November 20, 2017
Hydrogenation of Organic Matter as a Terminal Electron Sink Sustains High CO2:CH4 Production Ratios During Anaerobic Decomposition
Environmental metabolomics suggest a mechanism that controls methane production from peatlands.
In freshwater wetlands such as peatlands, soils become anoxic at the surface and the majority of organic matter is decomposed through microbial consortia that are believed to primarily terminate in methanogenesis or methane (CH4) production. In peat from high-latitude Sphagnum-dominated peatlands that are critical to the global carbon cycle, state-of-the-art environmental metabolomics measurements revealed extensive hydrogenation of organic matter, which may serve as a predominant mechanism for producing carbon dioxide (CO2) without CH4, thereby explaining why less CH4 is produced relative to CO2 in many northern peatlands.
Based on evidence of organic matter hydrogenation in field samples and peat incubations, the researchers hypothesize new pathways for organic matter degradation in peatlands, whereby electrons are deposited to the organic matter itself rather than to CH4. This mechanism has also been observed to reduce CH4 production in the cow rumen. An examination of past research on animal hosts suggests many parallels between the chemical and microbiological hydrogenation of organic matter between peatlands and the rumen. Because CH4 has a sustained flux warming potential about 45 times higher than that of CO2, mechanisms that alter CH4 production ratios during peat mineralization have important implications for environmental change.
These results highlight the utility of an “environmental metabolomics” approach that takes advantage of analytical chemistry assets at the U.S. Department of Energy’s (DOE) Environmental Molecular Sciences Laboratory (EMSL), for identifying microbial processes in organic matter decomposition that have importance in human and animal health as well as in the role of wetlands in environmental change.
Peatlands store one-third of soil organic carbon (SOC). It has been hypothesized that environmental change will increase the amount of CH4 produced from organic matter decomposition. In the inorganic electron acceptor deficient environment of Sphagnum-dominated peatlands, classical models of anaerobic decomposition suggest that peat mineralization should produce CO2 and CH4 in equal quantities (i.e., CO2:CH4 = 1). While this ratio has been observed during anaerobic decomposition in many wetlands or aquatic environments (e.g., landfills, lake sediments, and some fens), numerous investigations from Sphagnum-dominated bogs across the globe have found CO2:CH4 to be much greater than 1. A research team from Georgia Institute of Technology (Georgia Tech) used cutting-edge metabolomics techniques, which take advantage of advanced analytical chemistry instruments at EMSL, to provide evidence for ubiquitous hydrogenation of diverse unsaturated compounds that serve as organic electron acceptors in peat. Thereby, the necessary electron balance is provided to sustain CO2:CH4 production >1. In contrast to previously proposed mechanisms, this mechanism adds electrons to C-C double bonds in SOC, thereby serving as (1) a terminal electron sink, (2) a mechanism for degrading complex unsaturated organic molecules, and (3) a means to alleviate the toxicity of unsaturated aromatic acids. the scientists propose that organic matter hydrogenation is a major mechanism that modulates the amount of methane that is released from peatlands. Their results have important implications for environmental change, because of the divergent greenhouse warming potential of the two important greenhouse gases emitted from peatlands, CH4 and CO2.
Georgia Institute of Technology
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
Funding was provided by the U.S. Department of Energy (DOE) Office of Science under contract #DE-AC05-00OR22725. Work conducted by JEK, JPC, and RMW was supported by Contract No. DE-SC0012088 and by LP-M, CMZ, JKK, and SDB by Contract No. DE-SC0008092 from the Terrestrial Ecosystem Science (TES) program of the Office of Biological and Environmental Research (BER), within the DOE Office of Science, under Contract No. DE-SC0012088. BER also funds the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility.
Wilson, R. M., M. M. Tfaily, V. I. Rich and J. K. Keller, et al. "Hydrogenation of organic matter as a terminal electron sink sustains high CO2:CH4 production ratios during anaerobic decomposition." Organic Geochemistry 112 22–32 (2017). https://doi.org/10.1016/j.orggeochem.2017.06.011.