Figure 1.

(a) Calcification rate and (b) night-time pH measured in extracellular calcifying fluid (ECF), between calicoblastic epithelium and forming skeleton surface, as seawater pH decreases in simulated ocean acidification (OA, pH 8.1 → 7.2) experiments in three coral genera and species: Stylophora pistillata (Sp, light blue), Pocillopora damicornis (Pd, green), and Acropora hyacinthus (Ah, purple). These are selected, replotted data from Venn et al., 2019.² (a) During the day (open circles) the calcification rate is constant for Stylophora, but it decreases with OA for Pocillopora and Acropora. At night, calcification decreases with OA for all genera, but especially for Acropora, which goes below zero (black solid line); thus, the skeleton formed during the day dissolves at night. (b) The pH values in the ECF during the day, when photosynthesis is active, decrease to 7.8 identically for all three genera; thus, they are omitted here. Only night-time pH values in the ECF are shown, as they vary dramatically across the three genera. The solid lines are linear fits of the data; the 1:1 line (black dashed line) is where pH_ECF = pH_seawater. Clearly, as the seawater pH decreases, the ECF night-time pH decreases, but at slower rates for all genera compared to seawater. Stylophora ECF pH is the slowest, Pocillopora intermediate, and Acropora the fastest, that is, closest to the seawater pH decrease with OA.