Poster, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 25.02.2007 - 28.02.2007
Abstract:
In aerated forest soils degradation of endogenous organic matter under anoxic conditions lead to the accumulation of organic acids, especially acetate. However, the composition of the in situ involved prokaryotic community is still unknown. In the past 20 years by gene marker-based studies a huge diversity of prokaryotes with unknown ecological function could be detected in soils. Therefore, the present study aimed to identify prokaryotes involved in anaerobic degradation of glucose in an acidic forest soil (Coulissenhieb, Fichtelgebirge, Germany). In an initial experiment, a soil sample was diluted with water and these slurries were incubated anoxically for 65 h at room temperature. Afterwards 0.5 mM glucose were added. The further incubation temperature was 15 °C. Carbon derived from glucose was recovered in 28 % in acetate formed within 261 h (11 days). Beside acetate other fermentation products (propionate, H2, CO2 and formate) were produced. The anaerobic respirations methanogenesis, denitrification and DNRA could not be detected within 11 days. Similar results could be retrieved from soil slurry incubations of two other aerated forest soils from Germany. Afterwards a slurry from a soil sample of the Coulissenhieb was incubated pulsing with [13C6]-glucose to retain concentrations between 0.3 and 1.0 mM. In contrast to the initial experiment, propionate could not be detected. RNA was extracted at different time points and subjected to CsTFA density gradient centrifugation to separate 13C-enriched RNA from the unlabeled ones. For the identification of the glucose-degrading prokaryotes the 13C-enriched 16S rRNA of the gradients were analyzed. Community structure during labelling was detected with 16S rRNA gene-targeted T-RFLP (terminal restriction fragment length polymorphism analysis). By 16S rRNA gene clone libraries and subsequent assignation of these clone sequences to TRFs detected in the 13C-enriched 16S rRNA involved prokaryotes were phylogenetically identified. All 13C-labeled genotypes affiliated with the Enterobacteriaceae (Gammaproteobacteria) and were most closely related to Rahnella aquatilis and Ewingella americana. Both species are known to ferment glucose by mixed acid fermentation with acetate as one major endproduct. These results indicate that, under the given conditions, the facultative aerobic Enterobacteriaceae play an important role in the turnover of glucose via fermentation.