Porites’ coral calcifying fluid chemistry regulation under normal- and low-pH seawater conditions in Palau Archipelago: Impacts on growth properties

Ongoing ocean acidification is known to be a major threat to tropical coral reefs. To date, only few studies have evaluated the impacts of natural long-term exposure to low-pH seawater on the chemical regulation and growth of reef-building corals. This work investigated the different responses of the massive Porites coral living at normal (pHsw ~ 8.03) and naturally low-pH (pHsw ~ 7.85) seawater conditions at Palau (Fig. 1) over the last decades.

Fig. 1. Coral sampling locations in Palau, Micronesia. (A) Satellite image of the main islands of Palau. (B) Sampling sites where both Porites colonies were sampled in 2018 at the control site (grey) and the low-pH site (pink). Both sites are characterized by different seawater carbonate chemistry properties. Short Drop Off corresponds to the site where the in situ temperatures were measured (wtc.coralreefpalau.org).

Our results show that both Porites colonies maintained similar carbonate properties (pH_cf, [CO₃^2-]_cf, DIC_cf, and W_cf) within their calcifying fluid since 1972. However, the Porites skeleton of the more acidified conditions revealed a significantly lower density (~ 1.21 ± 0.09 g·cm^-3) than the skeleton from the open-ocean site (~ 1.41 ± 0.07 g·cm^-3) (Fig. 2).

Fig. 2. Growth and calcification parameters of both skeletal Porites colonies from the control (grey) and the low-pH (pink) sites at Koror (Palau). Values in grey or pink correspond to the slope for any significant linear regression ± standard error. (A) Extension rate (cm·yr^-1), (B) density (g·cm^-3), and (C) calcification rate (g·cm^-2·yr^-1). Grey and pink lines represent variations within estimated error (area fill) in the Porites colonies extracted in the control and low-pH sites respectively.

Overall, both Porites colonies exerted a strong biological control to maintain stable calcifying fluid carbonate chemistry that favored the calcification process, especially under low pH conditions. However, the decline in skeletal density observed at low-pH provides critical insights into Porites vulnerability to future global change (Fig. 3).

Fig. 3. Schematic summary of the responses to ocean acidification measured in the massive Porites corals of Palau over the period 1972-2015, where DpH = pHcf – pHsw.

Reference: Marine Canesi, E. Douville, L. Bordier, A. Dapoigny, G. E. H. Coulibaly, P. Montagna, E. Béraud, D. Allemand, S. Planes, P. Furla, E. Gilson, S. Roberty, D. Zoccola, S. Reynaud, 2024.Porites’ coral calcifying fluid chemistry regulation under normal- and low-pH seawater conditions in Palau Archipelago: Impacts on growth properties. Sci. Total Environ., 2024, 911, DOI : 10.1016/j.scitotenv.2023.168552