Archaeum

The dicarboxylate/4-hydroxybutyrate cycle starts from acetyl-CoA, which is reductively carboxylated to pyruvate. Pyruvate is converted to PEP and then car-boxylated to oxaloacetate. The latter is reduced to succinyl-CoA by the reactions of an incomplete reductive citric acid cycle. Succinyl-CoA is reduced to 4-hydroxybu-tyrate, the subsequent conversion of which into two acetyl-CoA molecules proceeds in the same way as in the 3-hydroxypropionate/4-hydroxybutyrate cycle. The cycle can be divided into part 1 transforming acetyl-CoA, one C02 and one bicarbonate to succinyl-CoA via pyruvate, PEP, and oxaloacetate, and part 2 converting succinyl-CoA via 4-hydroxybutyrate into two molecules of acetyl-CoA. This cycle was shown to function in Igrticoccus hospitalis, an anaerobic autotrophic hyperther-mophilic Archaeum (Desulfurococcales) [40]. Moreover, this pathway functions in Thermoproteus neutrophilus (Thermoproteales), where the reductive citric acid cycle was earlier assumed to operate, but was later disproved (W.H. Ramos-Vera et al., unpublished results). 

A variety of bacteria and other microorganisms, such as the archaeum Ferriplasma acidarmanus, may be actively involved in the oxidation of arsenopyrite (Gihring et al., 1999 Cruz et al., 2005 Barrett et al., 1993). Specifically, (Gihring et al., 1999) collected Thiobacillus caldus and Ferriplasma acidarmanus from acid mine drainage at Iron Mountain, California, USA. The mine drainage had a temperature of approximately 42 °C, a pH of 0.7, and contained about 50 mg L 1 of arsenic. T. caldus growths on the surfaces of arsenopyrite actually hindered the oxidation of the mineral, whereas F. acidarmanus was very tolerant of arsenic and accelerated the dissolution of arsenopyrite (Gihring et al., 1999). 

Hafenbradl D., Keller M., Dirmeier R., Rachel R., RoBnagel P., Burggraf S., Huber H., and Stetter K. O. (1996) Ferroglobus placidus gen. nov., sp. nov. a novel hyperthermophilic archaeum that oxidizes Fe at neutral pH under anoxic conditions. Arcli. Microbiol. 166, 308-314. 

Volkl P, Huber R, Drobner E, Rachel R, Burggraf S, Trincone A, Stetter KO (1993) Pyrobaculum aerophilum sp. nov., a novel nitrate-reducing hyperthermophilic archaeum. Appl Environ Microbiol 59 2918-2926... 

Huber H, Gallenberger M, Jahn U, Eylert E, Berg lA, Kockelkom D, Eisenreich W, Fuchs G (2008) A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignococcus hospitalis. Proc Natl Acad Sci USA 105 7851-7856... 

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