Correlation between fixation of high-concentration CO₂ and glutamate accumulation in Sulfurovum lithotrophicum 42BKT^T

Glutamate was identified as a compatible solute for Sulfurovum lithotrophicum 42BKT^T, which is a marine chemolithoautotrophic bacterium and performs CO₂ fixation through the reductive tricarboxylic acid (TCA) cycle. In a medium of 20 g/L or higher concentration of NaCl, sufficient to induce osmotic stress, the intracellular level of glutamate rapidly increased on exposure to high levels of NaHCO₃. Correlation of the glutamate accumulation with the level of NaHCO₃ was determined and compared to that with the level of NaCl. Glutamate synthesis driven by NaHCO₃ was measured at 1.29 mg Glu/mg protein/(Osmol/L), and 18.4 times higher than that driven by NaCl (0.07 mg Glu/mg protein/(Osmol/L)). Thus, the synthesis of glutamate from 2-oxoglutarate, which is the first product of CO₂ assimilation in the reductive TCA cycle, was triggered at the exposure to the high level of NaHCO₃ and appeared to support S. lithotrophicum 42BKT^T via resistance to osmotic stress and CO₂ toxicity.