Ecophysiology and diversity of Anaeromyxobacter spp. and implications for uranium bioremediation

Uranium has been released into the environment due to improper practices associated with mining and refinement for energy and weapons production. Soluble U(VI) species such as uranyl carbonate can be reduced to form the insoluble U(IV) mineral uraninite (UO2) via microbial respiratory processes. Formation of UO2 diminishes uranium mobility and prevents uranium-laden groundwater from being discharged into surface water; however, oxygen and other oxidants re-solubilize UO2. Many organisms have been shown to reduce uranium, but variations in microbial physiology (e.g., growth rate, oxygen sensitivity, the range of substrates that are utilized, and electron donor availability) change the dynamics of microbial uranium reduction in situ and affect uraninite stability. Determining the physiology and prevalence of relevant uranium-reducing organisms is critical to adequately monitor and optimize the microbial uranium reduction process for bioremediation applications. Anaeromyxobacter dehalogenans is a metal-reducing delta-Proteobacterium in the myxobacteria family that displays remarkable respiratory versatility and efficiently reduces U(VI). The approach of this research was to enhance characterization of A. dehalogenans by identifying unique genetic traits, describing variability within the species, and examining the environmental distribution of A. dehalogenans strains. Genome analysis revealed that A. dehalogenans shares many traits with the myxobacteria including type IV pilus-based motility and an aerobic-like electron transport chain, including NADH dehydrogenase and cytochrome oxidase subunits with sequence similarity to the aerobic myxobacteria. In addition, the genome revealed genes that share sequence similarity with strict anaerobes and other metal-reducing organisms, consistent with observed respiratory versatility in A. dehalogenans. Physiological examination of microaerophilism in A. dehalogenans strain 2CP-C reveal growth at sub-atmospheric oxygen partial pressure. Experimental evidence is also presented that verifies surface motility in A. dehalogenans strain 2CP-C. Isolation efforts yielded several A. dehalogenans strains native to uranium-contaminated environments. Physiological characterization of the novel isolates demonstrated that strain-level variation in the 16S rRNA gene coincides with metabolic changes that can be linked to the loss of specific gene homologs. Anaeromyxobacter spp. were present at the Oak Ridge Integrated Fieldscale Subsurface Research Challenge (IFC) site and multiplex qPCR tools designed using a minor-groove binding probe gave insights into strain and species differences in the community. Anaeromyxobacter spp. at the Oak Ridge IFC site appear to be primarily attached to the solid substrate in situ, consistent with surface motility mechanisms. Further, oxygen intrusion at the Oak Ridge IFC site corresponded to an increase in the sediment-associated Anaeromyxobacter community. Finally, 16S rRNA gene sequences were identified which suggest a novel Anaeromyxobacter species that is responsible for uranium reduction at the Oak Ridge IFC site. This research contributes new knowledge of the ecophysiology of a widely distributed, metal-reducing bacterial group capable of uranium immobilization. The characterization of Anaeromyxobacter spp. helps to elucidate the dynamics of biological cycling of metals at oxic-anoxic interfaces, like those at the Oak Ridge IFC, and contributes to the broader study of microbial ecology in groundwater and sediment environments.