The Physiological Response of Contrasting Coccolithophore Species to Ocean Alkalinity Enhancement

Abstract

Environmental impacts related to the Ocean Alkalinity Enhancement (OAE) on marine biota remain underexplored. Ocean Alkalinity Enhancement aims to increase the ocean's total alkalinity (TA), shifting the carbonate buffer system to prompt air-sea gas exchange and CO2 drawdown. These conditions might be favorable for calcifiers, leading to increased removal of alkalinity in CaCO3 and reversing some of the intended benefit of the OAE. Here, we parameterize the impact of increased ocean alkalinity on two dominant end-member coccolithophore species: Gephyrocapsa huxleyi and Coccolithus braarudii. The growth rate of each species increased significantly with increased alkalinity, likely driven by increasing resupply rates of CO2 from higher HCO3− concentrations. Both species increased growth rates relative to the control even at the lowest alkalinity treatments (∼3,000 μmol kg−1), which could lead to population expansion under air-equilibrated OAE, and higher population levels of calcification. At higher TA (i.e., >3,000 μmol kg−1), rates of calcification were increasingly limited by CO2 supply to the faster growing cells which resulted in malformation suggestive that cell division is prioritized over calcification when CO2-limited. Divergent species-specific responses may arise because large and heavily calcified C. braarudii have a far greater carbon demand, and rely on CO2 for calcification compared to the smaller, lightly calcified rapidly-growing G. huxleyi which have the additional capacity to use HCO3− when CO2-limited. Our study suggests constraints to ensure safe ecosystem boundaries (i.e., alkalinity 3,000 μmol kg−1), and provides mechanistic insights to understand the impacts of carbonate chemistry on physiology and calcite production by major calcifiers.