Photosynthetic symbioses in Cnidaria are major contributors to the structural and trophic foundation of shallow water coral reef ecosystems at tropical and subtropical latitudes. Reef-building corals form a symbiosis with photosynthetic dinoflagellates

INTRODUCTION Photosynthetic symbioses in Cnidaria are major contributors to the structural and trophic foundation of shallow water coral reef ecosystems at tropical and subtropical latitudes. Reef-building corals form a symbiosis with photosynthetic dinoflagellates belonging to the genus Symbiodinium, which reside intracellularly in host endodermal cells (Muscatine, 1990; Trench, 1993). This endosymbiosis provides corals with access to a valuable source of photosynthetically fixed carbon that is used for host respiration and other essential processes (Muscatine, 1990; Venn et al., 2008; Yellowlees et al., 2008). The relationship between corals and their symbiotic dinoflagellates is sensitive to changes in the marine environment associated with climate change (Hoegh-Guldberg, 1999; HoeghGuldberg et al., 2007). Elevations in seawater temperature and ocean acidification can exert physiological stress on both coral and dinoflagellate partners (Lesser, 2007; Anthony et al., 2008; Crawley et al., 2010). Much research on coral biology is currently directed towards improving our understanding of how and why corals are sensitive to environmental change, but the field is currently impeded by a limited knowledge of the basics of the cell physiology underpinning the coral–dinoflagellate symbiosis (Weis et al., 2008; Weis and Allemand, 2009). Intracellular cytosolic pH (pHi) is a fundamental parameter of cell physiology and is potentially very important to the cell biology of the coral–dinoflagellate symbiosis. pHi affects most aspects of cell biology, including protein synthesis, enzyme activity and cell signalling. As such, it has a strong influence over the physiology of all organisms and most organisms seek to minimise variations in pHi by a system of intracellular buffers and membrane transporters in order to maintain steady-state metabolism (Busa and Nuccitelli, 1984; Casey et al., 2010). When changes in pHi do occur, they are frequently linked with transitions such as changes in rates of cell metabolism and events such as cell activation and division (Roos and Boron, 1981; Busa, 1986; Casey et al., 2010). In algae, pHi also significantly increases on exposure to light because of the activity of photosynthesis (Smith and Raven, 1979; Kurkdjian and Guern, 1989). For example, differences of 0.4–0.5 pH units have been observed between photosynthesising and non-photosynthesising cells of the giant single-celled algae Chaetomorpha darwinii (Raven and Smith, 1980) and the dinoflagellate Prorocentrum micans. The single previous study performed on cnidarian pHi suggests that coral pHi may also be responsive to light (Venn et al., 2009). It was observed that pHi in coral cells containing dinoflagellate symbionts exposed to light have a higher pHi than those kept in dark conditions. However, for a more complete understanding of the interactions between host pHi and photosynthesis, further research into pHi dynamics is required as this previous study was built on a single time point measurement with a fixed light intensity. Important advances in the understanding of intracellular pH regulation in many organisms including corals have been facilitated by the use of pH-sensitive intracellular dyes (Dubbin et al., 1993; Lemasters et al., 1999). Certain dyes [such as carboxyseminaphthorhodafluor-1 (SNARF-1)] emit fluorescence at two wavelengths, which can be calibrated to the concentration of SUMMARY The regulation of intracellular pH (pHi) is a fundamental aspect of cell physiology that has received little attention in studies of reef-building corals and symbiotic cnidarians. Here, we investigated the hypothesis that dynamic changes in the pHi of coral host cells are controlled by the photosynthetic activity of the coralʼs dinoflagellate symbionts. Using live cell imaging and the pHsensitive dye SNARF-1, we tracked pH in symbiont-containing and symbiont-free cells isolated from the reef coral Stylophora pistillata. We characterised the response of coral pHi in the presence of a photosynthetic inhibitor, the dynamics of coral pHi during light exposure and how pHi values vary on exposure to a range of irradiance levels lying within the coralʼs photosynthesis–irradiance response curve. Our results demonstrate that increases in coral pHi are dependent on photosynthetic activity of intracellular symbionts and that pHi recovers under darkness to values that match those of symbiont-free cells. Furthermore, we show that the timing of the pHi response is governed by irradiance level and that pHi increases to irradiancespecific steady-state values. Minimum steady-state pHi values of 7.05±0.05 were obtained under darkness and maximum values of 7.46±0.07 were obtained under saturating irradiance. As changes in pHi often affect organism homeostasis, there is a need for continued research into acid/base regulation in symbiotic corals. More generally, these results represent the first characterization of photosynthesis-driven pHi changes in animal cells.