E=OC^2 – Energy resolves Organic Carbon Complexity in anoxic waters

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Inland waters are an important component of the global carbon cycle because they transport and process large amounts of organic matter (OM) which they receive from the terrestrial biosphere. Dissolved OM affects the color and health of these waters, and supplies the aquatic food web with external energy. The fate of OM largely depends on its reactivity in these waters: Because oxidation and mineralization to CO2 is the ultimate OM sink process, the conventional paradigm considers oxygen availability as a critical control on OM reactivity and turnover. However, there is very active production, processing and transformation of OM also in anoxic (oxygen-deprived) waters. A range of pathways on which the chemical structure and composition of OM is transformed are exclusive to anoxia, including (1.) the preferential degradation of energy-rich OM fractions, (2.) disrupted recycling of microbial OM, and (3.) reaction with hydrogen (H2) from anaerobes. Currently, it is unclear under which conditions these pathways occur, if multiple pathways interact and how they affect the ultimate fate of carbon in inland waters. This lack of understanding persists, despite anoxic conditions are widespread in the freshwater corridor, and global warming and eutrophication have been predicted to further increase anoxia duration and range. The central aim of this project is thus to classify the ecological and biogeochemical drivers that transform DOM in anoxic environments. To this end, we develop a novel framework based on mechanistically meaningful, compound-level free energy (ΔG) characteristics of OM using large compound databases. We use this energy-based framework first to conceptualize distinct anoxic transformation pathways in isolation, and later to trace multiple pathways in complex environmental samples. This builds the foundation to find the locale- and substrate-specific factors (e.g., availability of energy, timescales of anoxia) that vary with OM compositional changes observed in anoxia. Finally, large environmental datasets will be analyzed to assess the landscape-level imprint that anoxia leaves in OM during its passage of the soil-to-ocean continuum. The outcome of this project will pave the way for a novel energy-centric perspective of OM cycling in aquatic systems, allowing for a more accurate analysis of its biotic and abiotic processing and to link OM transformation pathways across ecosystems and research communities.