Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (delta 11B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Omega sw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Omega cf), and hence their potential to acclimate to changing Omegasw. We show that lifelong exposure to low Omega sw did not enable the corals to acclimate and reach similar Omega cf as corals grown under ambient conditions. The lower Omega cf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Omega sw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.

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 2020-03-18.