve a list of fees for it to process. The design is similar to the idea of a microalgae biofilm that allows aggregation of the engineered C. necator on a surface for its reaction (3) (7). The price of PHB granule is estimated to be ~4.00 CAD/kg in an international market, while that price does not indicate the price for Canadian businesses and consumers. The price would also be subject to change in the downstream manufacture of those PHB granules. There is a list of incentives and grants that this project would be eligible for, and that is estimated to help reduce the pressure of financial setup cost in a large extent.

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Landfill sites and indigenous settlements comparison. Images modified from first nation map, ontario gov, landfill site map, Ontario gov, and CBC news.

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Marketing approach: A global solution with a localized focus

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Marketing approach: A global solution with a localized focus

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The indigenous community, Oneida Nation of the Thames, voiced out on landfill sites being installed in the vicinity of the settlement, and the environmental effect is unbearable. One purpose of the project is to address localized environmental protection, as protecting the treaty lands and its people is one priority in alleviating environmental injustice. The proposed methodology could negate the effect of the released biogas in the long run, which localized greenhouse gas emissions would harm local ecosystems first.

The produced PHB could be further manufactured into cutlery to substitute the existing non-decomposable ones, in response to the intense growing of the food delivery over Canada

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This project is designed by Ruochen Liu.

Special Thanks to FCSB for accepting the project as one of the EL projects!
Special Thanks to FCSB Samir Hamadache for providing tips and tricks!

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1. Young, R. (2022, July 6). Canada's plastic problem: Sorting fact from fiction. Oceana Canada. Retrieved November 29, 2022, from https://oceana.ca/en/blog/canadas-plastic-problem-sorting-fact-fiction/

2. Canada, E. and C. C. (2022, January 28). Government of Canada. Canada.ca. Retrieved November 29, 2022, from https://www.canada.ca/en/environment-climate-change/services/canadian-environmental-protection-act-registry/reducing-methane-emissions-canada-municipal-solid-waste-landfills-discussion.html

3. Chidambarampadmavathy, K., Karthikeyan, O. P., & Heimann, K. (2017). Sustainable bio-plastic production through landfill methane recycling. Renewable and Sustainable Energy Reviews, 71, 555–562. https://doi.org/10.1016/j.rser.2016.12.083

4. Sánchez-Andrea, I., Guedes, I. A., Hornung, B., Boeren, S., Lawson, C. E., Sousa, D. Z., Bar-Even, A., Claassens, N. J., & Stams, A. J. (2020). The reductive glycine pathway allows autotrophic growth of Desulfovibrio Desulfuricans. Nature Communications, 11(1). https://doi.org/10.1038/s41467-020-18906-7

5. Bar-Even, A., Noor, E., Flamholz, A., & Milo, R. (2013). Design and analysis of metabolic pathways supporting formatotrophic growth for electricity-dependent cultivation of microbes. Biochimica Et Biophysica Acta (BBA) - Bioenergetics, 1827(8-9), 1039–1047. https://doi.org/10.1016/j.bbabio.2012.10.013

6. biocyc genome database collection. BioCyc Pathway/Genome Database Collection. (n.d.). Retrieved November 29, 2022, from https://biocyc.org/

7. Mantzorou, A., & Ververidis, F. (2019). Microalgal biofilms: A further step over current microalgal cultivation techniques. Science of The Total Environment, 651, 3187–3201. https://doi.org/10.1016/j.scitotenv.2018.09.355