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 (δ¹¹B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Ω_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 (Ω_cf), and hence their potential to acclimate to changing Ω_sw. We show that lifelong exposure to low Ω_sw did not enable the corals to acclimate and reach similar Ω_cf as corals grown under ambient conditions. The lower Ω_cf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Ω_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.