Thrifty trick for bacterial plastic upcycling
Implementation of a newly discovered metabolic pathway increases the CO2 efficiency of PET-utilizing bacteria
Scientists at the Max Planck Institute in Marburg have developed a more efficient, carbon dioxide-conserving route to upcycle ethylene glycol, a constituent of the plastic PET. They enhanced the metabolism of the bacterium Pseudomonas putida with a novel route taken from marine microbes, resulting in improved growth. Their findings offer new chances for the microbial upcycling of PET, but also for the development of sustainable material recycling.
Plastics are omnipresent. In 2017, the annual world plastic production had reached 350 million tons. A significant amount of plastic accumulates in natural environments. Plastic pollution bears serious consequences for the health of organisms and the stability of ecosystems. At the same time, valuable raw materials are lost that could be re-used in a sustainable way.
PET building block ethylene glycol: a key substance in the recycling loop
A promising solution is microbial degradation and/or upcycling of plastic. Since the discovery of the PET‐degrading bacterium Ideonella sakaiensis in 2016, many efforts center around PET (polyethylene terephthalate), which is most commonly used for the production of water bottles. Ethylene glycol, the C2 molecule that is used to produce PET, also finds applications as antifreeze agent or solvent, and can be electrochemically generated from syngas, thus gaining increasing attention as a key component for a carbon neutral bioeconomy. Therefore, efforts to develop microbial strains with improved conversion capacities for ethylene glycol is not only important for microbial PET upcycling, but also in the broader context of establishing circular economic routes for the biotechnological use of this abundant chemical.
Researchers at the Max-Planck-Institute for Terrestrial Microbiology, the Max-Planck-Institute for Plant Physiology, and the University of Leiden have now made an important step towards a more sustainable material recovery cycle. Using synthetic biology and directed evolution approaches they engineered the cycle into the biotechnologically relevant bacterium Pseudomonas putida. The new metabolic pathway increased the capacity for utilization of ethylene glycol.
Their work builds on their earlier identification of a metabolic pathway for the efficient assimilation of C2 compounds, the beta-hydroxyaspartate cycle (BHAC), in marine bacteria. “The BHAC is an elegant cycle through which the carbon of ethylene glycol can be recycled without the loss of carbon dioxide. Thus, it is highly favorable with regards to carbon and energy balance. Our engineered strain is now able to assimilate this component of the plastic PET more efficiently”, says Lennart Schada von Borzyskowski, co-leading author who helped to conceptualize the study. He performed his experiments during his postdoctoral studies at the Max-Planck-Institute for Terrestrial Microbiology in Marburg in collaboration with the group of Arren Bar-Even at the Max-Planck-Institute for Plant Physiology in Golm.
Directed evolution enhances bacterial performance
“First, we used E. coli selection strains to confirm that the synthetically modified bacteria are generally capable of sustaining complete biomass synthesis via the non-native BHA shunt. The subsequent complete BHAC integration into P. putida immediately enabled growth of the engineered strain on ethylene glycol. It also resulted in changes of the metabolic network to connect the new pathway to the host central carbon metabolism. Furthermore, directed laboratory evolution of P. putida with the BHAC resulted in a strain with increased growth performance - 35% faster growth, 20% higher biomass yield - on ethylene glycol”, says Helena Schulz-Mirbach, co-leading author of the study.
"Establishing sustainable material cycles is probably the greatest challenge of our time," adds Tobias Erb, Director at the Max-Planck-Institute for Terrestrial Microbiology, who coordinated the study, ”The degradation of plastics without the release of CO2 is an important step towards closing the carbon cycle in a circular fashion.”
“The study highlights the potential of the BHAC as a ‘plug-and-play’ metabolic module for synthetic biology”, Lennart Schada von Borzyskowski, now Assistant Professor at the University of Leiden in the Netherlands, adds. “In recent work, we have started to test the BHAC also in other organisms, for instance the plant Arabidopsis thaliana. In this plant, we could already show that the BHAC can make photosynthesis more efficient by allowing the plant to keep more CO2. These findings are very promising proof-of-concept for further research in developing CO2-saving pathways in biotechnology and agriculture.”