Enumeration and metabolic product profiles of the anaerobic microflora in the mineral soil and litter of a beech forest

Although anaerobic microorganisms can be isolated from well-drained soils, the qualitative and quantitative composition of the anaerobic populations p otentially involved in the turnover of carbon in oxic soils is poorly resol ved. In the present study, most probable number (MPN) estimates demonstrate d that the number of anaerobes that were cultured from both forest (beech) mineral soil and litter approximated 10(7) to 10(8) (g dry wt. soil or litt er)(-1), a value that also approximated the number of acetate-producing ana erobes. When a complex mixture of substrates was provided, the substrate-pr oduct profiles of the MPN series indicated that the substrates that yielded the highest numbers of microbes were: yeast extract > sugars > H-2 > organ ic acids and alcohols > methoxylated aromatic compounds. Except for yeast e xtract, this sequence also reflected the temporal consumption of these subs trates in the MPN series. The cultured anaerobic population was dominated b y members of the Enterobacteriaceae and other facultative anaerobes; the fa cultative microorganisms appeared to determine the initial anaerobic activi ties of both forest soil and litter. Compared to mineral soil, litter conta ined more sugar-consuming anaerobes but lower numbers of butyrate-forming a naerobes. H-2, ethanol, butanediol, and aromatic methoxyl groups were utili zed by acetogenic bacteria. H-2-utilizing acetogens were a dominant group o f obligate anaerobes, attesting to the ability of acetogens to survive tran sient or sustained periods of aeration in habitats such as leaf litter. The numbers of cultured methanogens were negligible. Together with previous fi ndings, these results (i) demonstrate that the formation of acetate in soil s is dependent on the combined activity of facultative and obligately anaer obic microorganisms, and (ii) emphasize the potential importance of acetate as an intermediate during the turnover of carbon in habitats subject to st eep oxygen gradients. (C) 1999 Federation of European Microbiological Socie ties. Published by Elsevier Science B.V. All rights reserved.