Evaluation of kinetic controls on microbially-mediated reactions in an electron donor-limited environment

Complexities and heterogeneities are inherent to aqueous natural systems; however, research must overcome these complexities to maintain ecosystem and human health. An important component of research involves the evaluation of kinetic controls on biogeochemical reactions. This is because the activities of native microorganisms play a vital role in hastening degradation processes, which provides insight into predictions of the degradation potential of a system and thus has implications for natural attenuation and bioremediation. Linking geochemical and microbial indicators, this study assessed the importance of individual kinetic controls on rates of microbially-mediated oxidation-reduction reactions, specifically analyzing the role of: 1) the availability of simple carbon-based electron donors: 2) the availability of electron acceptors; and, 3) the abundances of microbial species mediating reduction processes. A series of microcosm experiments were conducted over 75 days, with native sediments and water from an electron donor-limited environment. Results found that addition of electron donor (lactate and acetate) stimulated reduction approximately 35 days earlier than in non-amended microcosms. Sediments were rich in both iron and sulfate minerals but iron reduction was favored over sulfate reduction. Maximum rates of iron reduction in the donor amended series were 3.45 mg/L day--1 (lactate-amended)and 1.02 mg/L day--1 (acetate-amended), respectively. The maximum iron reduction rate for the control was 0.72 mg/L day--1. The presence of lactate stimulated both sulfate and iron reduction, whereas acetate addition simulated only the latter, indicating that acetate promoted competition between microbial species. After depletion of donor-additions, reduction continued undisrupted, indicating biostimulation of the native microbial community.