Biochemical Evidence for Formate Transfer in Syntrophic Propionate-Oxidizing Cocultures of Syntrophobacter fumaroxidans and Methanospirillum hungatei

Abstract

The hydrogenase and formate dehydrogenase levels in Syntrophobacter fumaroxidans and Methanospirillum hungatei were studied in syntrophic propionate-oxidizing cultures and compared to the levels in axenic cultures of both organisms. Cells grown syntrophically were separated from each other by Percoll gradient centrifugation. In S. fumaroxidans both formate dehydrogenase and hydrogenase levels were highest in cells which were grown syntrophically, while the formate-H 2 lyase activities were comparable under the conditions tested. In M. hungatei the formate dehydrogenase and formate-H 2 lyase levels were highest in cells grown syntrophically, while the hydrogenase levels in syntrophically grown cells were comparable to those in cells grown on formate. Reconstituted syntrophic cultures from axenic cultures immediately resumed syntrophic growth, and the calculated growth rates of these cultures were highest for cells which were inoculated from the axenic S. fumaroxidans cultures that exhibited the highest formate dehydrogenase activities. The results suggest that formate is the preferred electron carrier in syntrophic propionate-oxidizing cocultures of S. fumaroxidans and M. hungatei.Methanogenic decomposition of complex organic matter is a widespread process, which accounts for a large fraction of the global methane emission (15). Examples of natural methanogenic habitats are freshwater environments such as wetlands, sediments, and rice paddy fields, as well as intestinal tracts of higher animals and insects (4,6,8,21,28). Methanogenic processes can be applied to treat industrial wastewaters in high-rate anoxic bioreactors (16,19,33). The microorganisms involved in methanogenic decomposition are usually immobilized in granular aggregates or biofilms, which is essential for a high conversion rate (23) and prevents biomass from being washed out of the reactor.The amount of energy available in methanogenic conversions is small, and therefore the microorganisms involved are forced to cooperate syntrophically (24, 28). In particular, oxidation of intermediary reduced organic compounds, such as ethanol, butyrate, and propionate, is energetically unfavorable. Nevertheless, the methanogens involved keep the concentrations of the oxidation products, acetate and H 2 (or formate), low enough to create a situation in which all partners involved gain energy. To dispose of reducing equivalents, acetogens reduce protons or bicarbonate. Both H 2 and formate have been proposed as electron carriers in syntrophic degradation. Several studies have provided evidence for H 2 transfer by demonstrating syntrophic growth with methanogens that oxidize only H 2 . Schmidt and Ahring (25) studied interspecies electron transfer in granules from a mesophilic anaerobic sludge bed reactor, and concluded that formate transfer was not important during propionate and butyrate oxidation in this system.On the other hand, Thiele and Zeikus (29) provided evidence that interspecies formate transfer was the dominant mechanism in a whey-processing digester,...

Effect of tungsten and molybdenum on growth of a syntrophic coculture of Syntrophobacter fumaroxidans and Methanospirillum hungatei

“…These speciWc activities were much higher than those in axenically grown cells of S. fumaroxidans and M. hungatei on formate (Table 1), consistent with Wndings reported previously (de Bok et al 2002a). In W-depleted media (medium I and II), FDH levels were signiWcantly lower in both the organisms (Table 1).…”

“…The FDH level in M. hungatei under these conditions (no metals added) was 0.02 U/mg protein. This is almost negligible compared to its level in medium without any limitations, considering that the eYciency of Percoll separation with these co-cultures is never 100% (de Bok et al 2002a). Moreover, this activity could be attributed to residual FDH from S. fumaroxidans cells in the Percoll separated M. hungatei cells.…”

“…The terminal reductases of a model syntrophic consortium consisting of the propionate-converting Syntrophobacter fumaroxidans and the hydrogen-and formate-scavenging M. hungatei were studied in detail de Bok et al 2002a). Both the microorganisms possess hydrogenases (H 2 ases) and formate dehydrogenases (FDHs), of which the activities are much higher during syntrophic growth on propionate compared to the respective pure cultures (de Bok et al 2002a). Two W-containing FDHs were puriWed and characterized from S. fumaroxidans (de Bok et al 2003).…”

“…Therefore, besides H 2 , formate is also considered to play a role in syntrophic propionate oxidation (Thiele and Zeikus 1988;Boone et al 1989;Stams and Dong 1995). The terminal reductases of a model syntrophic consortium consisting of the propionate-converting Syntrophobacter fumaroxidans and the hydrogen-and formate-scavenging M. hungatei were studied in detail de Bok et al 2002a). Both the microorganisms possess hydrogenases (H 2 ases) and formate dehydrogenases (FDHs), of which the activities are much higher during syntrophic growth on propionate compared to the respective pure cultures (de Bok et al 2002a).…”

Reversible interconversion of carbon dioxide and formate by an electroactive enzyme

“…The reducing equivalents are used to reduce protons to hydrogen or to reduce CO 2 to formate. During syntrophic growth S. fumaroxidans optimizes its metabolism by transferring the formate (and/or the hydrogen) to its syntrophic partner (16). Therefore, the tungsten-containing formate dehydrogenases of S. fumaroxidans must be both thermodynamically and kinetically efficient, and so they are good candidate electrocatalysts for the electrochemical reduction of CO 2 .…”