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• A version of the 3-hydroxypropionate cycle turned into observed to operate inside the cardio severe thermoacidophile archaeon Metallosphaera sedula. This pathway is referred to as the 3-hydroxypropionate/4-hydroxybutyrate cycle.

• Yet another variant of the 3-hydroxypropionate cycle is the dicarboxylate/4-hydroxybutyrate cycle. It become discovered in anaerobic archaea. It become proposed in 2008 for the hyperthermophile archeon Ignicoccus hospitalis.