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Study of the distribution of autotrophic CO₂ fixation cycles in Crenarchaeota

¹ Institut fuer Biologie II, Universitaet Freiburg;
² Universitaet Regensburg

Two new autotrophic carbon fixation cycles were recently described in Crenarchaeota. The 3-hydroxypropionate/4-hydroxybutyrate cycle using acetyl-coenzyme A (CoA)/propionyl-CoA carboxylase as the carboxylating enzyme was shown for (micro)aerobic members of the Sulfolobales. The dicarboxylate/4-hydroxybutyrate cycle using oxygen-sensitive pyruvate synthase and phosphoenolpyruvate carboxylase as carboxylating enzymes was found in members of the anaerobic Desulfurococcales and Thermoproteales. However, Sulfolobales harbor anaerobic and Desulfurococcales aerobic autotrophic representatives, raising the question which of the two cycles they use. We studied the mechanisms of autotrophic CO2 fixation in the strictly anaerobic Stygiolobus azoricus (Sulfolobales) and in the facultatively aerobic Pyrolobus fumarii (Desulfurococcales). The activities of all enzymes of the 3-hydroxypropionate/4-hydroxybutyrate cycle were shown in the anaerobic S. azoricus. In contrast, the aerobic or denitrifying P. fumarii possesses all enzyme activities of the dicarboxylate/4-hydroxybutyrate cycle. We conclude that autotrophic Crenarchaeota use one of the two cycles, and their distribution correlates with the 16S rRNA-based phylogeny of this group, rather than with the aerobic or anaerobic life style.

³ E-mail: ivan.berg{at}biologie.uni-freiburg.de