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

Abstract

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 pCO₂. 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.

Figure 1

Effects of seawater acidification in light (open symbols) and darkness (closed symbols) on extracellular calcifying medium pH (pH_ECM) (a–c) and calcifying cell pH (pH_I) (d–f) in Stylophora pistillata (column a–d), Pocillopora damicornis (column b–e) and Acropora hyacinthus (c–f). Dashed line represents equivalence with seawater pH. Data are means ± standard deviation. For pH_ECM (a–c), three way ANOVA and simple effects analysis identified conditions where pH_ECM is elevated in the light with respect to dark (indicated by asterisks). For calicoblastic pH_I (d–f), three way ANOVA identified a significant positive effect of light on pH_I across the species. See Table 2 and Supplementary 2 for statistical analysis and Table 1 for carbonate chemistry corresponding to each seawater pH.

Figure 2

Effects of seawater acidification in light (open symbols) and darkness (closed symbols) on calcification (a–c) net photosynthetic rate (d–f) and respiration rate (g–i) in Stylophora pistillata (a–g), Pocillopora damicornis (b–h) and Acropora hyacinthus (c–i). Data are means ± standard deviation. See Table 2 and Supplementary 2 for statistical analysis and Table 1 for carbonate chemistry corresponding to each seawater pH.