Anaerobic reduction of europium by a Clostridium strain as a strategy for rare earth biorecovery

The biorecovery of europium (Eu) from primary (mineral deposits) and secondary (mining wastes) resources is of interest due to its remarkable luminescence properties, important for modern technological applications. In this study, we explored the tolerance levels, reduction and intracellular bioaccumulation of Eu by a site-specific bacterium, Clostridium sp. 2611 isolated from Phalaborwa carbonatite complex. Clostridium sp. 2611 was able to grow in minimal medium containing 0.5 mM Eu(3+). SEM-EDX analysis confirmed an association between Eu precipitates and the bacterium, while TEM-EDX analysis indicated intracellular accumulation of Eu. According to the HR-XPS analysis, the bacterium was able to reduce Eu(3+) to Eu(2+) under growth and non-growth conditions. Preliminary protein characterization seems to indicate that a cytoplasmic pyruvate oxidoreductase is responsible for Eu bioreduction. These findings suggest the bioreduction of Eu(3+) by Clostridium sp. as a resistance mechanism, can be exploited for the biorecovery of this metal.