Redox-Dependent Molecular Signatures of Carbon Transformation and Persistence in incubated Dissolved Organic Matter

“Redox-Dependent Molecular Signatures of Carbon Transformation and Persistence in incubated Dissolved Organic Matter”

Rania Mobarak¹, Carsten Simon¹,Klaus Holger Knorr², Maximilian P. Lau³, Oliver J. Lechtenfeld¹

• BioGeoOmics, Department of Environmental Analytical Chemistry, Helmholtz Centre for Environmental Research-UFZ, 04318 Leipzig, Germany.
• Institute of Landscape Ecology- ILÖk, University of Münster, 48149 Münster, Germany
• Interdisciplinary Environmental Research Centre, Technische Universität Bergakademie Freiberg ,09599 Freiberg, Germany

Microbial transformation of dissolved organic matter (DOM) under anoxic conditions exerts a fundamental control on carbon persistence, redox coupling, and energy transfer in aquatic and peatland ecosystems. Although previous studies have demonstrated that anoxic microbial processing favors the accumulation of chemically reduced organic matter, the mechanistic links between DOM transformation pathways, electron acceptor availability, and microbial activity remain poorly constrained.

Here, we investigated DOM dynamics under controlled anoxic conditions using incubation experiments with natural DOM sourced from three ecosystems characterized by contrasting redox histories: (i) the hypolimnion of a holomictic lake, (ii) the monimolimnion of meromictic lake, and (iii) the permanently anoxic peat pore waters. Microbial inocula originating from each respective system were added to the DOM incubations, alongside parallel abiotic controls to disentangle biologically mediated from abiotic transformation processes. Anoxic conditions were established via N₂ purging and maintained throughout the 90-day incubation period, with all filtration and subsampling conducted inside an anoxic glove box to prevent oxygen intrusion.

Temporal changes in dissolved organic carbon (DOC) concentrations were monitored in conjunction with key inorganic electron acceptors, including nitrate, dissolved iron, and sulfate, to evaluate coupled carbon turnover and redox dynamics. Across all incubations, we observed pronounced initial changes in DOC and electron acceptor concentrations, followed by more gradual transformation phases, indicative of sustained microbial metabolism under anoxic conditions. Distinct temporal trajectories among ecosystems highlight the strong influence of prior redox exposure, electron acceptor availability, and intrinsic DOM quality on anoxic organic matter transformation pathways.

Ongoing molecular-level characterization using liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR-MS) will resolve changes in DOM molecular composition, oxidation state, and energetic characteristics in relation to microbial metabolism and terminal electron-accepting processes. Together, this integrative approach provides mechanistic insight into how redox conditions regulate DOM reactivity and carbon persistence in anoxic environments, with implications for predicting carbon storage and redox-mediated feedbacks under shifting oxygen and hydrological regimes.