It is increasingly apparent that adequately mitigating anthropogenic climate interference will require ocean carbon dioxide removal (CDR) strategies. Ocean alkalinity enhancement (OAE) is an abiotic ocean CDR approach that aims to increase the ocean’s CO2 uptake capacity through the dispersal of pulverized mineral or dissolved alkali into the surface ocean. However, OAE’s effect on marine biota is largely unexplored. Here, we investigate the impacts of moderate ( 700 μmol kg−1) and high ( 2700 μmol kg−1) limestone-inspired alkalinity additions on two biogeochemically and ecologically important phytoplankton functional group representatives: Emiliania huxleyi (calcium carbonate producer) and Chaetoceros sp. (silica producer). The growth rate and elemental ratios of both taxa showed a neutral response to limestone-inspired alkalinization. While our results are encouraging, we also observed abiotic mineral precipitation, which removed nutrients and alkalinity from solution. Our findings offer an evaluation of biogeochemical and physiological responses to OAE and provide evidence supporting the need for continued research into how OAE strategies affect marine ecosystems. Coccolithophores and diatoms are resilient to alkalinization, but precipitation at high alkalinities may reduce CDR’s efficacy.