Fig. 4.

General scheme representing the strategy used by four different carbon dioxide fixation pathways. These pathways have in common the formation of succinyl-CoA from acetyl-CoA and two inorganic carbons. The vertical arrows point to the carbon dioxide fixation products released from these metabolic cycles. The combination of the metabolic modules 1 and 4 results in the 3-hydroxypropionate/glyoxylate cycle, as studied in Chloroflexus sp. The combination of 1 and 3 yields the 3-hydroxypropionate/4-hydroxybutyrate cycle present in several Crenarchaeota. The combination of 2 and 5 yields the reductive citric acid cycle, which is present in various anaerobes or microaerobes. The combination of 2 and 3 yields the dicarboxylate/4-hydroxybutyrate cycle.