SIMULTANEOUS REDUCTION OF U(VI) AND Fe(III):

8 Dissimilatory metal reducing bacteria (DMRB) are capable of reducing contaminants such as 9 Cr(VI), Se(VI) and U(VI) during respiration, a process that has a pronounced impact on the 10 mobility of these contaminants in surface and subsurface environments. DMRB can also 11 reduce Fe(III), most commonly associated with solid phase (hydr)oxide minerals such as 12 ferrihydrite, goethite, or hematite. Although bacterial reduction of each of these metals on an 13 individual basis is well documented, utilization of multiple electron acceptors has received 14 little attention. To better understand these processes, the reduction of U(VI) in the presence 15 of Fe(III) was investigated Shewanella alga strain BrY and Shewanella putrefaciens strain 16 CN32 were used as model metal reducing organisms. Both S. putrefaciens and S. alga 17 reduced U(VI) and Fe(III) simultaneously throughout the course of a 400 h reaction, despite 18 energy yield considerations that predict uranium to be the more energetically favorable 19 electron acceptor. Both S. putrefaciens and S. alga rapidly form biofilms, which our data 20 suggest may be limiting substrate availability spatially within the biofilm. Respiration using 21 Fe(III) is diffusion limited while respiration on uranyl appears to be defined by the bacterial 22 enzymatic reduction rate. These findings reveal that biofilm formation and the associated 23 chemical gradients can limit substrate availability and impact microbially driven 24 bioremediation of metal contaminated surface environments. 25