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.

Figure 1. Typical pathway for methane formation in anaerobic habitats. In many northern wetlands, conversion of acetate to methane is impeded (denoted by Xs) and most methane is derived from hydrogen and carbon dioxide (although this pathway can be rather low as well.

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.

Figure 2. Acetate accumulates in bog pore water because it is not consumed by methanogenic bacteria. Most of this activity occurs in the upper ~7 cm pf peat and is not noted as acetate accumulation unless the water table is near the surface and peat is anaerobic (Duddleston et al., 2002)

There appears to be a connection between vegetation distribution and the pathway of methane formation in northern peatlands (Figure 3) and subtle increases in the abundance of sedges in Sphagnum-dominated sites may lead to sharp increases in the proportion of methane formed from acetate. Naturally occurring stable C isotope data also support the finding that vegetation changes correspond with changes in the pathway of methanogenesis (Chanton et al, 2006; Hines et al., submitted). We have also shown that these wetlands do not harbor significant populations of methanogenic bacteria capable of converting acetate to methane (Rooney-Varga et al., in press).

Figure 3. Ratios of the production of anaerobic end products in wetland peats display sharp changes from oligotrophic sites like sphagnum bogs to minerotrophic sites without sphagnum (listed as fens here). These ratios indicate that C in bogs flows to acetate and carbon dioxide more readily than to methane, but that this trend reverses in minerotrophic sites. Therefore, the pathway of methane formation changes. If wetlands were to shift towards more minertrophy, then a larger percentage of C will flow to methane, increasing methane flux as vegetation changes (Hines et al., 2008).

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 role of northern wetlands in affecting global climate will vary drastically over the next few decades. Studies show that even slight increases in temperature cause sharp changes in vegetation cover with a loss of mosses and an increase in sedges. Even a slight increase in sedges may lead to a sharp increase in the relative proportion of methane produced during decomposition, which can act as a positive feedback on temperature. Sedges appear to activate the conversion of acetate to methane. We hypothesize that methane emission in the north will respond to warming in a highly non-linear manner due to a change in the path of C flow that will favor methane. This variation in pathway could lead to increasing methane fluxes, well above current predictions based on temperature and vegetation alone. Connecting this change to vegetation will allow for future mapping of methane sources and predictions of emission increases that will accompany vegetation changes.

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.

Figure 4. Methane and acetate accumulates in peat incubated anaerobically, but dimethylsulfide (DMS) also accumulates since it too is not utilized by methanogenic bacteria. The phenomenon is ubiquitous as evidenced by its occurrence in Alaska and New Hampshire wetlands (and many others). Acetate and DMS are readily consumed when other electron acceptors are present (Hines et al. 2001).

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, and R. P. Kiene (2001), Carbon flow to acetate and C1 compounds in northern wetlands, Geophys. Res. Lett., 28, 4251-4254.

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