Kinetics of CO₂ fixation by Sulfurovum lithotrophicum 42BKT^T: Medium optimization and tolerance of CO₂ toxicity

CO₂ toxicity, ionic strength, and specific culture media components, such as thiosulfate have been reported to influence the growth of CO₂-fixing autotrophic bacteria. In this study, a chemostat culture was used to determine the kinetics of CO₂ fixation by S. lithotrophicum 42BKT^T. The medium composition was determined to be capable of reaching a steady state at a high concentration of thiosulfate, an energy source, without growth inhibition by the excessively high ionic strength and thiosulfate itself. CO₂ was supplied by saturating the medium with a 1 atm CO₂ gas or a gaseous mixture of 0.8 atm N₂ and 0.2 atm CO₂. The Monod equation successfully described the kinetics of CO₂ fixation; the maximum specific growth rate was 0.33 h⁻¹ regardless of CO₂ pressure while the Monod constant was 0.0379 g Na₂S₂O₃/L and 0.196 g Na₂S₂O₃/L under 0.2 atm and 1.0 atm CO₂, respectively. We found that when nutrients, including thiosulfate, were supplied sufficiently, the CO₂ fixation rate at 1 atm CO₂ was only slightly lower than that at 0.2 atm CO₂ (e.g. only by 4.7 % for 3.0 g Na₂S₂O₃/L). Therefore, the CO₂ fixation by S. lithotrophicum 42BKT^T is not subject to CO₂ toxicity at least up to 1.0 atm CO₂.