Simultaneous utilization of acetate and hydrogen by Geobacter sulfurreducens and implications for use of hydrogen as an indicator of redox conditions.

Dissolved hydrogen concentrations, in conjunction with other geochemical indicators, are becoming an accepted means to determine terminal electron acceptor processes (TEAPs) in groundwater aquifers. Aqueous hydrogen concentrations have been found to fall within specific ranges under methanogenic, sulfate-reducing, iron-reducing, and denitrification conditions. Although hydrogen is gaining in acceptance for determining subsurface TEAPs, there is a dearth of data with regards to the kinetic coefficients for hydrogen utilization in the presence or absence of an additional electron donor under different TEAPs. This study expands the kinetic data for hydrogen utilization through a series of batch experiments, which were conducted to study the utilization of acetate and hydrogen by Geobacter sulfurreducens under iron-reducing conditions. The results of these experiments indicate that the kinetic coefficients (cell yield and first-order degradation rate) describing the rate of hydrogen utilization by G. sulfurreducens under iron-reducing conditions correlate energetically with the coefficients found in previous experiments under methanogenic and sulfate-reducing conditions. In addition, with acetate and hydrogen as simultaneous electron donors, there is slight inhibition between the two electron donors for G. sulfurreducens, and this can be modeled through competitive inhibition terms in the classic Monod formulation. Finally, a key result of this study is that the TEAP-dependent hydrogen concentration in aquifers is not related solely to the microbial kinetics of the hydrogen-consuming organisms as previously suggested but is affected by the multi-substrate kinetics of hydrogen being consumed simultaneously with other electron donors as well as the availability of the electron acceptor.