Autotrophic CO2 fixation pathways in archaea (Crenarchaeota)

Representative autotrophic and thermophilic archaeal species of different families of Crenarchaeota were examined for key enzymes of the known autotrophic CO2 fixation pathways. Pyrobaculum islandicum (Thermoproteaceae) contained key enzymes of the reductive citric acid cycle. This finding is consistent with the operation of this pathway in the related Thermoproteus neutrophilus. Pyrodictium abyssi and Pyrodictium occultum (Pyrodictiaceae) contained ribulose 1,5-bisphosphate carboxylase, which was active in boiling water. Yet, phosphoribulokinase activity was not detectable. Operation of the Calvin cycle remains to be demonstrated. Ignicoccus islandicus and Ignicoccus pacificus (Desulfurococcaceae) contained pyruvate oxidoreductase as potential carboxylating enzyme, but apparently lacked key enzymes of known pathways; their mode of autotrophic CO2 fixation is at issue. Metallosphaera sedula, Acidianus ambivalens and Sulfolobus sp. strain VE6 (Sulfolobaceae) contained key enzymes of a 3-hydroxypropionate cycle. This finding is in line with the demonstration of acetyl-coenzyme A (CoA) and propionyl-CoA carboxylase activities in the related Acidianus brierleyi and Sulfolobus metallicus. Enzymes of central carbon metabolism in Metallosphaera sedula were studied in more detail. Enzyme activities of the 3-hydroxypropionate cycle were strongly up-regulated during autotrophic growth, supporting their role in CO2 fixation. However, formation of acetyl-CoA from succinylCoA could not be demonstrated, suggesting a modified pathway of acetyl-CoA regeneration. We conclude that Crenarchaeota exhibit a mosaic of three or possibly four autotrophic pathways. The distribution of the pathways so far correlates with the 16S-rRNA-based taxa of the Crenarchaeota.