Energetic Aspects of Methanogenic Feeding Webs

The degradation of organic matter by methanogenesis is the most complex and the most efficient way of transforming organic matter into an energetically useful product. It is the dominant process in every anoxic environment where nitrate, sulfate, oxidized iron, or manganese species have been reduced, and where only CO2 is available as a terminal electron acceptor. Moreover, methanogenesis is the process of organic matter degradation that most likely evolved first in the Earth's biosphere and therefore had the longest time for evolutionary optimization. The complex interplay .of different metabolic types (guilds) of microorganisms in the overall process represents an evolutionary strategy quite different from that of aerobic degradation. Whereas aerobic organisms typically oxidize complex organic substrates including polymers in one single cell all the way dow'n to CO2 and inorganic residues with oxygen as electron acceptor, anaerobic degradative processes depend on cooperation between different guilds of microbes, and the complexity increases from nitratedependent over ironor sulfate-dependent oxidation (typically a two-step process) to methanogenesis, in which at least three types of organisms have to cooperate for efficient substrate conversion. The technological advantage of methanogenic degradation of biomass is obvious: a complex methanogenic community can degrade basically all types of major constituents of biomass, including polysaccharides, proteins, lipids, and nucleic acids finally to methane plus CO2, with few exceptions. The formed product, a mixture of methane, CO2, and traces of hydrogen sulfide, is gaseous and separates easily from the fermentation broth, without specific efforts involving distillation or transfer across membranes, etc. The advantage of methanogenesis over other fuel-producing processes becomes obvious from a comparison of energetics (all calculations are from tables published in Thauer et aI., 1977): Aerobic oxidation: