Uranium in Biofilms

Biofilms show a complex architecture of heterogeneously distributed sessile bacteria embedded in extracellular polymeric substances (EPS), made of polysaccharides, proteins, lipoproteins and glycoproteins, which are interspersed by open water channels. Biofilms are made of 50-95% water and dissolved substances and are thus considered as hydrogels. Through these water channels nutrients and possibly toxic heavy metals from the surrounding bulk solution effectively infuse into the biofilms to the microorganisms and metabolites and exudates of the microorganisms are transported away.
The attachment of microbial cells to surfaces during biofilm formation leads to major changes in metabolism, resistance, and survivability and therefore the retention of radionuclides by biofilms is probably different from the interactions with single cell suspensions of only one type of bacterial species.
In this study particulate uranium in a multi-spezies biofilm grown at the solid/liquid interface was visualized within the biofilm and spectroscopically identified by a combination of confocal laser scanning microscopy (CLSM) and laser-induced fluorescence spectroscopy as uranium(VI) and metastable uranium(V). The particles showed either a characteristic fluorescence spectrum in the wavelength range of 415-475 nm, indicative for uranium(V), or in the range of 480-560 nm, which is typical for uranium(VI). Particles of uranium(V) as well as uranium-(VI) were simultaneously observed in the biofilms. These uranium particles were attributed for uranium(VI) to biologically mediated precipitation and for uranium(V) to redox processes taking place within the biofilm. The detection of uranium(V) in a multispecies biofilm was interpreted as a short-lived intermediate of the uranium(VI) to uranium-(IV) redox reaction. Its presence clearly documents that the uranium(VI) reduction is not a two electron step but that only one electron is involved.