Another study at this site examined a pilot-scale field bioremediation system which used ethanol as an electron donor to stimulate microbial communities and immobilize U(VI) by reduction to U(IV) (Luo et al., 2006; Wu et al., 2006b,c) . GeoChip 2.0 was used to examine the microbial communities during different phases of operation. A period of active U(VI) reduction occurred after initial start-up (days 137-304). During this period U(VI) reduction was relatively rapid and genes associated with denitrification, sulfate reduction, and Fe(III) reduction increased in abundance, suggesting that these populations were involved in U(VI) reduction . This active reduction was followed by a maintenance period during which the low level of U(VI) was maintained, and the denitrifying, sulfateand Fe(III)-reducing communities remained in higher abundance. Next, the stability of the bioreduced U(IV) was examined by allowing the system to enter periods of starvation (ethanol injections were halted) and reoxidation (dissolved O 2 entered the system). The functional communities showed distinct clustering patterns based on whether the system received ethanol or not, indicating a shift in community structure with the return of ethanol injections . While total gene numbers increased once ethanol injection was restarted, the relative abundance of each gene group changed little during and after starvation, indicating a functionally diverse community which could be stimulated after adverse conditions. Chemical oxygen demand (COD, i.e., ethanol) was the most important driver in determining community structure, but temperature, sulfate, and U(VI) were also important.