Unique Ecophysiology among U(VI)-Reducing Bacteria as Revealed by Evaluation of Oxygen Metabolism in Anaeromyxobacter dehalogenans Strain 2CP-C

摘要

Anaeromyxobacter spp. respire soluble hexavalent uranium, U(VI), leading to the formation of insoluble U(IV), and are present at the uranium-contaminated Oak Ridge Integrated Field Research Challenge (IFC) site. Pilot-scale in situ bioreduction of U(VI) has been accomplished in area 3 of the Oak Ridge IFC site following biostimulation, but the susceptibility of the reduced material to oxidants (i.e., oxygen) compromises long-term U immobilization. Following oxygen intrusion, attached Anaeromyxobacter dehalogenans cells increased approximately 5-fold from 2.2 × 10~(7) ± 8.6 × 10~(6) to 1.0 × 10~(8) ± 2.2 × 10~(7) cells per g of sediment collected from well FW101-2. In the same samples, the numbers of cells of Geobacter lovleyi , a population native to area 3 and also capable of U(VI) reduction, decreased or did not change. A. dehalogenans cells captured via groundwater sampling (i.e., not attached to sediment) were present in much lower numbers (<1.3 × 10~(4) ± 1.1 × 10~(4) cells per liter) than sediment-associated cells, suggesting that A. dehalogenans cells occur predominantly in association with soil particles. Laboratory studies confirmed aerobic growth of A. dehalogenans strain 2CP-C at initial oxygen partial pressures (pO_(2)) at and below 0.18 atm. A negative linear correlation [μ = (?0.09 × pO_(2)) + 0.051; R ~(2) = 0.923] was observed between the instantaneous specific growth rate μ and pO_(2), indicating that this organism should be classified as a microaerophile. Quantification of cells during aerobic growth revealed that the fraction of electrons released in electron donor oxidation and used for biomass production ( f_(s) ) decreased from 0.52 at a pO_(2) of 0.02 atm to 0.19 at a pO_(2) of 0.18 atm. Hence, the apparent fraction of electrons utilized for energy generation (i.e., oxygen reduction) ( f_(e) ) increased from 0.48 to 0.81 with increasing pO_(2), suggesting that oxygen is consumed in a nonrespiratory process at a high pO_(2). The ability to tolerate high oxygen concentrations, perform microaerophilic oxygen respiration, and preferentially associate with soil particles represents an ecophysiology that distinguishes A. dehalogenans from other known U(VI)-reducing bacteria in area 3, and these features may play roles for stabilizing immobilized radionuclides in situ .