The synthesis of carbonate minerals mediated by microorganisms provides some insight into the “dolomite problem.” In this paper, under the mediation of aerobic and moderately halophilic bacteria, carbonate was synthesized in solutions with Mg/Ca ratios of 3, 5, and 7, and the pH of the synthetic mineral solution was recorded every 24 hours, and changes in the main cations in the solution were analyzed. Understanding the changes observed in the solution pH and cation concentrations is of great significance for understanding the process and mechanism of carbonate mineral synthesis under the action of microorganisms. The experimental results show that the synthesized minerals are mainly calcium carbonate monohydrate, calcite, and protodolomite. The pH of the three solutions decreased significantly in the first four days, and the solution was acidic. The rate of pH decline decreased after four days, and the pH slowly increased after 10 days to nearly neutral or weakly alkaline. Both Ca2+ and Mg2+ concentrations showed a tendency of first decreasing and then slightly increasing. The slight increase in Ca2+ and Mg2 + concentrations in the later stage may be due to the fact that Ca2+ and Mg2+ were no longer consumed after the bacterial activity decreased, and cations such as H+, Na+, and K+ in the solution replaced some of the Ca2+ and Mg2+ adsorbed on the culture medium by cation exchange and adsorption. In the three solutions, the consumption of Ca2+ decreased with an increase in the Mg/Ca ratio, while the consumption of Mg2+ increased with an increase in the Mg/Ca ratio. Changes in the Sr2+ concentration were consistent with the overall trends observed for Ca2+ and Mg2+ concentrations, but there was a much better correlation with Ca2+, which may be related to the fact that Sr2+ more easily replaces Ca2+ than Mg2+ when it enters the carbonate lattice.