Abstract
The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid (pHcf). Recent work suggests pHcf homeostasis under short-term exposure to pCO2 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’ pHcf (using boron isotopes). Porites maintained a similar pHcf (~8.24) at both a control (pH 8.1) and seep-influenced site (pH 7.9). Internal pHcf 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 pHcf 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 pHcf represents a key mechanism to persist in future oceans. Only beyond end-of-century pCO2 conditions do they face their current physiological limit of pH homeostasis and pHcf begins to decrease.
Highlights
The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid
To date our understanding of the fate of corals in the face of ocean acidification is based on controlled laboratory studies[1,2], mesocosm studies mimicking coral community composition[3,4,5], and field sites that function as natural ocean acidification analogues[6,7,8]
Culturing experiments have revealed that a reduction in seawater pHT is not directly reflected in the skeletal boron isotopic composition[11,12,13], as the decline in skeletal δ11B, and internal pH of the calcifying fluid (pHcf), is less than the change in seawater pHT
Summary
We derived the first δ11B-pHT relationship for tropical corals collected along a natural pHT gradient (Fig. 1A). Fabricius et al.[6] only measured growth at vent sites with seawater pHT levels not lower than 7.75 (expected seawater pHT values for the end of the century), not covering the seawater pHT range of this study We used their measured growth ratio and compared it to the relative growth rate calculated by the IpHRAC model based on our derived pHcf. The similarity in growth (G) between our intermediate site and the control site (Gintermediate/Gcontrol = 1.23 to 0.91; Supplementary Table S6) corroborates the lack of calcification response observed in a previous study[6]. After short-term exposure to near-future seawater pHT conditions, corals responded with an up-regulation of genes involved in ion transport (in particular Ca2+-transporters like Ca-ATPase that affects internal pHcf, and bicarbonate transporters)[26] Such a response might help to maintain the internal pHcf and calcification rate. It is essential to understand what allows corals in a certain environment to acclimate, and whether other species in other regions have the same capacity to adjust to future changes
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