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 (pHCF) 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 pHCF is closely coupled to calcification responses only in 3 of the 10 species, all 10 species elevate pHCF above pHsw under elevated pCO2. This result suggests that these species may expend additional energy regulating pHCF under future OA. This strategy of elevating pHCF above pHsw 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.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2019) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2020-05-6.