Pathways, methanogenic, relative

The decomposition of complex organic substances by microbial marine populations results in the in situ production of biogenic methane, which is found in trace amounts in all fresh and salt waters. The relative importance of acetate assimilation and CO reduction as primary methanogenic pathways is discussed in the chapter by R.A. Burke and W.M. Sacke. In the past, these pathways were studied by the use of C labelled substrates. 

Individual lipids have been isotopically analyzed in only three Archaea, all of them methanogens (see entries for Methanosarcina barkeri, Methanococcoides burtonii, and Methanobacterium thermoautotrophicum in Table 5). All reflect strong depletion of in lipids relative to biomass. Methanogens are commonly described as fixing carbon by use of the acetyl-CoA pathway. This can provide the feedstock required for synthesis of lipids, but C3 and C4 carbon skeletons are required for the synthesis of amino acids (which, in the form of proteins, account for most of the biomass). In methanogens, these are produced by additional C02-fixing steps (White 1995, p. 261). If the isotope effects associated with those reactions are much smaller than those associated with the production of acetyl-CoA, the isotopic contrast between the lipids and the biomass can be accounted for. 

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