Ralf Conrad
Max-Planck-Institut für Terrestrische Mikrobiologie
Karl-von-Frisch-Str.
D-35043 Marburg, Germany
Email: conrad@mailer.uni.marburg.de

In presence of O₂, organic matter produced by primary productivity is largely recycled to CO₂. In absence of O₂, organic matter is decomposed to CO₂ plus CH₄. Structural material of microorganisms and plants consists largely of carbohydrates (C₆H₁₂O₆). Anaerobic conversion of C₆H₁₂O₆ to 3 CO₂ + 3 CH₄ is much more exergonic (D G^o´ = -418 kJ) than its conversion to 6 CO₂ + 12 H₂ (D G^o´ = -26 kJ) or to 2 acetate + 2 CO₂ + 4 H₂ (D G^o´ = -216 kJ). The latter reaction in principle represents a complete fermentation reaction. The difference in free enthalpy between production of H₂ and production of CH₄ is a potential ecological niche for methanogenic microrganisms and has probably been exploited early in evolution. Methanogens belong to the Archaea, one of the three domains of Life. A new hypothesis [11] suggests that the Eukarya, the third domain of Life, have developed from symbiosis between the two other domains, the Archaea and Bacteria. Nowadays, the microbial CH₄ production is the dominant source in the global CH₄ cycle, while microbial H₂ production is negligible in the atmospheric H₂ budget.

Methanogenic archaea mostly use three types of reactions for energy generation [16]: (1) reduction of CO₂ with H₂ to CH₄; (2) disproportionation of acetate to CO₂ and CH₄; (3) disproportionation of methyl compounds to CO₂ plus CH₄. The metabolism involves many unique enzymes and coenzymes. Energy is gained from the generation of a membrane potential by translocation of protons across the cellular membrane. Biomass is formed by assimilation of CO₂ into acetyl-CoA as the first step.

The anaerobic degradation of organic matter (e.g. carbohydrates) is achieved by a complex microbial community consisting of several physiotypes [18]: (1) hydrolytic plus fermenting bacteria which degrade polysacharides to CO₂, H₂ and fatty acids/alcohols; (2) homoacetogenic bacteria which degrade polysacharides exclusively to acetate or convert H₂ plus CO₂ to acetate; (3) syntrophic bacteria which degrade fatty acids/alcohols to CO₂, H₂ and acetate; (4) methanogenic archaea which convert the formed H₂/CO₂ and acetate to CH₄. Interestingly, microorganisms that degrade polysaccharides directly to CO₂ and CH₄ have never evolved. The reason may be that reaction (1) and reaction (2) are much more exergonic than reaction (4) which therefore is a comparatively poor ecological niche. Fermenting bacteria would have not much additional profit if they would produce CH₄ instead of fatty acids/alcohols as end product. In fact, the further degradation of fatty acids/alcohols is quite problematic and is thermodynamically impossible in absence of methanogenesis, since the syntrophic reaction (3) is endergonic under standard conditions and proceeds only if the H₂ partial pressure is kept below a certain threshold. This is achieved by H₂-consuming methanogens. The H₂-consuming methanogens, on the other hand, require a minimum of H₂ to be able to metabolize. Therefore, the H₂ concentration in anaerobic environments has to be tightly controlled.

Indeed, H₂ concentrations in anaerobic environment vary within narrow limits as soon as steady state has been reached [3,4]. The H₂ partial pressure in the environment is close to the lower threshold of methanogenic archaea which corresponds to a D G of about —20 kJ mol^-1 CH₄ formed from H₂/CO₂. This amount of free enthalpy is just sufficient to allow the generation of 1/4 - 1/3 ATP, the minimum energy quantum in metabolism. Anaerobic environments that are dominated by sulfate or iron reduction typically exhibit lower H₂ concentrations [10], since bacteria exploiting these reactions require less H₂ to conserve a similar amount of energy than the methanogens [6,15]. When soils are submerged, CH₄ production is initiated and maintained whenever H₂ concentrations allow the conservation of sufficient energy [17].

With environmental H₂ concentrations close to the lower limit of methanogenic metabolism the H₂ is usually sufficiently low to allow the exergonic conversion of most fatty acids and alcohols to acetate, CO₂ and H₂. Degradation of propionate, however, is a critical reaction and thermodynamic conditions in the environment are frequently not permissive for this reaction [9,13]. Nevertheless, propionate degradation is frequently found to occur despite too high H₂ concentrations [8]. This discrepancy is explained by inhomogeneous conditions in the environment, in particular by microbial aggregates consisting of syntrophic H₂-producing and methanogenic H₂-consuming microorganisms which create a mini-environment with optimized conditions for H₂ cycling and microbial energy conservation, despite unfavorable conditions in the bulk environment. There is experimental evidence for the existence of such microbial aggregates [1,5].

