Engineering Thermophilic Microorganisms to Selectively Extract Strategic Metals from Low Temperature Geothermal Brines

Abstract

Geothermal brines can contain comparatively high concentrations of rare earth elements, 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 robust to feasibly extract these elements from geothermal brines in a cost-effective manner. Here we test whether engineered thermostable surface layer (S-layer) proteins can unite the affinity, selectivity, and robustness needed to act as a low-cost, selective, and reusable metal adsorbent. S-Layer proteins self-assemble into 2D crystalline lattices on the surface of many microorganisms, including the thermophile Geobacillus stearothermophilus. To create a robust, selective sorbant, we expressed S-layer proteins from Geobacillus stearothermophilus fused to various highly selective metal-binding domains in Escherichia coli. When purified, these engineered proteins retain their ability to crystallize into 2D sheets. More critically, the Zn-binding engineered proteins sheets can bind Zn2+ more tightly at 40°C and more selectively than their native counterparts. Likewise, the Gd-binding variants can bind Gd3+ more tightly at 40°C. 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.