Few studies have dealt so far with methanogenic pathways and populations in subarctic and arctic soils. We studied the effects of temperature on rates and pathways of CH4 production and on the relative abundance and structure of the archaeal community in a mildly acidic peat from a permafrost region in Siberia (67 degrees N). We monitored the production of CH4 and CO2 over time and measured the consumption of Fe(II), ethanol and volatile fatty acids. All experiments were performed with and without specific inhibitors [2-bromoethanesulfonate (BES) for methanogenesis and CH3F for acetoclastic methanogenesis]. The optimum temperature for methanogenesis was between 26 degrees C and 28 degrees C [4.3 micromol CH4 (g dry weight)(-1) day(-1)], but the activity was high even at 4 degrees C [0.75 micromol CH4 (g dry weight)(-1) day(-1)], constituting 17% of that at 27 degrees C. The population structure of archaea was studied by terminal restriction fragment length polymorphism analysis and remained constant over a wide temperature range. Acetoclastic methanogenesis accounted for about 70% of the total methanogenesis. Most 16S rRNA gene sequences clustered with Methanosarcinales, correlating with the prevalence of acetoclastic methanogenesis. In addition, sequences clustering with Methanobacteriales were recovered. Fe reduction occurred in parallel to methanogenesis. At lower and higher temperatures Fe reduction was not affected by BES. Because butyrate was consumed during methanogenesis and accumulated when methanogenesis was inhibited (BES and CH3F), it is proposed to serve as methanogenic precursor, providing acetate and H2 by syntrophic oxidation. In addition, ethanol and caproate occurred as intermediates. Because of thermodynamic constraints, homoacetogenesis could not compete with hydrogenotrophic methanogenesis.

The effects of temperature on rates and pathways of CH 4 production and on the abundance and structure of the archaeal community were investigated in acidic peat from a mire in northern Scandinavia (68°N). We monitored the production of CH 4 ؊1 · day ؊1 ). The theoretical lower limit for methanogenesis was calculated to be at ؊5°C. The optimum temperature for growth as revealed by real-time PCR was 25°C for both archaea and bacteria. The population structure of archaea was studied by terminal restriction fragment length polymorphism analysis and remained constant over a wide temperature range. Hydrogenotrophic methanogenesis accounted for about 80% of the total methanogenesis. Most 16S rRNA gene sequences that were affiliated with methanogens and all McrA sequences clustered with the exclusively hydrogenotrophic order Methanobacteriales, correlating with the prevalence of hydrogenotrophic methanogenesis. Fe reduction occurred parallel to methanogenesis and was inhibited by BES, suggesting that methanogens were involved in Fe reduction. Based upon the observed balance of substrates and thermodynamic calculations, we concluded that the ethanol pool was oxidized to acetate by the following two processes: syntrophic oxidation with methanogenesis (i) as an H 2 sink and (ii) as a reductant for Fe(III). Acetate accumulated, but a considerable fraction was converted to butyrate, making volatile fatty acids important end products of anaerobic metabolism.

Abstract. Ljubljana marsh in Slovenia is a 16 000 ha area of partly drained fen, intended to be flooded to restore its ecological functions. The resultant water-logging may create anoxic conditions, eventually stimulating production and emission of methane, the most important greenhouse gas next to carbon dioxide. We examined the upper layer (∼30 cm) of Ljubljana marsh soil for microbial processes that would predominate in water-saturated conditions, focusing on the potential for iron reduction, carbon mineralization (CO 2 and CH 4 production), and methane emission. Methane emission from water-saturated microcosms was near minimum detectable levels even after extended periods of flooding (>5 months). Methane production in anoxic soil slurries started only after a lag period of 84 d at 15 • C and a minimum of 7 d at 37 • C, the optimum temperature for methanogenesis. This lag was inversely related to iron reduction, which suggested that iron reduction out-competed methanogenesis for electron donors, such as H 2 and acetate. Methane production was observed only in samples incubated at 14-38 • C. At the beginning of methanogenesis, acetoclastic methanogenesis dominated. In accordance with the preferred substrate, most (91%) mcrA (encoding the methyl coenzyme-M reductase, a key gene in methanogenesis) clone sequences could be affiliated to the acetoclastic genus Methanosarcina. No methanogens were detected in the original soil. However, a diverse community of iron-reducing Geobacteraceae was found. Our results suggest that methane emission can remain transient and low if water-table fluctuations allow reoxidation of ferrous iron, sustaining iron reduction as the most important process in terminal carbon mineralization.

Abstract. Ljubljana marsh in Slovenia is a 16 000 ha area of partly drained fen, intended to be flooded to restore its ecological functions. The resultant water-logging may create anoxic conditions, eventually stimulating production and emission of methane, the most important greenhouse gas next to carbon dioxide. We examined the upper layer (~30 cm) of Ljubljana marsh soil for microbial processes that would predominate in water-saturated conditions, focusing on the potential for iron reduction, carbon mineralization (CO2 and CH4 production), and methane emission. Methane emission from water-saturated microcosms was near minimum detectable levels even after extended periods of flooding (>5 months). Methane production in anoxic soil slurries started only after a lag period and was inversely related to iron reduction, which suggested that iron reduction out-competed methanogenesis for electron donors, such as H2 and acetate. Methane production was observed only in samples incubated at 14–38°C. At the beginning of methanogenesis, acetoclastic methanogenesis dominated. In accordance with the preferred substrate, most (91%) mcrA (encoding the methyl coenzyme-M reductase, a key gene in methanogenesis) clone sequences could be affiliated to the acetoclastic genus Methanosarcina. No methanogens were detected in the original soil. However, a diverse community of iron-reducing Geobacteraceae was found. Our results suggest that methane emission can remain transient and low if water-table fluctuations allow re-oxidation of ferrous iron, sustaining iron reduction as the most important process in terminal carbon mineralization.