In many anaerobic environments, degradation of organic matter to gaseous products is dominated by iron reduction (many submerged soils) or sulfate reduction (marine sediments) rather than by methanogenesis. However, these reactions are basically dependent on the presence of O₂ which has allowed the formation of sufficient Fe(III) and sulfate during Earth´s history and nowadays ensures that Fe(III) and sulfate are permanently regenerated by oxidation of reduced Fe(II) and sulfide. For example, in submerged soils, most of the available Fe(III) and sulfate are reduced during the first week of flooding, followed by CH₄ production which becomes dominant as soon as Fe(III) and sulfate are depleted [14]. Nevertheless, some reduction of iron and sulfate is going on permanently, since reduced Fe(II) and sulfide are oxidized with O₂ on the surface of rice roots. The O₂ is diffusing via the plant aerenchyma system into the roots, some of it leaking into the soil environment. The availability of O₂ also allows the oxidation of CH₄ by aerobic methanotrophic bacteria [2,7]. In fact, about 60% of the globally produced CH₄ is oxidized by these bacteria before CH₄ is able to reach the atmosphere [12]. Without O₂, these processes would not be possible.

During Earth´s history O₂ became available with the evolution of photosynthetic cyanobacteria that use light as energy source and water as reductant for CO₂ assimilation (oxygenic photosynthesis). Until this event, primary production was achieved by either anoxygenic photosynthesis using light as energy source and inorganic reductants (H₂, Fe(II), H₂S) for CO₂ fixation, or by anaerobic chemolithotrophic microorganisms, such as methanogens, which use H₂ (geochemical source) for both energy generation and assimilation of CO₂. Before evolution of oxygenic photosynthesis, the global budget of biomass carbon must have been limited by the availability of inorganic reductants.

References:

Conrad, R. (1989) Activity of methanogenic bacteria in anoxic sediments: Role of H₂-syntrophic methanogenic bacterial associations, in Recent Advances in Microbial Ecology (Hattori, T., Ishida, Y., Maruyama, Y., Morita, R. Y. and Uchida, A., Eds.), pp. 118-122. Japan Scientific Societies Press, Tokyo.

Conrad, R. (1993) Mechanisms controlling methane emission from wetland rice fields, in The Biogeochemistry of Global Change: Radiative Trace Gases (Oremland, R.S., Ed.), pp. 317-335. Chapman & Hall, New York.

Conrad, R. (1996) Anaerobic hydrogen metabolism in aquatic sediments, in Cycling of Reduced Gases in the Hydrosphere. Mitt. Internat. Verein. Limnol., Vol. 25 (Adams, D. D., Seitzinger, S. P. and Crill, P. M., Eds.), pp. 15-24. Schweitzerbart'sche Verlagsbuchhandlung, Stuttgart.

Conrad, R. (1999) Contribution of hydrogen to methane production and control of hydrogen concentrations in methanogenic soils and sediments [review]. FEMS Microbiol. Ecol. 28, 193-202.

Conrad, R., Phelps, T. J. and Zeikus, J. G. (1985) Gas metabolism evidence in support of juxtapositioning between hydrogen producing and methanogenic bacteria in sewage sludge and lake sediments. Appl. Environ. Microbiol. 50, 595-601.

Cord-Ruwisch, R., Seitz, H. J. and Conrad, R. (1988) The capacity of hydrogenotrophic anaerobic bacteria to compete for traces of hydrogen depends on the redox potential of the terminal electron acceptor. Arch. Microbiol. 149, 350-357.

Krylova, N. I. and Conrad, R. (1998) Thermodynamics of propionate degradation in methanogenic paddy soil. FEMS Microbiol. Ecol. 26, 281-288.

Krylova, N. I., Janssen, P. H. and Conrad, R. (1997) Turnover of propionate in methanogenic paddy soil. FEMS Microbiol. Ecol. 23, 107-117.

Lovley, D. R. and Goodwin, S. (1988) Hydrogen concentrations as an indicator of the predominant terminal electron-accepting reactions in aquatic sediments. Geochim. Cosmochim. Acta 52, 2993-3003.

Martin, W. and Müller, M. (1998) The hydrogen hypothesis for the first eukaryote. Nature 392, 37-41.

Reeburgh, W. S., Whalen, S. C. and Alperin, M. J. (1993) The role of methylotrophy in the global methane budget, in Microbial Growth on C1 Compounds (Murrell, J.C. and Kelly, D.P., Eds.), pp. 1-14. Intercept, Andover.

Rothfuss, F. and Conrad, R. (1993) Thermodynamics of methanogenic intermediary metabolism in littoral sediment of Lake Constance. FEMS Microbiol. Ecol. 12, 265-276.

Roy, R., Klüber, H. D. and Conrad, R. (1997) Early initiation of methane production in anoxic rice soil despite the presence of oxidants. FEMS Microbiol. Ecol. 24, 311-320.

Seitz, H. J., Schink, B., Pfennig, N. and Conrad, R. (1990) Energetics of syntrophic ethanol oxidation in defined chemostat cocultures. 1. Energy requirement for H₂ production and H2 oxidation. Arch. Microbiol. 155, 82-88.

Thauer, R. K. (1998) Biochemistry of methanogenesis - a tribute to Stephenson, Marjory. Microbiology - UK 144, 2377-2406.

Yao, H. and Conrad, R. (1999) Thermodynamics of methane production in different rice paddy soils from China, the Philippines and Italy. Soil Biol. Biochem. 31, 463-473.

Zinder, S. H. (1993) Physiological ecology of methanogens, in Methanogenesis: Ecology, Physiology, Biochemistry & Genetics (Ferry, J.G., Ed.), pp. 128-206. Chapman & Hall, New York.