The four decades of the now classic studies by Harland G. Wood and Lars G. Ljungdahl lead to the resolution of the autotrophic acetyl-CoA 'Wood/Ljungdahl' pathway of acetogenesis. This pathway is the hallmark of acetogens, but is also used by other bacteria, including methanogens and sulfate-reducing bacteria, for both catabolic and anabolic purposes. Thus, the pathway is wide spread in nature and plays an important role in the global turnover of carbon. Because most historical studies with acetogens focused on the biochemistry of the acetyl-CoA pathway, the metabolic diversity and ecology of acetogens remained largely unexplored for many years. Although acetogens were initially conceived to be a somewhat obscure bacteriological group with limited metabolic capabilities, it is now clear that acctogens are arguably the most metabolically diverse group of obligate anaerobes characterized to date. Their anaerobic metabolic arsenal includes the capacity to oxidize diverse substrates, including aromatic, C1, C2, and halogenated compounds, and engage a large number of alternative energy-conserving, terminal electron-accepting processes, including classic fermentations and the dissimilation of inorganic nitrogen. In this regard, one might consider acetogens on a collective basis as the pseudomonads of obligate anaerobes. By virtue of their diverse metabolic talents, acetogens can be found in essentially all habitats. This review evaluates the metabolic versatilities of acetogens relative to both the engagement (regulation) of the acetyl-CoA pathway and the ecological roles likely played by this bacteriogical group.

Two anaerobic acid-tolerant bacteria, CK58T and CK74 T , were isolated from acidic beech litter and acidic peat-bog soil, respectively. Both bacteria were spore-forming, motile rods with peritrichous flagella.

Sporomusa silvacetica sp. nov. DG-lT (= DSMZ 10669T) (T = type strain) was isolated from well-drained, aggregated forest soil (pH 6.0) in east-central Germany. The cells were obligately anaerobic, slightly curved rods and were motile by means of laterally inserted flagella on the concave side of each cell. Typical cells were approximately 3.5 by 0.7 pm. Cells stained weakly gram positive, but thin sections revealed a complex multilayer cell wall. Spores were spherical and distended the sporangia. Growth and substrate utilization occurred with ferulate, vanillate, fructose, betaine, fumarate, 2,3-butanediol, pyruvate, lactate, glycerol, ethanol, methanol, formate, and H,-CO,. With most substrates, acetate was the primary reduced end product and was produced in stoichiometries indicative of an acetyl-coenzyme A pathway-dependent metabolism. Fumarate was dismutated to succinate and acetate. Methoxyl and acrylate groups of various aromatic compounds were 0-demethylated and reduced, respectively. Yeast extract was not required for growth. Cells grew optimally at approximately 30°C and pH 6.8; under these conditions and with fructose as the substrate, the doubling time was approximately 14 h. The lowest temperature that supported growth was between 5 and 10°C. The carbon monoxide dehydrogenase and hydrogenase activities were approximately 9 and 102 pmol min-' mg of protein-', respectively. A type b cytochrome was detected in the membrane. The G+C content was approximately 43 mol%. Phylogenetic analysis of the 16s ribosomal DNA indicated that DG-lT was most closely related to members of the genus Sporomusa in the Clostridium subphylum of the gram-positive bacteria.Low-molecular-weight aliphatic organic acids are present in mineral forest soil solutions and are believed to play roles in soil formation, solubility of toxic metals, and plant growth (23,24,30,49,58,59). In this regard, acetate is a dominate organic acid detected in mineral soils (59), and it has been proposed that the acetate in mineral soils is produced primarily through the collective action of facultatively and obligately anaerobic microorganisms (35,63). Although well-drained soils are not considered typical acetogenic habitats, supplementation of forest (34, 35), prairie (63), and tundra (46) soils with H, or CO results in the utilization of substrates and the production of acetate in stoichiometries approximating those expected for H,-or CO-dependent acetogenesis. In addition, acetogenic consortia are readily enriched from mineral forest soils (35,48) and leaf litter (36). To further evaluate the occurrence of acetogens in well-drained, aggregated soils, an acetogen was isolated from a beech forest in east-central Germany. The collective characteristics of this isolate (strain DG-lT [T = type strain]) are not consistent with the characteristics of any previously described acetogenic bacterium, and it is proposed that this organism should be placed in a new species, Sporomusa silvacetica. MATERIALS AND METHODSSoil collection. Forest soil (a silty loam...

Everglades sediments (wetland soils) near sources of agricultural runoff had low redox potentials, were blackened with sulfide, and displayed high porewater phosphorus (total) concentrations and high water column conductivities. These sediments yielded 10 3-to 10 4-fold-higher numbers of culturable anaerobes, including methanogens, sulfate reducers, and acetate producers, than did sediments from Everglades and Lake Okeechobee comparative control sites not as directly associated with agricultural runoff. These observations demonstrated that there was a general, rather than specific, enhancement of the anaerobic microflora in the sediments most likely influenced by agricultural runoff. Despite these differences in microfloral patterns, methylmercury and total mercury levels were similar among these contrasting sediments. Although available sulfate and phosphorus appeared to stimulate the productivity of sulfate reducers in Everglades sediments, the number of culturable sulfate reducers did not directly correspond to the concentration of sulfate and phosphorus in porewaters. Microcosms supplemented with sulfate, nitrate, and phosphate altered the initial capacities of the sediment microflora to produce acetate and methane from endogenous matter. For sediments nearest sources of agricultural runoff, phosphorus temporarily enhanced acetate formation and initially suppressed methane production, sulfate enhanced acetate formation but did not significantly alter the production of methane, and nitrate totally suppressed the initial production of both methane and acetate. In regards to the latter, microbes capable of dissimilating nitrate to ammonium were present in greater culturable numbers than denitrifiers. In microcosms, acetate was a major source of methane, and supplemental hydrogen was directed towards the synthesis of acetate via CO 2-dependent acetogenesis. These findings demonstrate that Everglades sediments nearest agricultural runoff have enhanced anaerobic microbial profiles and that the anaerobic microflora are poised to respond rapidly to phosphate, sulfate, and nitrate input.