At the present time five different autotrophic pathways have been described (Thauer, 2007; Nakagawa and Takai, 2008) and we chose these for comparison, all having been well established for carbon assimilation: (i) the reductive pentose phosphate pathway (Calvin and Bassham, 1962), (ii) the reductive tricarboxylic acid (rTCA) cycle (Buchanan and Arnon, 1990), (iii) the reductive acetyl-CoA pathway (Ljungdahl, 1986), and (iv) the 3-hydroxypropianate cycle (Ishii et al., 2004) and (v) its variant the 4-hydroxy butyrate cycle (Berg et al, 2007).
The reductive acetyl-CoA pathway is characterized by the presence of the key enzyme complex acetyl CoA synthetase, which also catalyzes the oxidation of CO to [CO.sub.2].