Identification of fluorescent U(V) and U(VI) microparticles in a multispecies biofilm by confocal laser scanning microscopy and fluorescence spectroscopy

Fluorescent uranium(V) and uranium(VI) particles were observed for the first time in vivo by a combined laser fluorescence spectroscopy and confocal laser scanning microscopy approach in a 43 µm thick living multispecies biofilm grown on biotite plates. These particles differ between 1-7 µm in diameter and were located at the bottom of the biofilm in the boundary region of high and low bacterial density. Laser fluorescence spectroscopy was used to identify these particles. The particles showed either a characteristic fluorescence spectrum in the wavelength range of 415 to 475 nm, indicative for uranium(V), or in the range of 480 to 560 nm, which is typical for uranium(VI). Particles of uranium(V) as well as uranium(VI) were simultaneously observed in the biofilm. 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 was involved.