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Applied and Environmental Microbiology, May 2003, p. 2942-2949, Vol. 69, No. 5
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.5.2942-2949.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Stable Carbon Isotope Fractionation by Sulfate-Reducing Bacteria

Kathleen L. Londry* and David J. Des Marais

Exobiology Branch, NASA Ames Research Center, Moffett Field, California

Received 6 September 2002/ Accepted 24 January 2003

Biogeochemical transformations occurring in the anoxic zones of stratified sedimentary microbial communities can profoundly influence the isotopic and organic signatures preserved in the fossil record. Accordingly, we have determined carbon isotope discrimination that is associated with both heterotrophic and lithotrophic growth of pure cultures of sulfate-reducing bacteria (SRB). For heterotrophic-growth experiments, substrate consumption was monitored to completion. Sealed vessels containing SRB cultures were harvested at different time intervals, and {delta}13C values were determined for gaseous CO2, organic substrates, and products such as biomass. For three of the four SRB, carbon isotope effects between the substrates, acetate or lactate and CO2, and the cell biomass were small, ranging from 0 to 2{per thousand}. However, for Desulfotomaculum acetoxidans, the carbon incorporated into biomass was isotopically heavier than the available substrates by 8 to 9{per thousand}. SRB grown lithoautotrophically consumed less than 3% of the available CO2 and exhibited substantial discrimination (calculated as isotope fractionation factors [{alpha}]), as follows: for Desulfobacterium autotrophicum, {alpha} values ranged from 1.0100 to 1.0123; for Desulfobacter hydrogenophilus, the {alpha} value was 0.0138, and for Desulfotomaculum acetoxidans, the {alpha} value was 1.0310. Mixotrophic growth of Desulfovibrio desulfuricans on acetate and CO2 resulted in biomass with a {delta}13C composition intermediate to that of the substrates. The extent of fractionation depended on which enzymatic pathways were used, the direction in which the pathways operated, and the growth rate, but fractionation was not dependent on the growth phase. To the extent that environmental conditions affect the availability of organic substrates (e.g., acetate) and reducing power (e.g., H2), ecological forces can also influence carbon isotope discrimination by SRB.

* Corresponding author. Present address: Department of Microbiology, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Phone: (204) 474-7199. Fax: (204) 474-7603. E-mail: londryk{at}cc.umanitoba.ca.

Applied and Environmental Microbiology, May 2003, p. 2942-2949, Vol. 69, No. 5
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.5.2942-2949.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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