A study of methane-related processes in freshwater ecosystems: methanogenesis, anaerobic methane oxidation, and interactions with other terminal metabolisms

This dissertation seeks to understand the seasonal controls of methane cycling in freshwater sediments. Using a combination of field measurements, radiotracer incubations, porewater characterization, lipid biomarker analysis, and stable carbon isotopes, pronounced seasonal variations in microbial carbon turnover were documented in a freshwater sediment and in two peat wetlands. Constraints of the methane budget in shallow (< 40 cm) sediments revealed a seasonal imbalance between methane fluxes and methane production that may be relieved through tidal pumping of methane-laden porewaters derived from adjacent high marsh through the creekbank. Rate measurements of sulfate reduction and the anaerobic oxidation of methane (AOM), two processes not typically considered relevant in low salinity habitats, revealed their importance in freshwater settings. Seasonal variations in AOM may be driven by fluctuations in hydrogen and acetate dynamics generated by variations in other microbial metabolisms (e.g. sulfate reduction and methanogenesis). Lipid biomarker analysis revealed the presence of sulfate-reducing bacteria and archaea associated with methane cycling. However, seasonal variations in microbial metabolisms were not associated with changes in the lipid distribution. Stable carbon isotope analyses revealed the imprint of AOM on the signatures of methane and dissolved inorganic carbon. The influence of methanotrophy, however, was not as pronounced in the microbial lipid signatures. A potential AOM isotopic signal may have been diluted by methanogenesis and other autotrophic and heterotrophic processes, which may mask a clear methanotrophic signature. While sulfate reduction activity is sufficient to support all observed AOM activity, no conclusive evidence was found to link these processes. Long-term enrichments of coastal sediments with various electron acceptors demonstrated a positive influence of sulfate and ferric citrate additions on AOM. Other electron acceptors such as nitrate and manganese may also support AOM in these coastal settings. These studies advance the understanding of the seasonal controls on methane emissions, methane production, and methane consumption via AOM in freshwater ecosystems. Future efforts are aimed at closer examinations of these mediating factors, especially temperature changes and substrate availability.