Syntrophic oxidation of butyrate
In anoxic environments, syntrophic microbial communities are important for decomposition of organic matter to CO2 and CH4. Among the most difficult steps are the fermentative degradation of short-chain fatty acids such as propionate and butyrate. Conversion of these metabolites to acetate, CO2, formate, and hydrogen is endergonic under standard conditions and occurs only if methanogens keep the concentrations of these intermediate products low. Hence, the methanogenic oxidation of butyrate to acetate requires a tight cooperation between the syntrophically fermenting Syntrophomonas wolfei and the methanogenic archaeobacterium Methanospirillum hungatei. Butyrate degradation includes an oxidation step of comparably high redox potential, i. e. oxidation of butyryl-CoA to crotonyl-CoA, that requires investment of energy to release the electrons as hydrogen or formate. Although investigated for several decades, the biochemistry of these reactions is still not completely understood. So far, genome analysis of the butyrate-oxidizing S. wolfei reveals the presence of energy-transforming protein complexes. Furthermore, a reversed electron transport system in S. wolfei was postulated to shift electrons from butyryl-CoA oxidation to the redox potential of NADH for H2 generation. Within this research project we investigate the electron activation mechanisms that are involved in butyrate degradation in S. wolfei in co-culture with M. hungatei.
Syntrophic oxidation of ethanol
The fermentation product ethanol is formed in every anoxic habitat and is also degraded in anoxic environments such as freshwater and marine sediments. For an understanding of carbon transformation processes in ecology, the understanding of the anaerobic degradation pathways is of major importance. Pelobacter acetylenicus and P. carbinolicus ferment ethanol in syntrophic association with the methanogenic archaeobacterium Methanospirillum hungatei under anoxic conditions. The fermentation of ethanol to acetic acid plus H2 as carried out by the Pelobacter strains is energetically feasible only if the partial pressure of dihydrogen is kept low (<104 atm) via consumption by the syntrophic partner organism. The degradation pathway within P. acetylenicus has been studied with respect to the basic carbon flow. This project focuses on the reactions associated with ATP formation and the possible involvement of reversed electron transport processes to understand the energetic aspects of syntrophic ethanol fermentation.
Syntrophic oxidation of acetate
In this project, we investigate the energy conservation in syntrophically acetate-oxidizing bacteria. Under anoxic conditions and in the absence of external electron acceptors organic matter is fermented to methane and carbon dioxide. In this stepwise process primary fermenting bacteria form carbon dioxide, hydrogen, acetate and C1 compounds which are further used by methanogens to accumulate methane. Homoacetogenic bacteria connect the pool between acetate and one-carbon compounds and hydrogen. However their role in this process is not well understood.
At standard conditions the conversion of acetate to carbon dioxide and hydrogen is unfavorable (∆G’o = +95 kJ/mol). However in the presence of a hydrogen-utilizing methanogen the oxidation of acetate becomes possible, because the formation of methane from hydrogen and carbon dioxide yield the energy (∆G’o = -131 kJ/mol) to enable acetate oxidation. This special case of symbiotic cooperation between two metabolically different types of bacteria which depend on each other for degradation of a certain substrate is called syntrophy.
Acetate is oxidized via the Wood-Liungdahl pathway. One mole of ATP is consumed in the acetate kinase reaction, but one mole of ATP is gained in the formyl-H4F synthetase reaction. Therefore, the net ATP gain by SLP is zero, and ATP must also be formed by ion gradient-driven phosphorylation. However less is known which enzymes are involved in the formation of a proton/sodium ion gradient.