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Anaerobic degradation of isobutyrate by methanogenic enrichment cultures and by a Desulfococcus multivorans strain
Authors:Marion Stieb  Bernhard Schink
Affiliation:(1) Fakultät für Biologie, Universität Konstanz, Postfach 5560, D-7750 Konstanz, Federal Republic of Germany;(2) Present address: Space Development Division, Dornier System GmbH, D-7990 Friedrichshafen 1, FRG;(3) Present address: Lehrstuhl Mikrobiologie I, Auf der Morgenstelle 28, D-7400 Tübingen, FRG
Abstract:Methanogenic enrichment cultures with isobutyrate as sole source of carbon and energy were inoculated with sediment and sludge samples from freshwater and marine origin. Over more than 20 transfers, these cultures fermented 2 mol isobutyrate with 1 mol CO2 via an intermediate formation of n-butyrate to 4 mol acetate and 1 mol CH4. The primary isobutyrate-fermenting bacteria could not be purified. From one of the marine enrichment cultures, a sulfate-reducing bacterium was isolated which oxidized isobutyrate with sulfate completely to CO2. Based on its physiological and morphological properties, this strain was assigned to the known species Desulfococcus multivorans. It also oxidized many other fatty acids without significant release of short-chain intermedeates. The enzymes involved in isobutyrate degradation by this bacterium were assayed in cell-free extracts. The results indicate that isobutyrate is activated to its CoA derivative and oxidized via methylmalonate semialdehyde to propionyl-CoA. Propionyl-CoA is further converted via the methylmalonyl-CoA pathway to acetyl-CoA which is finally cleaved by the CO-dehydrogenase system. It is evident that this is not the pathway used by the fermenting bacteria prevailing in the methanogenic enrichment cultures. There results are discussed on the basis of energetical considerations.
Keywords:Fatty acid degradation  Syntrophic associations  Isobutyrate  Sulfate-reducing bacteria  Isobutyrate-butyrate isomerization  CO-Dehydrogenase
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