Bacterial Metabolites and Particle Size Determine Cerium Oxide Nanomaterial Biotransformation

Soil is a major receptor of manufactured nanomaterials (NMs) following unintentional releases or intentional uses. Ceria NMs have been shown to undergo biotransformation in plant and soil organisms with a partial Ce(IV) reduction into Ce(III), but the influence of environmentally widespread soil bacteria is poorly understood. We used high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) with an unprecedented detection limit to assess Ce speciation in a model soil bacterium (Pseudomonas brassicacearum) exposed to CeO2 NMs of different sizes and shapes. The findings revealed that the CeO2 NM's size drives the biotransformation process. No biotransformation was observed for the 31 nm CeO2 NMs, contrary to 7 and 4 nm CeO2 NMs, with a Ce reduction of 64 +/- 14% and 70 +/- 15%, respectively. This major reduction appeared quickly, from the early exponential bacterial growth phase. Environmentally relevant organic acid metabolites secreted by Pseudomonas, especially in the rhizosphere, were investigated. The 2keto-gluconic and citric acid metabolites alone were able to induce a significant reduction in 4 nm CeO2 NMs. The high biotransformation measured for <7 nm NMs would affect the fate of Ce in the soil and biota.

Automated summary

The influence of environmentally widespread soil bacteria on the biotransformation of ceria nanomaterials (NMs) has been poorly understood. This study used high-energy resolution fluorescence detected X-ray absorption spectroscopy to investigate the speciation of Ce in a model soil bacterium exposed to different sizes and shapes of CeO2 NMs. The findings revealed that the size of the CeO2 NMs drives the biotransformation process, with smaller particles exhibiting higher rates of reduction. The study also identified specific organic acid metabolites secreted by the bacterium that can induce significant reduction in CeO2 NMs.