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 (δB) of biogenic calcium carbonate to investigate the extent to which organisms' ability to regulate pH at their site of calcification (pH) determines their calcification responses to OA. We report comparative δB analyses of 10 species with divergent calcification responses (positive, parabolic, threshold, and negative) to OA. Although the pH is closely coupled to calcification responses only in 3 of the 10 species, all 10 species elevate pH above pH under elevated pCO. This result suggests that these species may expend additional energy regulating pH under future OA. This strategy of elevating pH above pH 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.