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Although northern peatlands contribute significantly to natural methane emissions, recent studies of the importance and type of methanogenesis in these systems have provided conflicting results. Mechanisms controlling methanogenesis in northern peatlands remain poorly understood, despite the importance of methane as a greenhouse gas. We used 16S rRNA gene retrieval and denaturing gradient gel electrophoresis (DGGE) to analyse archaeal communities in 15 high-latitude peatland sites in Alaska and three mid-latitude peatland sites in Massachusetts. Archaeal community composition was analysed in the context of environmental, vegetation and biogeochemical factors characterized in a parallel study. Phylogenetic analysis revealed that Alaskan sites were dominated by a cluster of uncultivated crenarchaeotes and members of the families Methanomicrobiaceae and Methanobacteriaceae, which are not acetoclastic. Members of the acetoclastic family Methanosarcinaceae were not detected, whereas those of the family Methanosaetaceae were either not detected or were minor. These results are consistent with biogeochemical evidence that acetoclastic methanogenesis is not a predominant terminal decomposition pathway in most of the sites analysed. Ordination analyses indicated a link between vegetation type and archaeal community composition, suggesting that plants (and/or the environmental conditions that control their distribution) influence both archaeal community activity and dynamics.

Dimethylsulfide (CH(3)SCH(3)) is formed in anoxic freshwater sediments by biological methylation of methanethiol (CH(3)SH). We measured thiol methylation potential in low-pH, Sphagnum peat sediments from Alaska and Alabama by adding ethanethiol (CH(3)CH(2)SH) to peat slurries and quantifying the rate of ethylmethylsulfide (CH(3)CH(2)SCH(3)) formation. Thiol methylation potential ranged from 12 to 154 nM h(-1) and was significantly related to dimethylsulfide accumulation rates (P=0.0007; r(2)=0.48). Addition of methanol or syringic acid stimulated thiol methylation potential and dimethylsulfide accumulation rate, suggesting that these compounds could be methyl donors. Addition of acetate or its metabolic precursors (glucose or Sphagnum plant material) inhibited thiol methylation potential, but not carbon dioxide or methane production. Inhibition of methanogenesis with either 2-bromoethanesulfonic acid or KNO(3) consistently inhibited thiol methylation potential and dimethylsulfide accumulation. These results suggest that methanogens play a role in thiol methylation and therefore dimethylsulfide formation.

Laboratory incubations, gas and solute analyses, and stable isotope methods were used to investigate the pathway of methanogenesis in 25 wetland peats of varying vegetation composition along a latitudinal gradient in Alaska. Sites were divided into gross vegetation classes indicative of tropic status: mostly Sphagnum (class 1); Sphagnum plus vascular plants (i.e., Carex) (class 2); mostly vascular plants, but still containing Sphagnum (class 3), and; sites dominated by vascular plants with no visible Sphagnum species (class 4). The magnitude of CO2, acetate and CH4 as end products of anaerobic metabolism varied greatly, but ratios of end product formation indicative of differences in the pathway of C flow and methanogenesis corresponded with vegetation classes, especially at the extremes, e.g., acetate-C accounted for 67% of total C production in Sphagnum-rich sites (class 1) decreasing to 13% in sites devoid of sphagna (class 4). Conversely, CH4 comprised only 0.4% of products in class 1 sites, but increased to 14% in class 4. Total respiration rates (sum of all three products) varied by only a factor of ~-~2 among vegetation classes (200-440 nmol ml-1 day-1), but rates differed greatly if acetate formation was not included suggesting that belowground C cycling can be much more rapid than previously thought. Apparent fractionation factors (α = δ13CDIC + 1000/δ13CCH4 + 1000) that estimate methanogenic pathway, i.e., the relative contribution of CO2 reduction or acetate as precursors of methane, varied from ~1.030 to ~1.080 and agreed with incubation end product ratios, underscoring the importance of the presence or absence of vascular plants and Sphagnum mosses in affecting the pathway of anaerobic C flow. We contend that methanogenesis in general, including CO2 reduction, is impeded in northern wetlands compared to the production of other C compounds and that its importance decreases with oligotrophy. The connection with vegetation suggests that climate change scenarios leading to increases in vascular plant cover in northern wetlands may shift methanogenic pathways toward increased acetotrophy and increased methane formation, which is a positive feedback on warming. [ABSTRACT FROM AUTHOR]