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The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO₂. We compared the effects of six partial pressures of CO₂ (P_CO2) from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted P_CO2 levels and net calcification was quantified. All eight species calcified at the highest P_CO2 in which the calcium carbonate aragonite saturation state was ∼1. Calcification decreased linearly as a function of increasing partial P_CO2 in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was …