Symbiosis increases coral tolerance to ocean acidification

Abstract

Increasing the acidity of ocean waters will directly threaten calcifying marine organ- isms such as reef-building scleractinian corals, and the myriad of species that rely on corals for protection and sustenance. Ocean pH has already decreased by around 0.1 pH units since the beginning of the industrial revolution, and is expected to decrease by another 0.2–0.4 pH units by 2100. This study mimicked the pre-industrial, present, and near-future levels ofpCO 2 using a precise control system (±5 %pCO 2 ), to as- sess the impact of ocean acidification on the calcification of recently-settled primary polyps ofAcropora digitifera, both with and without symbionts, and adult fragments with symbionts. The increase inpCO 2 of 100 µatm between the pre-industrial period and the present had more effect on the calcification rate of adultA. digitiferathan the anticipated future increases of several hundreds of micro-atmospheres ofpCO 2 . The primary polyps with symbionts showed higher calcification rates than primary polyps without symbionts, suggesting that (i) primary polyps housing symbionts are more toler- ant to near-future ocean acidification than organisms without symbionts, and (ii) corals acquiring symbionts from the environment (i.e. broadcasting species) will be more vul- nerable to ocean acidification than corals that maternally acquire symbionts.