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Applied and Environmental Microbiology, February 2006, p. 1322-1329, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1322-1329.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Evidence for Autotrophy via the Reverse Tricarboxylic Acid Cycle in the Marine Magnetotactic Coccus Strain MC-1

Timothy J. Williams,1 Chuanlun L. Zhang,2,3 James H. Scott,4 and Dennis A. Bazylinski1*

Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011,1 Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802,2 Department of Marine Sciences, University of Georgia, Athens, Georgia 30602,3 Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd. NW, Washington, D.C. 20015-13054

Received 25 September 2005/ Accepted 30 November 2005

Strain MC-1 is a marine, microaerophilic, magnetite-producing, magnetotactic coccus phylogenetically affiliated with the {alpha}-Proteobacteria. Strain MC-1 grew chemolithotrophically with sulfide and thiosulfate as electron donors with HCO3–/CO2 as the sole carbon source. Experiments with cells grown microaerobically in liquid with thiosulfate and H14CO3–/14CO2 showed that all cell carbon was derived from H14CO3–/14CO2 and therefore that MC-1 is capable of chemolithoautotrophy. Cell extracts did not exhibit ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO) activity, nor were RubisCO genes found in the draft genome of MC-1. Thus, unlike other chemolithoautotrophic, magnetotactic bacteria, strain MC-1 does not appear to utilize the Calvin-Benson-Bassham cycle for autotrophy. Cell extracts did not exhibit carbon monoxide dehydrogenase activity, indicating that the acetyl-coenzyme A pathway also does not function in strain MC-1. The 13C content of whole cells of MC-1 relative to the 13C content of the inorganic carbon source ({Delta}{delta}13C) was –11.4 Formula . Cellular fatty acids showed enrichment of 13C relative to whole cells. Strain MC-1 cell extracts showed activities for several key enzymes of the reverse (reductive) tricarboxylic acid (rTCA) cycle including fumarate reductase, pyruvate:acceptor oxidoreductase and 2-oxoglutarate:acceptor oxidoreductase. Although ATP citrate lyase (another key enzyme of the rTCA cycle) activity was not detected in strain MC-1 using commonly used assays, cell extracts did cleave citrate, and the reaction was dependent upon the presence of ATP and coenzyme A. Thus, we infer the presence of an ATP-dependent citrate-cleaving mechanism. These results are consistent with the operation of the rTCA cycle in MC-1. Strain MC-1 appears to be the first known representative of the {alpha}-Proteobacteria to use the rTCA cycle for autotrophy.

* Corresponding author: Mailing address: Department of Biochemistry, Biophysics, and Molecular Biology, 207 Science I, Iowa State University, Ames, IA 50011. Phone: (515) 294-2561. Fax: (515) 294-6019. E-mail: dbazylin{at}iastate.edu.

Applied and Environmental Microbiology, February 2006, p. 1322-1329, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1322-1329.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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