Chemical conditions favoring photosynthesis-induced CaCO3 precipitation and implications for microbial carbonate formation in the ancient ocean

Chemical conditions favoring photosynthesis-induced CaCO3 precipitation (PCP) was examined to provide basic knowledge for understanding ancient ocean chemistry that enabled microbial carbonate formation. First, numerical simulations were conducted to examine the property of photosynthetic increase in CaCO3 saturation state (ΔΩ), an indicator for PCP introduced by previous studies. These simulations revealed that ΔΩ attained a high value at high Ca2+ concentration, low ionic strength, and optimum pH and dissolved inorganic carbon (DIC) where a low-DIC effect and CO2/CO32− buffering were insignificant. Second, microelectrode measurements were conducted using cyanobacteria-dominated stromatolite to examine the property of actual PCP. Although Ca2+ concentration and ionic strength similarly affected actual PCP, the influences of pH and DIC were quite different from what was expected from simulations: significant PCP occurred even at high DIC (up to ∼300 mmol L−1) where the ΔΩ increase was suppressed by CO2/CO32− buffering. Instead, actual PCP reflected the photosynthetically achieved saturation state (Ωaft), which is the sum of ΔΩ and initial saturation state (Ωbef). Thus, the chemical conditions favoring PCP is an optimum pH-DIC condition where ΔΩ achieves a high value and/or a high pH-DIC condition where Ωbef achieves a high value, in addition to a sufficiently high Ca2+ concentration and low ionic strength. The microelectrode measurements also revealed that the photosynthetic pH increase did not always reflect the occurrence and significance of PCP.