Anthropogenic CO 2 is a major driver of present environmental change in most ecosystems 1, and the related ocean acidification is threatening marine biota 2. With increasing pCO 2, calcification rates of several species decrease 3, although cases of upregulation are observed 4. Here, we show that biological control over mineralization relates to species abundance along a natural pH gradient. As pCO 2 increased, the mineralogy of a scleractinian coral (Balanophyllia europaea) and a mollusc (Vermetus triqueter) did not change. In contrast, two calcifying algae (Padina pavonica and Acetabularia acetabulum) reduced and changed mineralization with increasing pCO 2, from aragonite to the less soluble calcium sulphates and whewellite, respectively. As pCO 2 increased, the coral and mollusc abundance was severely reduced, with both species disappearing at pH< 7.8. Conversely, the two calcifying and a non …