Species differences in carbon drawdown during marine phytoplankton growth

Ocean alkalinity enhancement (OAE) has been proposed as a mitigation method for negative carbon emission. Its effects on marine phytoplankton communities would depend on species differences in tolerance to high pH, which results from phytoplankton photosynthetic drawdown of dissolved inorganic carbon (DIC). In this study, 20 marine phytoplankton species were grown in sealed batch cultures and DIC, pH and chlorophyll a (Chl-a) were measured at the peaks of biomass. These results revealed a wide range of species differences. The drawdown DIC (ΔDIC) vs. increases in pH (ΔpH) graph resembled a Michaelis-Menten curve: significantly linear for ΔDIC < ~1000 μM and starting to plateau at ΔDIC > 1000 μM. This indicated that two mechanisms were operating: CO₂ limitation at ΔpH < 1.41 and biologically-mediated precipitation-CO₂ released carbon uptake at ΔpH > 1.41. These findings suggest that the potential effects of OAE on the phytoplankton communities would depend on the species differences in oceans.