Advancing the understanding of the aquatic methane cycle through cutting-edge isotopic approaches and methane oxidation analysis

• Applicant: Dr. Moritz Schroll
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561132391

Methane (CH4) is a key climate-relevant gas, significantly contributing to global warming and playing an important role in atmospheric chemistry. Recent studies have identified aquatic systems as major sources of CH4, potentially responsible for up to 50% of global emissions. However, substantial uncertainty remains about the magnitude of these emissions, particularly concerning their spatial and temporal drivers. This issue is especially relevant in Arctic environments, where currently only a limited number of studies exist. Therefore, accurately characterizing and classifying CH4 sources in aquatic environments is essential to advance our understanding of their role in the global CH4 budget. Current CH4 source classification methods use stable isotope ratios, such as stable carbon (delta13C) and hydrogen (delta2H) isotope values of CH4 (visualized as 13C vs. 2H plots) and geochemical Bernard ratios, representing the molar ratios of CH4 relative to ethane and propane vs. delta13C-CH4 values (Bernard plots). Both diagrams classify different CH4 sources by their distinct ranges of delta13C- and delta2H-CH4 values as well as Bernard ratios, due to differences in their underlying production mechanisms. However, a major limitation arises from concurrent CH4 oxidation (MOx) by methanotrophic bacteria, which is widespread in aquatic environments. This process alters CH4 concentrations and stable isotope values, as well as ethane and propane levels, though oxidation of these gases is currently overlooked with respect to CH4 source classification. Thus, MOx complicates the source classification of CH4 which may further lead to data misinterpretation. To overcome these limitations, novel tools and improved methods for source classification are needed. One promising advancement is the delta(2,13) parameter, which is based on delta13C- and delta2H values of CH4 but corrects for isotope fractionation alteration caused by MOx. Thereby, the delta(2,13) parameter has the potential to significantly improve source classification accuracy and to enhance the assessment of contributions from different CH4 sources in aquatic systems. However, its currently limited application and the lack of knowledge of its influencing factors challenge its reliability and demand a systematic investigation. The overall aim of AMIOX is to enhance our understanding of aquatic CH4 cycling by improving the classification of CH4 sources and sinks in temperate and Arctic aquatic environments. This will be achieved by introducing and combining the novel delta(2,13) parameter with revised Bernard and 13C vs. 2H-CH4 plots. To reach these goals, I will investigate the influence of MOx from three widespread methanotroph species on delta(2,13) values and Bernard ratios in laboratory studies under different environmental conditions. Finally, I will apply these findings in the field to better characterize and advance the understanding of the CH4 cycle in temperate lakes in Germany and Arctic lakes in Greenland.

• DFG Programme: Research Grants
• International Connection: Denmark
• Co-Investigators: Dr. Steffen Greiner; Professor Dr. Hans-Peter Grossart; Professor Dr. Frank Keppler
• Cooperation Partner: Professor Dr. Jesper Riis Christiansen