Is Microbially Produced Dissolved Organic Carbon a Missing Link in Natural Attenuation?

Abstract

Our research program characterizes dissolved organic carbon (DOC) generated in association with in-situ hydrocarbon biodegradation. While this DOC has been thought to consist primarily of degradation intermediates, high-resolution mass spectrometry (HRMS) analysis of samples collected from the USGS's Bemidji research site indicates that most of the chemical formulae identified in DOC collected from impacted wells are also found in unimpacted water samples. We recently proposed that the DOC consists primarily of microbial products generated as a result of de novo synthesis by organisms growing on carbon supplied by the oil and is chemically consistent with naturally occurring DOC. In this note, we further evaluated our HRMS results focusing on two electrochemical characteristics: identification of potential redox pairs and the distribution of the average oxidation state of the carbon in HRMS-identified formulae. We hypothesize that microbially produced DOC acts as a pool of carbon compounds with a continuum of oxidation states that serves as a reversible electron buffer with the ability to accept, store, and donate electrons depending on redox conditions. This allows hydrocarbon oxidation that is disconnected in time and space from the reduction of the terminal electron acceptors (TEAs), such as oxygen, Fe³⁺, or sulfate. Given that concentrations of DOC may be high relative to concentrations of dissolved hydrocarbons and TEAs, it suggests that microbially produced DOC may be a missing link in understanding the carbon and electron balance at sites undergoing natural attenuation.