From holocene to anthropogenic impact: Surpassing coral's pH up-regulation capacity under ocean acidification

Corals' regulation of internal calcifying fluid (CF or cf) chemistry is crucial for their extraordinary calcification capacity, endowing them with a certain ability to cope with environmental changes such as anthropogenic ocean acidification (OA) and warming. However, it remains unclear whether the impacts of these changes on corals have substantially surpassed their regulation capacity, particularly in comparison to the CF chemistry responses to natural climate variability with minor or no human perturbation. In this study, we reconstructed the pH, dissolved inorganic carbon, and carbonate ion concentrations in coral CF (pH_cf, DIC_cf, and [CO₃²⁻]_cf) during the Mid- to Late-Holocene, by analyzing the skeletal δ¹¹B and B/Ca of 80 Porites spp. from eastern Hainan Island in the South China Sea (SCS). Our records indicate considerable inter-colony variations in CF chemistry, with maximum disparities reaching 0.18 units for pH_cf and 1664 μmol/kg for DIC_cf. With this in mind, we found no clear responses of coral DIC_cf to the climate fluctuations during the past ∼5500 years, nor evident differences in pH_cf and [CO₃²⁻]_cf across pre-industrial natural epochs. However, pH_cf and [CO₃²⁻]_cf of modern corals have significantly declined compared to fossil corals. Further analyzes compiling global data on Porites spp. also confirm this pronounced pH_cf decrease in modern corals, suggesting the limitations of pantropical corals to counteract OA by up-regulating pH_cf. Importantly, these fossil and modern corals reveal a clear long-term pH_cf descending trend parallel to atmospheric CO₂ changes, supporting the reliability of coral δ¹¹B in recording long-term changes in seawater pH (pH_sw).