The acid-base relevant molecules carbon dioxide (CO2), protons (H + ), and bicarbonate (HCO3 -) are substrates and end products of some of the most essential physiological functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acidbase homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H + and HCO3have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments 2 presents organisms with distinct acid-base challenges that are not found in terrestrial environments. These include a relatively high CO2 relative to O2 solubility that prevents internal CO2/HCO3accumulation to buffer pH, a lower O2 content that may favor anaerobic metabolism, and variable environmental CO2, pH and O2 levels that require dynamic adjustments in acid-base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example, acidifying mechanisms can enhance O2 delivery by red blood cells, lead to ammonia trapping for excretion or buoyancy purposes, or lead to CO2 accumulation to promote photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms can serve to promote calcium carbonate skeletal formation. This non-exhaustive review summarizes some of the distinct acid-base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.

Reef-building corals maintain an intracellular photosymbiotic association with dinoflagellate algae. As the algae are hosted inside the symbiosome, all metabolic exchanges must take place across the symbiosome membrane. Using functional studies in Xenopus oocytes, immunolocalization, and confocal Airyscan microscopy, we established that Acropora yongei Rh (ayRhp1) facilitates transmembrane NH 3 and CO 2 diffusion and that it is present in the symbiosome membrane. Furthermore, ayRhp1 abundance in the symbiosome membrane was highest around midday and lowest around midnight. We conclude that ayRhp1 mediates a symbiosomal NH 4 + -trapping mechanism that promotes nitrogen delivery to algae during the day—necessary to sustain photosynthesis—and restricts nitrogen delivery at night—to keep algae under nitrogen limitation. The role of ayRhp1-facilitated CO 2 diffusion is less clear, but it may have implications for metabolic dysregulation between symbiotic partners and bleaching. This previously unknown mechanism expands our understanding of symbioses at the immediate animal-microbe interface, the symbiosome.

The calcium carbonate skeletons of corals provide the underlying structure of coral reefs; however, the cellular mechanisms responsible for coral calcification remain poorly understood. In osteoblasts from vertebrate animals, a Na+/Ca2+ exchanger (NCX) present in the plasma membrane transports Ca2+ to the site of bone formation. The aims of this study were to establish whether NCX exists in corals and its localization within coral cells, which are essential first steps to investigate its potential involvement in calcification. Data mining identified genes encoding for NCX proteins in multiple coral species, a subset of which were more closely related to NCXs from vertebrates (NCXA). We cloned NCXA from Acropora yongei (AyNCXA), which, unexpectedly, contained a peptide signal that targets proteins to vesicles from the secretory pathway. AyNCXA subcellular localization was confirmed by heterologous expression of fluorescently tagged AyNCXA protein in sea urchin embryos, which localized together with known markers of intracellular vesicles. Finally, immunolabeling of coral tissues with specific antibodies revealed AyNCXA was present throughout coral tissue. AyNCXA was especially abundant in calcifying cells, where it exhibited a subcellular localization pattern consistent with intracellular vesicles. Altogether, our results demonstrate AyNCXA is present in vesicles in coral calcifying cells, where potential functions include intracellular Ca2+ homeostasis and Ca2+ transport to the growing skeleton as part of an intracellular calcification mechanism.