Evidence for autotrophic CO2 fixation via the reductive tricarboxylic acid cycle by members of the ε subdivision of proteobacteria

Author Posting. © American Society for Microbiology, 2005. This article is posted here by permission of American Society for Microbiology for personal use, not for redistribution. The definitive version was published in Journal of Bacteriology 187 (2005): 3020-3027, doi:10.1128/JB.187.9.3020-3027.2005.Based on 16S rRNA gene surveys, bacteria of the ε subdivision of proteobacteria have been identified to be important members of microbial communities in a variety of environments, and quite a few have been demonstrated to grow autotrophically. However, no information exists on what pathway of autotrophic carbon fixation these bacteria might use. In this study, Thiomicrospira denitrificans and Candidatus Arcobacter sulfidicus, two chemolithoautotrophic sulfur oxidizers of the ε subdivision of proteobacteria, were examined for activities of the key enzymes of the known autotrophic CO2 fixation pathways. Both organisms contained activities of the key enzymes of the reductive tricarboxylic acid cycle, ATP citrate lyase, 2-oxoglutarate:ferredoxin oxidoreductase, and pyruvate:ferredoxin oxidoreductase. Furthermore, no activities of key enzymes of other CO2 fixation pathways, such as the Calvin cycle, the reductive acetyl coenzyme A pathway, and the 3-hydroxypropionate cycle, could be detected. In addition to the key enzymes, the activities of the other enzymes involved in the reductive tricarboxylic acid cycle could be measured. Sections of the genes encoding the {alpha}- and ß-subunits of ATP citrate lyase could be amplified from both organisms. These findings represent the first direct evidence for the operation of the reductive tricarboxylic acid cycle for autotrophic CO2 fixation in {varepsilon}-proteobacteria. Since {varepsilon}-proteobacteria closely related to these two organisms are important in many habitats, such as hydrothermal vents, oxic-sulfidic interfaces, or oilfields, these results suggest that autotrophic CO2 fixation via the reductive tricarboxylic acid cycle might be more important than previously considered.This study was supported by the National Science Foundation “Ecological and Evolutionary Physiology” program (grant IBN-0131557) and the NASA Astrobiology Institute (“From Early Biospheric Metabolism to the Evolution of Complex Systems”; grant NNA04CC04A)