Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate

Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO₂ in sea water, which causes declines in carbonate ion concentration CO₃²⁻] and increases in bicarbonate ion concentration HCO₃⁻]. To address this topic, we manipulated CO₃²⁻] and HCO₃⁻] to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. CO₃²⁻] played a significant role in light and dark calcification of P. rus, whereas HCO₃⁻] mainly affected calcification in the light. Both CO₃²⁻] and HCO₃⁻] had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with CO₃²⁻]. Our results show that the negative effect of declining CO₃²⁻] on the calcification of corals and algae can be partly mitigated by the use of HCO₃⁻ for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.