BayCEER Workshop 2008

Anaerobic Consumers of Monosaccharides in a Moderately Acidic Fen as Revealed by RNA-Stable Isotope Probing

Acidic wetlands are important reservoirs of global soil carbon and emit the greenhouse gas methane.  50-80% of organic matter in wetlands is lignocellulose, consisting mainly of xylan (xylose monomers), and cellulose (glucose monomers). Degradation of such compounds proceeds via fermentation and methanogenesis. Thus, 16S rRNA-based stable isotope probing (16S rRNA-SIP) with ¹³C-labeled xylose and glucose was used to identify xylose- and glucose-fermenting Bacteria, and also Archaea, in a moderately acidic fen.  In anoxic fen slurries, xylose and glucose were converted to volatile fatty acids, CO₂, H₂, and CH₄ at pH 4-5, indicating that the fen harbors acid-tolerant glucose- and xylose-using fermenters, as well as acid-tolerant methanogens. Acidaminococcacea, Aeromonadaceae, Clostridiaceae, Pseudomonadaceae, and Enterobacteriaceae utilized glucose- or xylose-derived carbon, suggesting that highly diverse facultative aerobes and obligate anaerobes are involved in carbon flow under anoxic conditions.  Acidobacteriaceae-related 16S rRNA was still detected after 13 days of anoxic incubation, indicating survival of Acidobacteriaceae under anoxic conditions.  16S rRNA of uncultured Euryarchaeota (i.e., Methanosarcinaceae, Methanobacteriaceae) and group I.3 Crenarchaeota was identified, indicating that novel Archaea are active in anoxic fen slurries.  Fermentation products are primary drivers for methanogenesis when electron acceptors other than CO₂ are absent, and diverse xylose- and glucose-utilizing facultative aerobes and obligate anaerobes provide trophic links to potentially novel acid-tolerant methanogens in the moderately acidic fen.