@ARTICLE{TreeBASE2Ref14914, author = {Barbara J Campbell and S. Craig Cary}, title = {Abundance of Reverse Tricarboxylic Acid Cycle Genes in Free-Living Microorganisms at Deep-Sea Hydrothermal Vents}, year = {2004}, keywords = {}, doi = {}, url = {}, pmid = {}, journal = {Applied and Environmental Microbiology}, volume = {70}, number = {}, pages = {}, abstract = {Since the discovery of hydrothermal vents over 25 years ago, the Calvin-Bassham-Benson (Calvin) cycle has been considered to be the principal carbon fixation pathway in this microbial-based ecosystem. However, based on recent molecular data of cultured free-living and non-cultured episymbiotic members of the epsilon subdivision of Proteobacteria and earlier carbon isotope data of primary consumers, an alternative autotrophic pathway may predominate. Here, genetic and culture-based approaches demonstrated the abundance of the reverse tricarboxylic acid cycle compared to the Calvin cycle in microbial communities from two geographically distinct deep-sea hydrothermal vents. PCR with degenerate primers for three key genes in the reverse tricarboxylic acid cycle as well as form I and form II of ribulose 1,5-bisphosphate carboxylase/oxygenase (Calvin cycle marker gene) was utilized to demonstrate the abundance of the reverse tricarboxylic acid cycle in diverse vent samples. These genes were also expressed in at least one chimney sample. Diversity, similarity matrix and phylogenetic analyses of cloned samples as well as amplified gene products from autotrophic enrichment cultures suggest that the majority of autotrophs that utilize the reverse tricarboxylic acid cycle are members of the epsilon Proteobacteria. These results parallel previously published 16S molecular surveys which demonstrate the dominance of epsilon Proteobacteria in free-living hydrothermal vent communities. Epsilon Proteobacteria are ubiquitous in many other microaerophilic to anaerobic sulfidic environments as well, such as the deep subsurface. Therefore, the reverse tricarboxylic acid cycle may be a major autotrophic pathway in these environments and significantly contribute to global autotrophic processes.} }

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