Multispecies Biofilms Transform Selenium Oxyanions into Elemental Selenium Particles: Studies Using Combined Synchrotron X-ray Fluorescence Imaging and Scanning Transmission X-ray Microscopy

Selenium (Se) is an element of growing environmental concern, because low aqueous concentrations can lead to biomagnification through the aquatic food web. Biofilms, naturally occurring microbial consortia, play numerous important roles in the environment, especially in biogeochemical cycling of toxic elements in aquatic systems. The complexity of naturally forming multispecies biofilms presents challenges for characterization because conventional microscopic techniques require chemical and physical modifications of the sample. Here, multispecies biofilms biotransforming selenium oxyanions were characterized using Xray fluorescence imaging (XFI) and scanning trarismission X-ray microscopy (STXM) These complementary synchrotron techniques required minimal sample preparation and were applied correlatively to the same biofilm areas. Sub-micrometer XFI showed distributions of Se and endogenous metals, while Se Kedge X-ray absorption spectroscopy indicated.the presence of elemental Se (Se-0). Nanoscale carbon K-edge STXM revealed the distributions of microbial cells, extracellular polymeric substances (EPS), and lipids using the protein, saccharide, and lipid signatures, respectively, together with highly localized Se-0 using the Se L-iii edge. Transmission electron microscopy showed the electron-dense particle diameter to be 50-700 rim, suggesting Se-0 nanoparticles. The intimate association of Se-0 particles with protein and polysaccharide biofilm components has implications for the bioavailability of selenium in the environment.