Abstract. Bioavailable organic carbon in aquifer recharge waters and sediments can fuel microbial reactions with implications for groundwater quality. A previous incubation experiment showed that sedimentary organic carbon (SOC) mobilized off sandy sediment collected from an arsenic-contaminated and methanogenic aquifer in Bangladesh was bioavailable; it was transformed into methane. We used high-resolution mass spectrometry to molecularly characterize this mobilized SOC, reference its composition against dissolved organic carbon (DOC) in surface recharge water, track compositional changes during incubation, and advance understanding of microbial processing of organic carbon in anaerobic environments. Organic carbon mobilized off aquifer sediment was more diverse, proportionately larger, more aromatic, and more oxidized than DOC in surface recharge. Mobilized SOC was predominately composed of terrestrially derived organic matter and had characteristics signifying that it evaded microbial processing within the aquifer. Approximately 50 % of identified compounds in mobilized SOC and in DOC from surface recharge water contained sulfur. During incubation, after mobilized SOC was converted into methane, new organosulfur compounds with high S-to-C ratios and a high nominal oxidation state of carbon (NOSC) were detected. We reason that these detected compounds formed abiotically following microbial reduction of sulfate to sulfide, which could have occurred during incubation but was not directly measured or that they were microbially synthesized. Most notably, microbes transformed all carbon types during incubation, including those currently considered thermodynamically unviable for microbes to degrade in anaerobic conditions (i.e., those with a low NOSC). In anaerobic environments, energy yields from redox reactions are small and the amount of energy required to remove electrons from highly reduced carbon substrates during oxidation decreases the thermodynamic favorability of degrading compounds with a low NOSC. While all compound types were eventually degraded during incubation, NOSC and compound size controlled the rates of carbon transformation. Large, more thermodynamically favorable compounds (e.g., aromatics with a high NOSC) were targeted first, while small, less thermodynamically favorable compounds (e.g., alkanes and olefinics with a low NOSC) were used last. These results indicate that in anaerobic conditions, microbial communities are capable of degrading and mineralizing all forms of organic matter, converting larger energy-rich compounds into smaller energy-poor compounds. However, in an open system, where fresh carbon is continually supplied, the slower degradation rate of reduced carbon compounds would enable this portion of the organic carbon pool to build up, explaining the apparent persistence of compounds with a low NOSC in anaerobic environments.
Abstract. Bioavailable organic carbon in aquifer-recharge waters and sediments can fuel microbial reactions with 10 implications for groundwater quality. A previous incubation experiment showed that sedimentary organic carbon (SOC) mobilized off sandy sediment collected from an arsenic-contaminated and methanogenic aquifer in Bangladesh was bioavailable; it was fermented into methane. We used high-resolution mass spectrometry to molecularly characterize this mobilized SOC, reference its composition against dissolved organic carbon (DOC) in aquifer recharge water, track compositional changes during incubation, and advance understanding of how composition relates to bioavailability in 15 anaerobic conditions. Mobilized SOC was more diverse and proportionately larger, more aromatic and more oxidized than DOC in surface recharge. In all samples, ~50% of identified compounds contained sulfur. After SOC was fermented into methane, new organosulfur compounds with high S-to-C ratios and high nominal oxidation state of carbon (NOSC) were detected. We conjecture these detected compounds were microbially synthesized to biochemically support methane production or they formed abiotically following microbial sulfate reduction, which could have occurred during incubation 20 but was not directly measured. Microbes transformed all carbon types during incubation, including those considered molecularly recalcitrant (e.g., condensed aromatics) and thermodynamically unfavourable to oxidize (e.g., low NOSC).While all compound types were eventually degraded, NOSC and compound size controlled the rates of carbon transformation. Large energy-rich compounds (e.g., aromatics with high NOSC) were targeted first while small energy-poor compounds (e.g., alkanes and olefinics with low NOSC) persisted. Preferential use of aromatic compounds, which are 25 typically considered molecularly recalcitrant, demonstrates that in the anaerobic conditions of the incubation, thermodynamic favourability of carbon oxidation rather than molecular structure controlled the rate of carbon degradation by microbes.Biogeosciences Discuss., https://doi
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