Metal stress-modulated extracellular polymeric substances enable enhanced and selective scandium adsorption in Methylobacterium extorquens.

Problem: The recovery of scandium (Sc), a critical rare earth element, is hampered by its dispersed geological occurrence and the economic and environmental drawbacks of conventional methods. While microbial extracellular polymeric substances (EPS) are promising biosorbents, their practical application is limited by insufficient target-specificity in complex matrices.

Solution: Inspired by bacterial adaptive responses, this study employed targeted scandium stress to modulate the composition and structure of EPS secreted by Methylobacterium extorquens, thereby enhancing the selective adsorption of scandium.

Methods: M. extorquens was cultivated under scandium stress to generate an adapted strain (Mex4). Its adsorption performance, including capacity and selectivity, was systematically evaluated. The EPS was characterized using multifaceted spectroscopic (FTIR, 3D-EEM), amino acid, and proteomic analyses to elucidate the enhancement mechanisms.

Results: The resulting stress-adapted strain, Mex4, exhibited a scandium adsorption capacity of 79 mg/g-threefold higher than the unadapted strain. EPS were identified as the dominant component in the adsorption mechanism, achieving a capacity of 220 mg/g and selectivity for Sc over competing ions (separation factor up to 38.5). The enrichment of specific functional groups (thiol, carboxyl) within the EPS matrix may have contributed to the selectivity. Compositional analysis revealed an increased abundance of protein-like constituents under scandium stress, with notable increases in cysteine, glutamic acid, and glycine, indicating their critical role in metal chelation and retention.

Significance: This work demonstrates that metal-induced stress is a powerful strategy for remodeling the molecular architecture of microbial EPS to achieve enhanced adsorption capacity and selectivity.