Bicarbonate requirement for elimination of the lag period of Hydrogenomonas eutropha

Abstract

Carbon dioxide and oxygen concentrations have a profound effect on the lag period of chemoautotrophically grown Hydrogenomonas eutropha. Minimum lag periods and high growth rates were obtained in shaken flask cultures with a prepared gas mixture containing 70% H(2), 20% O(2), and 10% CO(2). However, excessively long lag periods resulted when the same gas mixture was sparged through the culture. The lag period was shortened in sparged cultures by decreasing both the pO(2) and the pCO(2), indicating that gas medium equilibration had not occurred in shaken cultures. The lag period was completely eliminated at certain concentrations of O(2) and CO(2). The optimum pO(2) was 0.05 atm, but the optimum pCO(2) varied according to the pH of the medium and physiological age of the inoculum. At pH 6.4, the pCO(2) required to obtain immediate growth of exponential, postexponential, and stationary phase inocula at equal specific rates was 0.02, 0.05, and 0.16 atm, respectively. With each 0.3-unit increase in the pH of the medium, a 50% decrease in the CO(2) concentration was needed to permit growth to occur at the same rate. The pCO(2) changes required to compensate for the pH changes of the medium had the net effect of maintaining a constant bicarbonate ion concentration. Initial growth of H. eutropha was therefore indirectly related to pCO(2) and directly dependent upon a constant bicarbonate ion concentration.