S. Napieralski, W.R Henson, C. Immethun, B. Heater, C. Hart, H. Zurier, S. Banta, K. Kucharzyk
Battelle Memorial Institute,
United States
Keywords: biotechnology, protein engineering, rare earth element separations, microbiology
Summary:
Current rare earth element (REE) mining operations function under considerable global attention, since >80% of the world’s supply is monopolized by China, leaving global markets vulnerable to supply disruptions. One impediment to a more diversified REE supply chain is the difficulty in achieving economical and environmentally sustainable REE extraction and separation from ore deposits and REE-containing waste. To alleviate supply vulnerability and diversify the global REE production chain, new processing technologies that enable environmentally benign extraction and purification of REEs must be developed. Biologically based approaches are particularly attractive, however fundamental research is necessary to advance understanding and ultimately the application of leaching and separation of REEs by microorganisms and/or biomolecules. Our objective is thus to develop a microbial chassis with the ability to grow and survive in extreme conditions to extract metals from REE ores. To address challenges with growth and survival of microorganisms, we will develop a microbial chassis that can thrive in increasingly extreme conditions in the presence of REEs. Our approach starts with the acidophile Acidithiobacillus. Ferrooxidans. With an ability to solubilize metals in mining operations, A. ferrooxidans can grow in pH as low as 1.3 and derive metabolic energy from the oxidation of Fe2+ to Fe3+ or sulfur species. Since genetic transformations by conjugation, antibiotic selection at low pH, and a limited catalog of genetic parts have made engineering efforts difficult in extremophiles, our team has worked to expand the number of plasmids, promoters, and selection conditions enabling the creation of more than 50 unique genetically modified strains of A. ferrooxidans. These past experiences will guide us and be extended for REE leaching and processing by additional extremophilic taxa including acidophiles with increased tolerance to low pH and high temperature, e.g., A. caldus and Acidianus brierleyi, as well as alkaliphilic taxa including Alkalihalobacillus halodurans and Bacillus megaterium. Additional efforts are underway to enrich for novel REE solubilizing microorganism both acidic and alkaline hot springs from Yellowstone National Park. In addition to exploration of biologically enhanced REE leaching, biologically based separation of REEs offers an attractive alternative to conventional solvent-based separations. The discovery of REE binding proteins, such as the well characterized protein Lanmodulin (LanM), provides a blueprint and proof of concept for development of protein based REE separation and purification systems. Our team has discovered the repeats in toxins (RTX) domain of secreted microbial proteins is capable high affinity REE binding under acidic conditions. Thus, we will explore the RTX domain as an additional novel system for engineering/evolving highly selective REE binding in addition to LanM. Our combined approach to REE bioleaching under disparate conditions of pH and temperature, coupled with protein-based recovery and separations will thus yield an environmentally sustainable, scalable, modular, feedstock agnostic solution to diversifying the REE supply chain.