Effects of light and darkness on pH regulation in three coral species exposed to seawater acidification

The resilience of corals to ocean acidification has been proposed to rely on regulation of extracellular calcifying medium pH (pH(ECM)), but few studies have compared the capacity of coral species to control this parameter at elevated pCO(2). Furthermore, exposure to light and darkness influences both pH regulation and calcification in corals, but little is known about its effect under conditions of seawater acidification. Here we investigated the effect of acidification in light and darkness on pH(ECM), calcifying cell intracellular pH (pH(I)), calcification, photosynthesis and respiration in three coral species: Stylophora pistillata, Pocillopora damicornis and Acropora hyacinthus. We show that S. pistillata was able to maintain pH(ECM) under acidification in light and darkness, but pH(ECM) decreased in P. damicornis and A. hyacinthus to a much greater extent in darkness than in the light. Acidification depressed calcifying cell pH(I) in all three species, but we identified an unexpected positive effect of light on pH(I). Calcification rate and pH(ECM) decreased together under acidification, but there are inconsistencies in their relationship indicating that other physiological parameters are likely to shape how coral calcification responds to acidification. Overall our study reveals interspecies differences in coral regulation of pH(ECM) and pH(I) when exposed to acidification, influenced by exposure to light and darkness.