Figure 1. Boron isotopic signature and corresponding internal pH_cf of Porites corals from the Papua New Guinea (PNG) pCO₂ seeps.

(A) Average δ¹¹B values measured in 14 massive Porites coral skeletons collected along a seawater pH_T (pH in total scale) gradient at the PNG seeps. (B) Coral skeletal δ¹¹B signature translated into internal calcifying fluid pH (pH_cf). Black filled circles and error bars are means ±1 SE per colony (n = 15–20 samples per colony). Individual colonies at each site are offset horizontally for clarity. Black lines: Regression analysis following a second-order polynomial fit (thick black line) with 95% confidence interval (thin black lines). Grey symbols and lines represent literature data of laboratory findings for tropical corals¹¹,¹²,¹³ (¹¹open symbol: Stylophora pistillata lateral growth and filled symbol: Stylophora pistillata apical growth, ¹²open symbol: Acropora nobilis and filled symbol: Porites cylindrica, ¹³open symbol: Stylophora pistillata and filled symbol: massive Porites sp.). Letters (a, b, c and d) indicate results of the post hoc test when there was a significant site effect (p < 0.01). Statistical test can be found in the supplements Table S4. Yellow dashed lines indicate internal pH conditions when organisms are not up-regulating pH_cf.