The regulation of intracellular pH is a fundamental aspect of cell physiology that has received little attention in studies of the phylum Cnidaria, which includes ecologically important sea anemones and reef building corals. Like all organisms, cnidarians must maintain pH homeostasis to counterbalance reduction in intracellular pH, which can arise due to changes in either intrinsic or extrinsic parameters. Corals and sea anemones face natural daily changes in internal fluids where extracellular pH can range from 8.9 during the day to 7.4 at night. Furthermore, cnidarians are likely to experience future CO2-driven declines in seawater pH, a process known as ocean acidification. Here, we carried out the first mechanistic investigation to determine how cnidarian pHi responds to decreases in extracellular and intracellular pH. Using the anemone Anemonia viridis, we employed confocal live cell imaging and a pH-sensitive dye to track the dynamics of pHi after intracellular acidosis induced by acute exposure to decreases in seawater pH and NH4Cl prepulses. The investigation was conducted on cells that contained intracellular symbiotic algae (Symbiodinium sp.) and on symbiont-free endoderm cells. Experiments using inhibitors and sodium-free seawater indicate a potential role of Na+/H+ plasma membrane exchangers (NHE) in mediating pHi recovery following intracellular acidosis in both cell types. We also measured the buffering capacity of cells and obtained values between 20.8 and 43.8 mM/pH unit, comparable with other invertebrates. Our findings provide the first steps towards a better comprehension of acid-base regulation in these basal metazoans, for which information on cell physiology is extremely limited.
Laurent J., Venn A., Tambutté E., Ganot P., Allemand D. & Tambutté S., in press. Regulation of intracellular pH in cnidarians: response to acidosis in Anemonia viridis. FEBS Journal. Article (subscription required).