Figure 1. The phylogenetic tree of archaea. Only one representative of the Nanoarchaeota is known, Nanoarchaeum equitans, see Figure 5. The phylum Korarchaeota was postulated on the basis of unique DNA sequences. Members have been observed in mixed cultures, but pure cultures are not yet available.

Figure 2. The sulfur cycle.1, polysulfide reductase. Polysulfide contains a chain of sulfur atoms between HS and SH; 2, sulfur reductase; 3, sulfide: cytochrome c oxidoreductase; 4, sulfur oxygenase; 5, sulfite reductase; 6, adenylylsulfate (APS) reductase; 7, ATP sulfurylase; 8, APS kinase; 9, PAPS reductase; 10, cysteine desulfurase.

Figure 3. The two methanogenic pathways. (a) Formation of methane and CO ₂ from methanol. Membrane-integrated complexes: 1, coenzyme F ₄₂₀H ₂ dehydrogenase; complex containing cytochromes and methanophenazin; 2, heterodisulfide reductase; 3, sodium ion/proton antiporter; 4, A ₁A ₀ ATPase, which translocates protons as well as sodium ions; 5, methyl-H ₄MPT: CoM-SH methyl transferase. Soluble enzyme systems: 6, methyl-CoM reductase; 7, methylene-H ₄MPT reductase; 8, methylene-H ₄MPT dehydrogenase; 9, methenyl-H ₄MPT cyclohydrolase; 10, formyl-MF: H ₄MPT formyltransferase; 11, formyl-MF dehydrogenase. MF, methanofuran; H ₄MPT, tetrahydromethanopterin. Note that the oxidation of one molecule of methanol generates 3 × 2H that are used to reduce three molecules of methanol via methyl-CoM to methane. Acetate molecules are converted by the carbon monoxide/acetyl-CoA synthase to methyl groups and CO. CO is oxidized to CO ₂ and the methyl groups enter the pathway at the level of methyl-H ₄MPT. (b) Methane formation from CO ₂ + H ₂. The biochemistry differs from Figure 3a) as follows: reaction 11 is driven by the heterodisulfide reductase (5); cytochromes and methanophenazin are not involved; reaction 4 is the only energy-conserving reaction.