Engineering Thermostable Proteins to Selectively Extract Strategic Metals from Low Temperature Geothermal Brines

Geothermal brines can contain comparatively high concentrations of rare earth elements (REEs), high-value metals, and/or critical metals that offer the opportunity to add value to geothermal energy production. However, existing technologies are either insufficiently selective or too expensive to feasibly extract these elements. Here we test whether engineered thermostable cell surface proteins can unite the affinity, selectivity, and robustness needed to act as a low-cost, selective, and reusable metal adsorbent. Using heterologously expressed S-layer proteins with a C-terminal metal-binding moiety purified by affinity chromatography from E. coli, we show that our engineered variant can bind Zn²⁺ more tightly at 40°C and more selectively than its native counterpart via ICP-MS. We also show that two other variants can bind Gd³⁺ more tightly at 40°C. Thus, the results presented here suggest that using thermophilic microorganisms to display engineered metal-binding proteins is an extremely promising approach for strategic metal extraction in low-temperature geothermal arena.