Regulation of calcification site pH is a polyphyletic but not always governing response to ocean acidification

The response of marine-calcifying organisms to ocean acidification (OA) is highly variable, although the mechanisms behind this variability are not well understood. Here, we use the boron isotopic composition (δ¹¹B) of biogenic calcium carbonate to investigate the extent to which organisms' ability to regulate pH at their site of calcification (pH_CF) determines their calcification responses to OA. We report comparative δ¹¹B analyses of 10 species with divergent calcification responses (positive, parabolic, threshold, and negative) to OA. Although the pH_CF is closely coupled to calcification responses only in 3 of the 10 species, all 10 species elevate pH_CF above pH_sw under elevated pCO₂. This result suggests that these species may expend additional energy regulating pH_CF under future OA. This strategy of elevating pH_CF above pH_sw appears to be a polyphyletic, if not universal, response to OA among marine calcifiers-although not always the principal factor governing a species' response to OA.