Syntrophism among prokaryotes

Abstract

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 CO 2 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.