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

PRIORITY RESEARCH DIRECTION:BIOGEOCHEMISTRY IN EXTREME SUBSURFACE ENVIRONMENTSFor the geoscience community, there is a need to incorporate and quantify the interactionsbetween microbial populations and their geochemical environment in models of natural andperturbed geologic systems. This is necessary because the interactions may control or stronglycouple with processes that need to be modeled as a part of important technological applications,including the remediation of contaminants and the promotion of precipitation reactions in thecase of CO 2 sequestration.Mechanisms of biogeochemical reactionsSpecific bacterial populations are known to catalyze the reactions of certain redox couples, e.g.,the respiration of acetate with iron(III) as an electron acceptor. Moreover, there are feedbackmechanisms between the geochemical environment and bacteria that lead to gradual changes inmicrobial functions as the environment changes. For example, the bacterial community mayswitch from iron(III) reduction to sulfate reduction as iron(III) reaches limiting (but non-zero)concentrations in a porous medium (Chappelle and Lovley 1992). A variety of competitive andcooperative bacterial community processes generally lead to a shift in predominant species whenan evolution of geochemical conditions occurs. The transition of the bacterial community fromone electron acceptor to another is an important topic with regard to bioremediation or thenatural attenuation of redox-sensitive radionuclides, but also in marine sediments where steepredox gradients control the cycling of Fe, Mn, and carbon (Thullner et al. 2005; Van Cappellenand Gaillard 1996).Current microbial-biogeochemical models for reactive transport lack a suitable mechanistictreatment of these processes at the cellular level. This seriously hampers our ability to predictbehavior under a range of conditions and environments. The few existing models that couple theprocesses are largely empirical and do not account for effects of local geochemical environments(especially nutrient and solute fluxes) on metabolic rates, even though these ultimately governthe overall rates of subsurface reactions. For example, these simulators typically include “yieldfactors” in their kinetic rate laws that specify the partitioning of the nutrients between growthand metabolism as a constant. Or the switchover from the use of one electron acceptor to another(e.g., iron(III) to sulfate) is handled with empirical “inhibition” functions that do not capture thedynamic interaction between the bacteria and their environment and between biotic and abioticchemical cycles (Brun and Engesgaard 2002; Thullner et al. 2005).Cellular level modelsAt the cellular level, a new class of computational models is currently under development. Theseare referred to as in silico models (Covert et al. 2003; Palsson 2000, 2002; Price et al. 2003). Themost widely used approach to date is the constraint-based approach, which eschews the fullcomputation of all of the metabolic pathways taking place at the cellular level in favor of areduced set of key metabolic and transport processes that honor that constraint space of theorganism and its local environment.Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 149