Fig. 2: MMs of Stylophora pistillata and their proportions.

A MM of a portion of a Stylophora pistillata area of coral skeleton grown at pH 8.05 and B pH 7.2. Pixels are 56 nm in (A) and 45 nm in (B). Pixels with only proportions below 0.5 are displayed in black in both MMs, whereas a pixel where one phase has a proportion of 0.5–1.0 is a colored pixel, as indicated by the color legend in (B). The X-ray absorption near-edge structure spectra (345–355 eV) of 4 precursors and aragonite are also shown in (B), color coded as MM pixels, except for the aragonite spectrum, which is plotted in white rather than blue. C, D Proportion profiles as a function of distance from the edge for each precursor and mature aragonite. In (C, D), each colored datapoint is the average proportion of each phase at each distance, calculated from 10⁶ pixels. The colored curve is the best-fitting exponential decay or exponential rise (Supplementary Fig. 2), and the R² indicates the goodness of fit. The color legend in (B) identifies biomineral precursors and mature aragonite phases by color and extends to all precursor maps and plots in this work. Magenta data points are open circles to distinguish them from red. The precursor proportions for each of the five components in each of the eight areas are shown in Supplementary Fig. 3.