Biological regulation of carbonate chemistry during diatom growth under different concentrations of Ca2+ and Mg2+

Algal photosynthesis increases pH to a level that can induce CaCO3 and Mg(OH)2 precipitation. However, the roles of Mg2+ and Ca2+ in regulating the pH-carbonate system during algal growth are not well understood. We examined effects of different [Mg2+] (50 to 50,000 μM) under different [Ca2+] (500 to 10,000 μM) on pH, dissolved inorganic carbon (DIC) and total alkalinity (TA) during batch culture of the diatom Phaeodactylum tricornutum. The results showed that growth rates and biomass were higher when [Mg2+] increased under a fixed [Ca2+]. DIC and TA were almost depleted, decreasing from 1968 to ~50 μmol kg−1, and from 2171 to ~500 μmol kg−1, respectively. Paradoxically, higher [Mg2+] produced lower maximum pH, which could not be accounted for by DIC consumption. Our analysis reveals that this unexpected lower pH but larger decrease in TA was largely driven by Mg(OH)2 formation. A reduction in TA decreases pH, which stimulates algal carbon uptake by shifting carbonate species from CO32− to bio-available forms (CO2 and HCO3−). The subsequent photosynthetic drawdown of DIC forces pH to rise again, which leads to more precipitation and again a decrease in TA. We built a conceptual model to explain this biological regulation, describing the dynamic feedback loop of diatom growth, DIC uptake, pH increase, mineral precipitation, TA decrease, pH reduction, and DIC uptake, which drives depletion of DIC and TA. This mechanism demonstrates that high-biomass algal growth can overcome carbon limitation in natural waters at the expense of lowering TA, particularly in eutrophic environments.