Basic Research Needs for Geosciences - Energetics Meetings and ...

PRIORITY RESEARCH DIRECTION:BIOGEOCHEMISTRY IN EXTREME SUBSURFACE ENVIRONMENTSRESEARCH APPROACHESThe fate and transport of radionuclides and metals (Charlet and Polya 2006; Kretschmar andSchaefer 2005; Lloyd and Oremland 2006; Roden and Scheibe 2005; Steefel et al. 2005), thecorrosion of nuclear waste forms and packages (Bruno and Ewing 2006; Burns and Klingensmith2006; Ewing 2006; Grambow 2006), the performance of engineered barrier systems, the storageof CO 2 in deep aquifers (Bachu et al. 1994; Bruant et al. 2002; Elliott et al. 2001; Gaus et al.2005; Gunter et al. 1993; Johnson et al. 2004; Rochelle et al. 2004), and the global elemental andnutrient cycles (Berner 1995; Berner et al. 1983; Van Cappellen and Gaillard 1996) are allinfluenced by biogeochemical processes. In many cases, the rates of these processes are directlymediated by microbial activity. In other cases, new reactions may occur or the extent of reactionsmay be altered relative to an analogous abiotic system through the participation of cells or theirbyproducts in the reactions. Microbially mediated environmental processes are rarely due toactivity of a single group of organisms, but instead are the result of a diverse group of organismsthat may reside as biofilms in the subsurface. Competitive and/or symbiotic communities may bepresent. Microbial metabolism and growth can directly or indirectly impact importantgeochemical processes, including electron transfer, precipitation and dissolution, and sorption.For example, bacteria can:• Drive the rates of thermodynamically favorable redox reactions, such as those of iron, sulfur,uranium, and plutonium, which in the absence of biological mediation would be kineticallyinhibited (Chappelle and Lovley 1990, 1992; Christensen et al. 2000; Jakobsen and Postma1999; Lovley and Chappelle 1995; Park et al. 2006b)• Produce nanosized precipitates with unique geochemical properties (Banfield and Navrotsky2001; Banfield and Zhang 2001; Suzuki et al. 2002)• Indirectly influence the rates and extent of sorption, dissolution, and precipitation processesvia biofilm formation, the exudation of extracellular materials, and the geochemistry of thepore solutionDespite the universally recognized role of microbes in regulating geochemical cycles, there isstill relatively little understanding of the actual mechanisms by which microbial communitiesinteract with their geochemical environment. Specific scientific questions include the following:• What environmental factors affect the metabolic pathways of individual bacterial species andtheir rates?• What are the controls on the rates of electron transfer across the cell wall in the case of redoxreactions?• How are the microbial community dynamics influenced by the flux of nutrients at the porescale, and how do these affect the partitioning of energy between growth and metabolism andthe competition or symbiosis between individual communities?• How do microbially mediated mineral precipitation or oxidation-reduction rates change asnutrient fluxes change, going for example from “banquet” to “starvation” levels?• How do biofilm properties, growth, and function affect extracellular geochemical reactionslike adsorption, precipitation and dissolution?• How does the pore environment affect the interactions between mobile aqueous and solidphases, immobile reactive phases, and the microbial communities that reside there?148 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems