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Applied and Environmental Microbiology, February 2007, p. 1174-1179, Vol. 73, No. 4
0099-2240/07/$08.00+0     doi:10.1128/AEM.01817-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

CO2 Uptake and Fixation by Endosymbiotic Chemoautotrophs from the Bivalve Solemya velum{triangledown}

Kathleen M. Scott1,2* and Colleen M. Cavanaugh1

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts,1 Department of Biology, University of South Florida, Tampa, Florida2

Received 1 August 2006/ Accepted 3 December 2006

Chemoautotrophic symbioses, in which endosymbiotic bacteria are the major source of organic carbon for the host, are found in marine habitats where sulfide and oxygen coexist. The purpose of this study was to determine the influence of pH, alternate sulfur sources, and electron acceptors on carbon fixation and to investigate which form(s) of inorganic carbon is taken up and fixed by the gamma-proteobacterial endosymbionts of the protobranch bivalve Solemya velum. Symbiont-enriched suspensions were generated by homogenization of S. velum gills, followed by velocity centrifugation to pellet the symbiont cells. Carbon fixation was measured by incubating the cells with 14C-labeled dissolved inorganic carbon. When oxygen was present, both sulfide and thiosulfate stimulated carbon fixation; however, elevated levels of either sulfide (>0.5 mM) or oxygen (1 mM) were inhibitory. In the absence of oxygen, nitrate did not enhance carbon fixation rates when sulfide was present. Symbionts fixed carbon most rapidly between pH 7.5 and 8.5. Under optimal pH, sulfide, and oxygen conditions, symbiont carbon fixation rates correlated with the concentrations of extracellular CO2 and not with HCO3– concentrations. The half-saturation constant for carbon fixation with respect to extracellular dissolved CO2 was 28 ± 3 µM, and the average maximal velocity was 50.8 ± 7.1 µmol min–1 g of protein–1. The reliance of S. velum symbionts on extracellular CO2 is consistent with their intracellular lifestyle, since HCO3– utilization would require protein-mediated transport across the bacteriocyte membrane, perisymbiont vacuole membrane, and symbiont outer and inner membranes. The use of CO2 may be a general trait shared with many symbioses with an intracellular chemoautotrophic partner.

* Corresponding author. Mailing address: University of South Florida, 4202 E. Fowler Ave., SCA 110, Tampa, FL 33620. Phone: (813) 974-5173. Fax: (813) 974-3263. E-mail: kscott{at}cas.usf.edu.

{triangledown} Published ahead of print on 8 December 2006.

Applied and Environmental Microbiology, February 2007, p. 1174-1179, Vol. 73, No. 4
0099-2240/07/$08.00+0     doi:10.1128/AEM.01817-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.





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