Microbial Processes Driven by the Redox Capacity of Natural Organic Matter Suppress the Emission of Greenhouse Gases in Wetland Sediments

Abstract

Microorganisms are the smallest living entities driving biogeochemical cycles on Earth. Within this context, anaerobic oxidation of methane (AOM) is a key microbial process, which suppresses the emission of huge amounts the potent greenhouse gas (GHG), methane (CH4), to the Earth’s atmosphere. The purpose of this doctoral thesis was to evaluate the role that the redox-active fraction of natural organic matter (NOM), also known as humic substances (HS), could play by eliciting microbial and chemical reactions with the potential to suppress the emission of GHGs. The first part of this research describes a novel mechanism for AOM in organotrophic environments, which is driven by the microbial reduction of the redox-active moieties in NOM (quinone functional groups). It was estimated that this mechanism could diminish the global emission of CH4 from wetlands by values ranging 1300 Tg of CH4 year-1. The second part of this thesis explores the role of HS as an electron shuttle fueling AOM with ferric iron (Fe(III)) as the terminal electron acceptor (TEA). Besides proving the positive effect that HS had on both CH4 oxidation and Fe(III) reduction, it was demonstrated that this humus-mediated process had the capacity to prompt carbon burial by eliciting the formation and precipitation of inert iron carbonate minerals (siderite). Following an analogous mechanism of extracellular electron transport (EET), the third part of this work, aimed to demonstrate how HS could link microbial metabolisms of GHG consumption by enabling quinone-mediated electron transfer (QUIET). As a result of this research, a novel microbial process was demonstrated: AOM linked to N2O reduction, mediated by HS (connecting the C and N cycles), in which members of the Methanocellaceae and Moraxellaceae families were potentially involved. Finally, the potential of HS triggering a cryptic sulfur (S) cycle and with implication in AOM was explored. It was proven that HS can oxidize sulfide coming from microbial sulfate-reducing processes conducting to partially oxidized inorganic S compounds formation, as well as S incorporation to the organic structure of HS. These reactions, however, negatively affected CH4 consumption rates, presumably by chemically reducing the redox-active functional moieties in HS. Altogether, the pieces of evidence collected in this thesis point out to the importance that the most abundant organic electron acceptors present in ecosystems, humic substances, may have in the suppression of the emission of GHGs from organotrophic environments by interconnecting the C, Fe, N and S cycles.