Source-specific carbon dioxide production and acetoclastic methanogenesis upon anaerobic degradation of dissolved organic matter

Lakes receive and process diverse sources of dissolved organic matter (DOM), which contribute differently to the production of carbon dioxide (CO₂) and methane (CH₄) within the system. Here, we isolated DOM from soil, algae, submerged macrophytes, and sewage and tracked their anaerobic fate for 56 days with high-frequency isotope and ultrahigh-resolution mass spectrometry analyses. Biolabile macrophyte-, sewage-, and algal-DOM lost approximately 80% of DOC within 56 days, leading to CO₂ and CH₄ concentrations of up to 1422 µmol L⁻¹ and 1.59 µmol L⁻¹, respectively, for the macrophyte-DOM group. These concentrations were 6 to 9-fold higher than those found for soil-DOM and 30 to 90-fold higher than their initial levels. Isotope fractionation (α_C) decreased from 1.05–1.10 on day 0 to 1.03–1.04 on day 56, and δ¹³C-CH₄ rose to –40‰, suggesting immediate acetoclastic methanogenesis. Random-forest models identified microbial humic-like fluorescence and aliphatic compounds as the primary predictors of CO₂ and CH₄ concentrations. Degradation of one milligram algal DOC yielded 47 µmol CO₂ and 0.04 µmol CH₄, surpassing soil-derived yields by 7- and 4-fold, respectively. Our study delivers biological and mechanistic evidence of DOM processing in aquatic sediments, contributing to a deeper understanding of the origins and fate of their CO₂ and CH₄ concentrations, that may differ substantially among different types of sediments.