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 (pH _sw  ~ 8.03) and naturally low-pH (pH _sw  ~ 7.85) seawater conditions at Palau over the last decades. Our results show that both Porites colonies maintained similar carbonate properties (pH _cf , [CO ₃ ^2- ] _cf , DIC _cf, and Ω _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 ). 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.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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