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

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.