Detection of U(V) in Pseudomonas stutzeri biofilms by confocal laser scanning microscopy

A biofilm of Pseudomonas stutzeri was cultured in a annular rotating reactor /1/ at near-neutral pH conditions on glass slides. After reaching a mature confluent biofilm uranium(VI) was added to the nutrient solution to obtain a final uranium(VI) concentration of 5 ´ 10-6 M. The resulting biofilm was studied by Confocal Laser Scanning Microscopy (CLSM) and a biofilm thickness of 43 µm was recorded. Particles of approximately 5 µm were detected in the biofilm. These particles were excited by an diode laser with an excitation wavelength of 405 nm and the resulting fluorescence signal was recorded. Two kinds of flourescence signals were detected. One with a characteristic fluorecence signal in the range of 490 to 560 nm which is indicative for U(VI) /2/ and a second one with a fluorescence signal from 420 to 470 nm. Since U(IV) requires an excitation wavelength of 245 nm and shows a completely different fluorescence spectrum the formation of uranium(IV) particles was ruled out. However, based on UV-vis measurements of U(V) containing standard solutions these signals were interpreted as U(V) species, indicating that redox processes must have occured within the biofilm.