Poster, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 2007-02-25 - 2007-02-28
Abstract:
Mineralization of organic matter in wetland soils is believed to proceed primarily via fermentation and methanogenesis, mainly because of the absence or limited availability of alternative electron acceptors. Such electron acceptors depend on the water table that in turn effects carbon flow to methane and associated populations. Activities and communities involved in the “intermediary ecosystem metabolism” of (i.e., in the processes that precede methanogenesis in) acidic fens are mostly unresolved. Tools to detect activity changes of such communities are not established to date. Consequently, the objectives of this study were (1) to determine activity parameters of fermenters and methanogens, and (2) to elucidate the bacterial and archaeal community structure by enumeration and molecular analyses of prokaryotes in fen material. Total cell numbers in the surface layer approximated 5 x 1010 cells gDW-1 and decreased with depth, as did the percentage of living cells. The MPN values of sugar-consuming anaerobes and methanogens ranged from 106-107 and 105-106 organisms gDW-1, respectively. Nonsupplemented fen material produced CO2, CH4, and organic acids. CO2- and CH4-production rates were highest in the upper layers (9.7 µmol CO2 and 0.18 µmol CH4 gDW-1 d-1, respectively) and decreased with increasing depth. The optimum temperature for the production of CO2 and CH4 was 30°C (Q10 was 1.2-2.4, and 2.4-3.4, respectively), and the optimum pH for the production of CO2 was near neutral. Glucose and potential methane precursors stimulated the production of CH4. H2/CO2 and formate were dominant metabolic intermediates and apparent precursors of CH4. Acetate produced in anoxic microcosms accumulated as an end product, and supplemental acetate did not significantly stimulate the production of CH4. When methanogenesis was inhibited with 2-bromo-ethane sulphonate, glucose, N-acetylglucosamine, and xylose stimulated the production of CO2 and other fermentation products (e.g., acetate, propionate, butyrate, and H2). Vmax calculated from CO2 production rates decreased with depth. Thus, the activities of acid-tolerant fermenters and acid-tolerant methanogens present in fen material were limited by several parameters, e.g., lack of substrate, low temperature, and acidic pH. Denaturing gradient gel analysis of 16S rRNA genes indicated that the acidic fen harbors Acidobacteria, Nitrospirales, Clamydiales, Clostridiales, α-, γ-, δ-Proteobacteria, Cyanobacteria, Crenarchaeota, Methanomicrobiales and Methanosarcinales. The collective results suggest that the acidic fen harbors acid tolerant fermenters (both facultative and obligate anaerobes) that convert monosaccharides to fermentation products, e.g., acetate, propionate, butyrate, formate, CO2, and H2. Vmax and Q10 values were useful activity parameters for fermenters. Formate and H2/ CO2 appear to be used by acid tolerant methanogens and acid tolerant acetogens that form CH4 and acetate, respectively. The extent to which alternative terminal electron-accepting processes and acetoclastic methanogenesis occur under in situ conditions in this fen were not resolved by this study.