Biotransformation and Interactions of Selenium with Mixed and Pure Culture Biofilms

Abstract

Biofilms have been reported to have an important role in the biogeochemical cycling of toxic elements in aquatic systems. Despite this important role, the complexity of biofilms and chemical and physical modifications that occur in conventional microscopic techniques hinder research progress. Thus, this research utilized a suite of less destructive synchrotron-based techniques to investigate selenium biotransformation in biofilms.

Natural biofilm collected from a coal mine-affected field site was examined with X-ray absorption spectroscopy (XAS), the results of which suggested the presence of methylated and elemental selenium. Biofilms generated in the laboratory using collected coal mine-affected water were investigated using confocal laser scanning microscopy (CLSM), (μ-)XAS and extended X-ray absorption fine structure (EXAFS). The results showed biotransformation of added oxyanion species (selenate, selenite, arsenate or arsenite). Micro X-ray fluorescence imaging (μ-XRF) and μ-XAS combined with CLSM revealed that selenium was highly localized in the biofilm in chemically reduced forms. Isolation and partial 16S rRNA gene sequencing suggested there were 4 principle bacterial genera responsible for these biotransformations of selenium. Further examination of multispecies biofilm and pure-culture biofilm (Arthrobacter sp SASK-Se22) exposed to high concentrations of selenate and selenite was carried out using synchrotron-based nano-scale scanning transmission X-ray microscopy (STXM) and nano-XRF imaging at the selenium L and K near-edges. These results demonstrated that selenium was biotransformed to nano-particulate elemental selenium and that the selenium particles were closely associated with lipid within the biofilm.

The distributions of selenate and selenite as well as elemental selenium within this complex biofilm structure were investigated using a novel combination of STXM and XRF on identical areas of biofilms. Transmission electron microscopy showed that the biogenic elemental selenium was sub-micron-sized, ranging from 50 to 700 nm in diameter, and was closely associated with microorganisms. The presence of submicron-sized elemental selenium may suggest possible applications of the biofilms in bioremediation or in the semiconductor industry where micro- to nano- sized selenium particles are in great demand.

Overall, this research demonstrated a novel application of synchrotron-based spectroscopic and imaging techniques to biofilm research, the results of which advance understanding of selenium biotransformation in multispecies and pure-culture biofilms.