Internal pH regulation facilitates in situ long-term acclimation of massive corals to end-of-century carbon dioxide conditions

The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid (pH(cf)). Recent work suggests pHcf homeostasis under short-term exposure to pCO(2) conditions predicted for 2100, but it is still unclear if pHcf homeostasis can be maintained throughout a corals lifetime. At CO2 seeps in Papua New Guinea, massive Porites corals have grown along a natural seawater pH gradient for decades. This natural gradient, ranging from pH 8.1-7.4, provides an ideal platform to determine corals' pH(cf) (using boron isotopes). Porites maintained a similar pH(cf) (similar to 8.24) at both a control (pH 8.1) and seep-influenced site (pH 7.9). Internal pH(cf) was slightly reduced (8.12) at seawater pH 7.6, and decreased to 7.94 at a site with a seawater pH of 7.4. A growth response model based on pH(cf) mirrors the observed distribution patterns of this species in the field. We suggest Porites has the capacity to acclimate after long-time exposure to end-of-century reduced seawater pH conditions and that strong control over pH(cf) represents a key mechanism to persist in future oceans. Only beyond end-of-century pCO(2) conditions do they face their current physiological limit of pH homeostasis and pH(cf) begins to decrease.