K-045. Thermodynamic Electron Equivalent Models of Geobacter Species

B. Sayyar1, K. Nevin2, D. Lovley2, R. Mahadevan1;
1Univ. of Toronto, Toronto, ON, CANADA, 2Univ. of Massachusetts, Amherst, MA.

Geobacter species have been shown to be important agents in the bioremediation of subsurface environments contaminated with organic and/or metal contaminants. In order to optimize the design of bioremediation strategies, genome-based in silico models of Geobacter metabolism are being coupled with hydrological and geochemical models of the subsurface. An important component of this modeling is accurately predicting the growth yield of Geobacter species on various substrates. Thermodynamic electron equivalent modelling (TEEM) is a method that was previously developed to permit prediction of the maximum bacterial yield with a variety of electron acceptors. In this approach, the available Gibb’s free energy associated with the chemical reactions driving microbial growth along with a growth efficiency parameter is used to determine the biomass yield. However, application of the TEEM method failed to accurately predict growth yields of Geobacter species with Fe(III) oxides, their most common electron acceptor in the subsurface. Further investigation revealed that this is due to fundamental differences between extracellular and intracellular electron transfer mechanisms. In most organisms, electron transfer to soluble electron acceptors that can enter the cell consumes protons generated from organic matter oxidation in the reduction of the electron acceptor. However, when electrons are transferred outside the cell, respiration does not provide an equivalent proton sink. This results in an energetic cost that is reflected in lower growth yields for extracellular electron transfer. Therefore, the TEEM modeling approach was modified to incorporate the effect of global proton balances. Experimental data from growth in batch and continuous cultures with a variety of electron donors were used to modify the model to predict the maximum bacterial yield of Geobacter species. This modification of the TEEM approach will greatly improve attempts to model the growth of Geobacter species in the subsurface during in situ bioremediation and should aid in modeling the growth of other microorganisms growing via extracellular electron transfer.