Community Diversity and Makeup Affect the Capacity for Bioconversion of Chemically Deconstructed PET Plastic Waste Into Biomass

Plastic waste management is an issue worldwide, particularly for locations with challenging logistics, such as remote locations, those impacted by natural disasters, and regions with armed conflict. These locations also struggle with food logistics and malnutrition. It is possible to tackle both issues by combining chemical deconstruction to depolymerize plastic waste with microbial conversion of the liquid product into biomass that can be used as an edible protein powder. The focus of this work is on the bioconversion of products from the chemical deconstruction of polyethylene terephthalate (PET) using ammonium hydroxide. Microbial communities have previously been shown to utilize the various products generated from the chemical deconstruction of PET (terephthalic acid (TPA), TPA monoamide, terephthalamide, and ethylene glycol). Bioreactors allow for large-scale continuous production of biomass, however, there are challenges to this process. First, the formation of biofilms causes issues during processing. This work demonstrates that increased airflow and increased agitation speed can reduce biofilm formation, however, there is a negative impact on biomass production when the shear stresses are too strong. Using microbial communities of lower diversity can also reduce biofilm formation. The second problem to address is how the different products from the chemical deconstruction of PET affect microbial growth. Using Monod kinetics to estimate parameters to characterize growth on various substrates, it was determined that for Rhodococcus sp. TE21C, terephthalic acid becomes inhibitory to growth as the concentration increases, while Paracoccus sp. RL32C was not able to utilize terephthalic acid as a substrate. Ethylene glycol was shown to not be inhibitory to both organisms at the concentrations evaluated. Using both substrates simultaneously in a mixture showed no effect on substrate interactions on growth of either organism. The final problem addressed here is how temperature perturbations affect biomass productivity. In locations where utilities can be easily disrupted or are restricted, keeping the temperature controlled may be difficult or impossible and variability in bioreactor temperature is expected to impact biomass production. This work shows that by using communities of higher diversity, the system is more stable and better able to recover in the middle of a long-term temperature perturbation.