Schink, Bernhard
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Syntrophy in Methanogenic Degradation
This chapter deals with microbial communities of bacteria and archaea which closely cooperate in methanogenic degradation and perform metabolic functions in this community that neither one of them could carry out alone. The methanogenic degradation of fatty acids, alcohols, most aromatic compounds, amino acids, and others is performed in partnership between fermenting bacteria and methanogenic Archaea. The energy available in these processes is very small, attributing only fractions of an ATP unit per reaction run to every partner. The biochemical strategies taken include in most cases reactions of substrate-level phosphorylation combined with various kinds of reversed electron transport systems in which part of the gained ATP is reinvested into thermodynamically unfavorable electron transport processes. Altogether, these systems represent fascinating examples of energy efficiency at the lowermost energy level that allows microbial life.
Conversion of biomass into marketed energy carriers and precursors for chemical synthesis
Methanogens : Syntrophic Metabolism
Syntrophy is a mutualistic interaction in which two metabolically different types of microorganisms are linked by the need to keep metabolites exchanged between the two partners at low concentrations to make the overall metabolism of both organisms feasible. In most cases, the cooperation is based on the transfer of hydrogen, formate, or acetate from fermentative bacteria to methanogens to make the degradation of electron-rich substrates thermodynamically favorable. Syntrophic metabolism proceeds at very low Gibbs’ free energy changes, close to the minimum free energy change needed to conserve energy biologically, which is the energy needed to transport one proton across the cytoplasmic membrane. Pathways for syntrophic degradation of fatty acids predict the net synthesis of about one-third of an ATP per round of catabolism. Syntrophic metabolism entails critical oxidation-reduction reactions in which H2 or formate production would be thermodynamically unfavorable unless energy is invested. Molecular insights into the membrane processes involved in ion translocation and reverse electron transport revealed that syntrophs harbor multiple systems for reverse electron transfer. While much evidence supports the interspecies transfer of H2 and formate, other mechanisms of interspecies electron transfer exist including cysteine cycling and possibly direct interspecies electron transfer as electric current via conductive pili or (semi)conductive minerals.
Methylosoma
Me.thy.lo.so'ma. N.L. neut. n. methylum, the methyl group; Gr. neut. n. soma, body; N.L. neut. n. Methylosoma methyl-using body. Proteobacteria / Gammaproteobacteria / Methylococcales / Methylococcaceae The genus Methylosoma belongs to the Gammaproteobacteria and consists so far of only one species, M. difficile. Cells are microaerobic and use only methane or methanol as substrate. Only a particulate methane monooxygenase was found. Closest phylogenetic relatives are the genera Methyloglobulus and Methylovulum. DNA G + C content (mol%): 49.9 (HPLC determination).
The Family Syntrophomonadaceae
The family Syntrophomonadaceae comprises the genera Syntrophomonas, Pelospora, Syntrophothermus, and Thermosyntropha. All these bacteria are strictly anaerobic and depend on reducing conditions for growth. They are Gram-positive with low DNA content, but in most cases the murein layer is thin and an outer membrane appears, resembling the cell wall architecture of Gram-negative bacteria. Also in Gram-staining, these bacteria mostly behave Gram-negative. Except for Pelospora, all members of this family degrade fatty acids of four carbon atoms or more by beta oxidation, in close association with hydrogen- or formate-utilizing partner organisms, and depend on this association for thermodynamic reasons. Most representatives of this family can be grown in pure culture with crotonate which is dismutated to acetate and butyrate. Pelospora sp. grows by decarboxylation of glutarate or succinate.
Syntrophism among prokaryotes
Syntrophism (or syntrophy) is a special kind of symbiosis between two metabolically different types of microorganisms which cooperate by short-distance metabolite transfer. Thus, both organisms together can carry out a metabolic function that neither one can do alone. Syntrophic associations play an essential role in the terminal steps of methane formation from biomass. Here, the partners involved include secondarily fermenting bacteria and methanogens, which together convert intermediates of biomass degradation (amino acids, alcohols, fatty acids, aromatic compounds, etc.) to methane and CO2 at the very end. The partners involved have to share extremely small increments of energy which are in the range of only fractions of an ATP equivalent, at minimum in the range of −20 kJ per mol reaction. In all cases of syntrophic (secondary) fermentations studied so far, ATP is formed via substrate-level phosphorylation, and part of this ATP is reinvested into reversed electron transport systems to release redox equivalents to the partner organism, either as molecular hydrogen or as formate. Also acetate transfer can have an impact on the total energy balance of the partners. The availability of complete genome sequences of syntrophically butyrate- and propionate-degrading syntrophs has advanced our understanding of the biochemistry of these processes considerably in the recent past. A special case is the sulfate-dependent oxidation of methane in marine sediments which, according to our present understanding, is catalyzed by a syntrophic association of methanogens operating in reverse and sulfate-reducing partners. Syntrophy is a wide-spread phenomenon in anoxic environments, and the study of their energy metabolism represents exciting samples of microbial life at minimum energy gains.