Not too fast: a critique on biodegradation of plastics, especially by marcoorganisms such as caterpillars

Caterpillars can consume plastics, but it is not a magic solution.

Caterpillars can consume plastics! Back in 2017, the world was getting to know this extraordinary phenomenon by global news coverage of findings of Bombelli et al. [1]. Their claims were that larvae of the greater wax moth, Galleria mellonella, consume polyethylene at an extraordinary rate, and moreover produce ethylene glycol as a major metabolite. News outlets started dreaming of recycling factories where caterpillars were fed with plastics and virgin, high value materials would come out.

Meanwhile, it is becoming clear from both earlier [2] and later work [3,4] that indeed various 'caterpillars', more specifically mealworms, and larvae of the greater and lesser wax moth, are able to degrade polyethylene as well as polystyrene, albeit at lower rates. Additionally, some methods/findings of the work of Bombelli et al. have been questioned later on [5], and it is still very doubtful whether any ethylene glycol would be produced at all. It is much more likely that any plastics that have been effectively degraded are converted into CO2, as demonstrated to multiple reports.

In our recently published work in Science of the Total Environment [6], we have shown that the consumption roughly matches the energy needs of larvae, taken into account that only half of the ingested plastics is converted (the remainder is still present as small flakes). Additionally, we have found no significant amounts of ethylene glycol, and have provided some extra findings about the why and how these macroorganisms do this.

However, in the same report, we share our findings about the technological prospects of these observations, resulting from our collaboration with the Technion in Haifa, Israel (prof. Sabrina Spatari). Whereas the biodegradation of polyethylene (or other plastics) by larvae is pretty awesome, and very remarkable given the well-known persistency of these plastics in the environment, it is very unlikely that this directly will lead to technological applications. Here's why:

Thus far it seems that these larvae do not grow on a diet of plastics alone; it is very expensive though to grow them solely for the purpose of waste conversion.

After the larvae have been in contact with waste plastics, it is unlikely that they will be allowed to re-enter the anthropic food chain, therefore there's no value-added outlet for proteins and lipids.

For bioremediation purposes, as suggested in popular media, it is unlikely that larvae would prefer plastics over more abundant and easier nutrition.

Last but not least, as a major product, CO2 is worthless. The larvae are believed to use the plastics predominantly for their own sustenance, thus for their energy needs. There are other methods to convert plastics into CO2 while capturing their energy content in a more efficient and useful way; they could also be incinerated.

The above is not a pledge to maximally incinerate plastics; when possible, recycling of plastics should be the go-to option from an environmental point of view. However, for those plastics that are so heavily mixed or polluted that recycling cannot result in products with acceptable quality in an economically sensible way, incineration (Waste-to-Energy) makes more sense than biodegradation. At the same time, recycling technologies are evolving to cope with more diverse, mixed and polluted plastics streams, through many ongoing R&D projects.

Are the findings not useful then? They are! In some scenarios where biodegradation is already the governing mechanism for waste treatment (think about composting plants, anaerobic digesters…), residual plastics in e.g. food waste are a real burden. These plastics risk to end up on agricultural land, alongside the compost and other secondary organic fertilizers. For specifically those technologies, it makes sense to investigate which (combinations) of micro-organisms are responsible for biodegrading plastics. Composting mixtures could then potentially be inoculated with plastic-degrading microor macro-organisms. Yet, to this end there's still much work to do.

Above all, our work should be seen as a call-out to popular media not to rush to flashy conclusions, as is often the case when biodegradation is involved. Biodegradation is not a magic solution to the problem of plastic waste. Don't forget that we invested a lot of energy in making those long and versatile polymeric molecules; it's such a waste seeing them degrade into worthless CO2.

Note

This article is based on the original research paper, which you are kindly invited to consult and cite:

Billen, P., Khalifa, L., Van Gerven, F., Tavernier, S., Spatari, S. Technological application potential of polyethylene and polystyrene biodegradation by macro-organisms such as mealworms and wax moth larvae, Science of the Total Environment 735 (2020), 139521 https:// doi.org/10.1016/j.scitotenv.2020.139521

Read the open access author’s copy can be found

References

Bombelli, P., Howe, C.J., Bertocchini, F., 2017. Polyethylene bio-degradation by caterpillars of the wax moth echterGalleria mellonella. Curr. Biol. 27, R283-R293. doi.org/10.1016/j.cu­b.2017.02.060.

Yang, J., Yang, Y.,Wu,W.M., Zhao, J., Jiang, L., 2014. Evidence of polyethylene biodegradation by bacterial strains from the guts of plastic-eating wax worms. Environmental Science & Technology 48, 13776-13784. doi.org/10.1021/es50­4038a.

Brandon, A.M., Gao, S.H., Tian, R., Ning, D., Yang, S.S., Zhou, J.,Wu,W.M., Criddle, C.S., 2018. Biodegradation of polyethylene and plastic mixtures in mealworms (larvae of Tenebrio molitor) and effects on the gut microbiome. Environmental Science & Technology 52, 6526-6532. doi.org/10.1021/acs.­est.8b02301.

Cassone, B.J., Grove, H.C., Elebute, O., Villanueva, S.M.P., LeMoine, C.M.R., 2020. Role of the intestinal microbiome in low-density polyethylene degradation by caterpillar larvae of the greater wax moth, Galleria mellonella. Proc. R. Soc. B 287, 20200112. https://doi.org/10.1098/rspb.2020.0112.

Weber, C., Pusch, S., Opatz, T., 2017. Polyethylene bio-degradation by caterpillars? Curr. Biol. 27, R731-R745. doi.org/10.1016/j.cu­b.2017.07.004.

Billen, P., Khalifa, L., Van Gerven, F., Tavernier, S., Spatari, S., 2020. Technological application potential of polyethylene and polystyrene biodegradation by macro-organisms such as mealworms and wax moth larvae, Sci. Total Environ. 735, 139521 doi.org/10.1016/j.sc­itotenv.2020.139521

Link: www.uantwerpen.be/en­/research-groups/ipracs/

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