Skip to main content

Quantifying Aerobic Methane Oxidation in the Ocean: Calibration and palaeo application of a novel proxy

Final Report Summary - AMOPROX (Quantifying Aerobic Methane Oxidation in the Ocean: Calibration and palaeo application of a novel proxy)

Methane is a key greenhouse gas produced via a range of natural and anthropogenic processes. Its release to the atmosphere is modulated by two natural microbial pathways. Aerobic oxidation by bacteria is an important process in terrestrial systems whilst research on the marine methane cycle has focused on anaerobic processes. At the start of this project, recent data from marine sediment cores based on the detection of biomarkers (biological molecules that can be unambiguously linked to source organisms and/or environments) suggested that aerobic methane oxidation (AMO) may play a more significant role in reducing the amount of methane emitted from marine sediments prior to reaching the atmosphere than previously thought. As the role of AMO in marine settings and methane cycling is still relatively poorly constrained the development of new biomarker tools was proposed to provide a more complete understanding of present and past methane fluxes linked to AMO to allow reconstruction of the history of methane cycling from the sedimentary record.

This project targeted complex lipids, known as aminobacteriohopanepolyols (BHPs), biosynthesised by aerobic methanotrophs. These biomarkers were found in marine sediments deposited at the mouths of tropical rivers including the Congo and Amazon. The sediment record from the Congo deep-sea fan dates back to over 2.5 million years and shows a pattern of fluctuating high concentrations of AMO biomarkers deposited under warm and humid climate conditions (interglacials) and much lower concentrations during colder and drier (glacial) conditions. Unexpectedly our investigations showed that rather than being released from marine sediments the source of the methane, and the biomarkers produced by organisms consuming this methane, is large continental wetland areas which are important features of tropical river catchments. Organic matter produced and stored in the wetlands, which expand during humid climate conditions, is then transported to the deep-sea fan via the river. This new data enhances our understanding of the role of AMO in terrestrial tropical systems under past climate regimes and will be useful for climate modelers who currently have no direct evidence, to allow then to calibrate models of both past and potentially future scenarios. However, as we were unable to identify similarly appropriate BHP biomarkers for AMO in marine settings not influenced by terrestrial input, the role of AMO in the past ocean system still remains unconstrained.

To improve understanding of the environmental and biological controls of these biomarkers, experiments were performed on sediments containing natural methanotroph communities. Conditions including methane concentration, temperature, acidity/alkalinity and salinity were varied and showed that multiple factors produced changes in the dominant organisms which in turn control the biomarker signature. The organisms performing AMO under simulated marine conditions showed profound differences to other conditions; the most diagnostic biomarker (known as aminopentol) was not produced. Further investigations of other modern marine settings, excluding those nearby large rivers, could not identify aminopentol. However, we did detect previously unidentified BHPs in both cultured methanotrophic organisms as well as in our marine settings, which show potential as biomarkers for marine AMO.

Organic carbon rich sediments deposited under conditions of low to zero oxygen were investigated for a possible link to the removal of oxygen during the AMO process. Although this link could not be fully confirmed, an exciting observation identifying a novel biomarker for a process known as anaerobic ammonium oxidation was made. This process is responsible for loss of nitrogen from marine environments leading to reduced primary production, affecting the carbon cycle. This biomarker, known as BHT-isomer, allows new insight into this important process under past climate conditions which will be carried forward in a new study at Newcastle University.