Browsing by Author "Reardon, Catherine L."

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Composition and diversity of microbial communities recovered from surrogate minerals incubated in an acidic uranium-contaminated aquifer
(2004-10) Reardon, Catherine L.; Cummings, David E.; Petzke, Lynn M.; Kinsall, Barry L.; Watson, David B.; Peyton, Brent M.; Geesey, Gill G.
Our understanding of subsurface microbiology is hindered by the inaccessibility of this environment, particularly when the hydrogeologic medium is contaminated with toxic substances. In this study, surrogate geological media contained in a porous receptacle were incubated in a well within the saturated zone of a pristine region of an aquifer to capture populations from the extant communities. After an 8-week incubation, the media were recovered, and the microbial community that developed on each medium was compared to the community recovered from groundwater and native sediments from the same region of the aquifer, using 16S DNAcoding for rRNA (rDNA)-based terminal restriction fragment length polymorphism (T-RFLP). The groundwater and sediment communities were highly distinct from one another, and the communities that developed on the various media were more similar to groundwater communities than to sediment communities. 16S rDNA clone libraries of communities that developed on particles of a specular hematite medium incubated in the same well as the media used for T-RFLP analysis were compared with those obtained from an acidic, uraniumcontaminated region of the same aquifer. The hematite-associated community formed in the pristine area was highly diverse at the species level, with 25 distinct phylotypes identified, the majority of which (73%) were affiliated with the β-Proteobacteria. Similarly, the hematite-associated community formed in the contaminated area was populated in large part by β-Proteobacteria (62%); however, only 13 distinct phylotypes were apparent. The three numerically dominant clones from the hematite-associated community from the contaminated site were affiliated with metal- and radionuclide-tolerant or acidophilic taxa, consistent with the environmental conditions. Only two populations were common to both sites.
Hydrogenase activity of mineral-associated and suspended populations of Desulfovibrio desulfuricans Essex 6
(2014-02) Reardon, Catherine L.; Magnuson, Timothy S.; Boyd, Eric S.; Leavitt, W. D.; Reed, D. W.; Geesey, Gill G.
The interactions between sulfate-reducing microorganisms and iron oxides influence a number of important redox-sensitive biogeochemical processes including the formation of iron sulfides. Enzymes, such as hydrogenase which catalyze the reversible oxidation of molecular hydrogen, are known to mediate electron transfer to metals and may contribute to the formation and speciation of ferrous sulfides formed at the cellâ€“mineral interface. In the present study, we compared the whole cell hydrogenase activity of Desulfovibrio desulfuricans strain Essex 6 growing as biofilms on hematite (hematite-associated) or as suspended populations using different metabolic pathways. Hematite-associated cells exhibited significantly greater hydrogenase activity than suspended populations during sulfate respiration but not during pyruvate fermentation. The enhanced activity of the hematite-associated, sulfate-grown cells appears to be dependent on iron availability rather than a general response to surface attachment since the activity of glass-associated cells did not differ from that of suspended populations. Hydrogenase activity of pyruvate-fermenting cells was stimulated by addition of iron as soluble Fe(II)Cl2 and, in the absence of added iron, both sulfate-reducing and pyruvate-fermenting cells displayed similar rates of hydrogenase activity. These data suggest that iron exerts a stronger influence on whole cell hydrogenase activity than either metabolic pathway or mode of growth. The location of hydrogenase to the cell envelope and the enhanced activity at the hematite surface in sulfate-reducing cells may influence the redox conditions that control the species of iron sulfides on the mineral surface.
Resolving biogeochemical phenomena at high spatial resolution through electron microscopy
(2008-06) Geesey, Gill G.; Borch, Thomas; Reardon, Catherine L.
Our understanding of microbe-metal interactions has advanced dramatically since the mid-1970s when little was known about the reactivity of bacterial cell wall components toward metal ions in the extracellular milieu. Although certain metals such as and Pb+ were known to react with components of bacterial cell walls and used to visualize their structure by electron microscopy (Garland et al., 1975), little physicochemical data were available on the specificity and sites of interactions (Humphrey & Vincent, 1966; Heptinstall et al., 1970; Irvin et al., 1975; Lambert et al., 1975; Raymond & MacLeod, 1975). Furthermore, there were no model systems to explorethe mechanisms of these interactions. This began to change when Beveridge and Murray used isolated cell walls of Bacillus subtilis to quantify metal ion binding to wall components. Beveridge demonstrated that cell walls concentrated cations such as Mg++, Na+, K+, Cu++ and Fe+++, but not Ba++, Li+ or Al+++ (Beveridge & Murray, 1976). Since these initial studies, Beveridge and his students and collaborators have contributed greatly to our understanding of the complex interactions between microbial cell surface polymers and metals in the environment. As fellow scientists working in this research area, we have developed a deep admiration of Beveridge’s scientific insight, technical skills and collegial demeanor. Not surprisingly, Beveridge’s research has had a significant impact on our research, as well as on the research of our collaborators and colleagues, and will likely influence the work of future generations of scientists working in the field of geobiology. Some examples are cited below.
Role of outer membrane c-type cytochromes MtrC and OmcA in Shewanella oneidensis MR-1 cell production, accumulation, and detachment during respiration on hematite
(2012-07) Mitchell, Isaac; Peterson, L.; Reardon, Catherine L.; Reed, S. B.; Culley, D. E.; Romine, Margaret F.; Geesey, Gill G.
The iron-reducing bacterium Shewanella oneidensis MR-1 has the capacity to contribute to iron cycling over the long term by respiring on crystalline iron oxides such as hematite when poorly crystalline phases are depleted. The ability of outer membrane cytochromes OmcA and MtrC of MR-1 to bind to and transfer electrons to hematite has led to the suggestion that they function as terminal reductases when this mineral is used as a respiratory substrate. Differences in their redox behavior and hematite-binding properties, however, indicate that they play different roles in the electron transfer reaction. Here, we investigated how these differences in cytochrome behavior with respect to hematite affected biofilm development when the mineral served as terminal electron acceptor (TEA). Upon attachment to hematite, cells of the wild-type (WT) strain as well as those of a Î”omcA mutant but not those of a Î”mtrC mutant replicated and accumulated on the mineral surface. The results indicate that MtrC but not OmcA is required for growth when this mineral serves as TEA. While an OmcA deficiency did not impede cell replication and accumulation on hematite prior to achievement of a maximum surface cell density comparable to that established by WT cells, OmcA was required for efficient electron transfer and cell attachment to hematite once maximum surface cell density was achieved. OmcA may therefore play a role in overcoming barriers to electron transfer and cell attachment to hematite imposed by reductive dissolution of themineral surface from cell respiration associated with achievement of high surface cell densities.