The challenges corals and symbiotic cnidarians face from global environmental change brings new urgency to understanding fundamental elements of their physiology. Intracellular pH (pHi) influences almost all aspects of cellular physiology but has never been described in anthozoans or symbiotic cnidarians, despite its pivotal role in carbon concentration for photosynthesis and calcification. Using confocal microscopy and the pH sensitive probe carboxy SNARF-1, we mapped pHi in short-term light and darkincubated cells of the reef coral Stylophora pistillata and the symbiotic anemone Anemonia viridis. In all cells isolated from both species, pHi was markedly lower than the surrounding seawater pH of 8.1. In cells that contained symbiotic algae, mean values of pHi were significantly higher in light treated cells than dark treated cells (7.41 ؎ 0.22 versus 7.13 ؎ 0.24 for S. pistillata; and 7.29 ؎ 0.15 versus 7.01 ؎ 0.27 for A. viridis). In contrast, there was no significant difference in pHi in light and dark treated cells without algal symbionts. Close inspection of the interface between host cytoplasm and algal symbionts revealed a distinct area of lower pH adjacent to the symbionts in both light and dark treated cells, possibly associated with the symbiosome membrane complex. These findings are significant developments for the elucidation of models of inorganic carbon transport for photosynthesis and calcification and also provide a cell imaging procedure for future investigations into how pHi and other fundamental intracellular parameters in corals respond to changes in the external environment such as reductions in seawater pH.cell biology ͉ cnidaria ͉ symbiosis ͉ photosynthesis ͉ climate change S ymbiotic cnidarians form the structural foundation of coral reefs, which rank among the most biodiverse and ecologically important ecosystems. Despite their environmental significance, key elements of coral physiology such as calcification and the symbiotic interactions between cnidarians and their intracellular algae (Symbiodinium) are poorly understood, largely due to knowledge gaps in fundamental aspects of cnidarian cell biology (1-3).Intracellular pH (pHi) modulates virtually all aspects of cell metabolism, including membrane channel functioning, the structural and functional properties of enzymes, intracellular signaling, and ion availability (4, 5). As a consequence, pHi must be tightly controlled and changes in pHi are minimized by intracellular buffers and regulation by ion carriers on the cell membrane (6, 7). In symbiotic cnidarians such as corals, pHi is thought to be linked to processes of photosynthesis and calcification by influencing the flux of ions between host, symbiont, and the surrounding environment (6,8). This particularly concerns the transport of dissolved organic carbon (DIC) between cell layers and the speciation of DIC between CO 2 , HCO 3 Ϫ , and CO 3 2Ϫ (9-11). Measurements of pHi in cnidarian cells are therefore an essential step for elucidating how DIC is transferred from the surroundin...

Carbonic anhydrases (CA) play an important role in biomineralization from invertebrates to vertebrates. Previous experiments have investigated the role of CA in coral calcification, mainly by pharmacological approaches. This study reports the molecular cloning, sequencing, and immunolocalization of a CA isolated from the scleractinian coral Stylophora pistillata, named STPCA. Results show that STPCA is a secreted form of ␣-CA, which possesses a CA catalytic function, similar to the secreted human CAVI. We localized this enzyme at the calicoblastic ectoderm level, which is responsible for the precipitation of the skeleton. This localization supports the role of STPCA in the calcification process. In symbiotic scleractinian corals, calcification is stimulated by light, a phenomenon called "lightenhanced calcification" (LEC). The mechanism by which symbiont photosynthesis stimulates calcification is still enigmatic. We tested the hypothesis that coral genes are differentially expressed under light and dark conditions. By real-time PCR, we investigated the differential expression of STPCA to determine its role in the LEC phenomenon. Results show that the STPCA gene is expressed 2-fold more during the dark than the light. We suggest that in the dark, up-regulation of the STPCA gene represents a mechanism to cope with night acidosis.

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