Phosphatase-mediated mitigation of rare earth element toxicity to Pseudomonas putida

Anthropogenic activities are driving an increasing flux of rare earth elements (REE) into environmental compartments, raising concerns about their biological impact, particularly on microorganisms that sustain ecosystem functioning. Here, we provide a systematic assessment of the toxicity of all 16 REE toward Pseudomonas putida KT2440, a soil bacterium that can use these metals as enzyme cofactors. Dose-response growth inhibition assays revealed high sensitivity to light REE. Toxicity correlated strongly with ionic radius, with IC₅₀ values ranging from 0.3 µM for lanthanum to 10 µM for scandium. Serial propagation of P. putida under gradually increasing REE stress yielded resistant populations, from which two stably resistant strains were isolated. Genome resequencing showed that both strains carried a single mutation in uxpB, encoding an alkaline phosphatase. Gene deletion and overexpression experiments, together with phosphatase activity measurements, confirmed the involvement of uxpB in REE resistance. Our findings reveal a previously unrecognized mechanism of tolerance to REE, suggesting that mutations enhancing phosphatase activity promote phosphate release from organic phosphorus compounds, thereby reducing REE bioavailability through phosphate-mediated complexation and/or immobilization.