Extremophilic microbes tolerate high concentrations of rare earth elements (REEs)

As demand for rare earth elements (REEs) increases, biotechnological solutions to their extraction and purification are becoming more critical. Identification of chassis organisms capable of surviving in high REE concentrations is a crucial step towards development of sustainable biotechnologies. In this study, we show that growth of mesophilic Escherichia coli is not significantly inhibited by < 300 µM REEs in complex media of, but in phosphate-depleted minimal media a half-maximum inhibitory concentration (IC₅₀) for REEs is 80 ± 100 µM. REE-utilizing mesophile Pseudomonas alloputida KT2440 is growth-impaired in presence of REEs, with an IC₅₀ of 100 ± 20 µM. In contrast, we demonstrate that extreme acidophiles survive in conditions of 500 µM REEs without apparent inhibition of growth. Indeed, we observe that hyperacidophilic bacteria Acidithiobacillus ferrooxidans, A. thiooxidans, and A. caldus appear to enter log phase earlier in presence of REEs and grow to significantly higher densities. Acidophilic archaeon Sulfulobus acidocaldarius, a hyperthermophile, is similarly not significantly inhibited by 500 µM REEs. These data indicate that acidophiles have broad-range stress tolerance mechanisms that apply to REE stress. Importance: Rare earth elements (REEs) are essential for a diverse and expanding range of high-technology applications, which are an important part of the industrial economy. Application of biomining technologies could introduce "greener" extraction and processing steps, however, many fundamental challenges must be addressed before a biological approach to REEs recovery and separation can be fully adopted at scale. Fundamentally, any chassis organism used for REE biomining must be able to tolerate high concentrations of REEs, and existing literature along with this study demonstrate that neutrophilic mesophiles are highly sensitive to REEs. In contrast, we demonstrate that mesophilic extreme acidophiles and thermophilic acidophiles exhibit inherent REE tolerance. This phenomenon highlights their potential for bioprocessing and corroborates existing evidence that the extremophile response to REE presence may allow for broader adaptation behavior in comparison to other model chassis strains.