Uncoupling of Terminal Decomposition During Anaerobic Degradation in Northern Wetlands
Collaborators: Jeff Chanton (Florida State University), Khrys Duddleston (University of Alaska Anchorage), Juliette Rooney-Varga (University of Massacusetts Lowell)
Northern peatlands are large natural sources of atmospheric methane, a powerful greenhouse gas. The production of methane is an anaerobic bacterial process, and in most instances about 2/3 is derived directly from acetate, the remainder from hydrogen and carbon dioxide.
Our recent studies funded by the National Science Foundation indicate that acetate is not converted quantitatively to methane in many northern wetlands, but is itself a product that accumulates (Figure 2), and eventually is converted to carbon dioxide, a much less powerful greenhouse gas (Hines et al., 2001; Duddleston et al., 2002). Therefore, even though northern wetlands are currently large emitters of methane, one of the major paths to its formation is not operating. However, this situation is poised on the brink of change because the relative formation of anaerobic end products, and therefore methane emissions, may vary in response to climate change.
We are testing the hypothesis that pathways of carbon flow at the terminus of anaerobic decomposition in northern wetlands vary in response to subtle changes in the abundance of sedges. The primary goal is to conduct field studies to establish the relationship between moss and sedge distribution (and other vascular plants) and pathways of anaerobic degradation. This is being accomplished through incubations of peat collected along vegetation gradients, biogeochemical measurements, and the use of stable isotope and radiocarbon methods capable of discerning the path of methanogenesis. Experiments are investigating possible causes for the lack of acetate consumption by methane producers including testing the ability of moss exudates to attenuate methanogenesis, use of reciprocal transplants, and incubations employing manipulations of pH, trace element availability and nutrients.
The inability of these types of wetlands to convert acetate to methane has ramifications for other compounds as well. Methanogens handle C-1 compounds (no C-C bonds) similarly to acetate (which to a bacterium is simply a carboxylated methyl group). Therefore, C-1 compound use is impeded as well. Figure 4 demonstrates this for dimethylsulfide, which accumulates in northern wetlands leading to significant fluxes of the gas into the atmosphere.
Chanton, J. P., D. Fields, and M. E. Hines (2006), Controls on the hydrogen isotopic composition of biogenic methane from high latitude terrestrial wetlands, J. Geophys. Res., 111, G04004, doi:04010.01029/02005JG000134.
Chanton, J.P., Glaser, P.H., Chasar, L.S., Burdige, D.J., Hines, M.E., Siegel, D.I., Tremblay, L.B. and Cooper, W.T. (2008), Radiocarbon evidence for the importance of surface vegetation on fermentation and methanogenesis in contrasting types of boreal peatlands. Global Biogeochem. Cycles 22, GB4022, doi:10.1029/2008GB003274
Duddleston, K. N., M. Kinney, R. P. Kiene, and M. E. Hines
(2002), Seasonal anaerobic biogeochemistry in a northern ombrotrophic
bog: Acetate as a dominant metabolic end product, Global Biogeochem.
Cycles, 16, 1063, doi:1010.1029/2001GB001402.
Hines, M. E., K. N. Duddleston, J. N. Rooney-Varga, D. Fields, and J. P. Chanton. 2008. Uncoupling of acetate degradation from methane formation in Alaskan wetlands: Connections to vegetation distribution. Global Biogeochem. Cycles, 22, GB2017, doi:10.1029/2006GB002903
Rooney-Varga, J.N., M.W. Giewat, K.N. Duddleston, J.P. Chanton, M.E. Hines. 2007. Links between Archaea community structure, vegetation type, and methanogenesis in Arctic peatlands. FEMS Microbiol. Ecol. 60:240–251