A microsensor study on the O2 consumption in a U(VI) contaminated multispecies biofilm

Multispecies biofilms were cultured in annular rotating biofilm reactors and subsequently exposed to U(VI) in ecological relevant concentration (5×10-5 M and 5×10-6 M). Such concentrations are comparable with uranium concentrations typically found in seepage waters of uranium tailings, e.g. in Saxony/Germany. The resulting response of the microbial biofilm community to the added U(VI) was then studied by electrochemical oxygen microsensors with tip diameters of 10 µm and by staining methods using the fluorogenic redox indicator 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and the DNA-binding fluochrome 4,6-diamidino-2-phenylindole (DAPI) in combination with confocal laser scanning microscopy (CLSM). The visualized ratio of CTC-formazan to DAPI intensity was used as an indication of the specific respiratory activity within the biofilms. In addition, 16S rDNA analysis and fluorescence in situ hybridization (FISH) investigations were carried out to study the effect of added uranium on the bacterial diversity.

The microsensor measurements revealed that the oxygen concentration in the multispecies biofilms exposed to uranium decreased faster with increasing biofilm depths in comparison to the uranium free biofilms. Analyses of the amplified 16S rDNA gene fragments showed that the addition of uranium induced no changes to the bacterial diversity in the multispecies biofilms. However, the analyses clearly indicated that a stable multispecies biofilms had developed. The metabolic activity, determined by CTC measurements increased in the upper layers of the biofilms by the addition of the uranium shown by faster oxygen consumptions. This indicates that the bacteria in the biofilms battle the toxic effects of aqueous uranium with an increased metabolic activity proven by the increased CTC activity and in particular by faster oxygen consumption in the biofilm profiles.