UC San Diego

Coral biomineralization is an important physiological process responsible for creating the massive reef ecosystems found throughout the world. Biomineralization requires the transport of Ca2+ and dissolved inorganic carbon (DIC) to the subcalicoblastic medium (SCM), which is the alkaline microenvironment located underneath coral tissues where the skeleton is built. Additionally, H+ are constantly produced as a byproduct of calcification and must be removed from the SCM to prevent acidification that would impede biomineralization, however, the coral cellular mechanisms that perform these processes are mostly unknown. The solute carrier transporter 9 gene family (SLC9) transports Na+ in exchange for H+ across a cellular membrane. It is found throughout the animal kingdom to be essential for pH regulation and acid/base homeostasis. For the first time, an SLC9 protein has been characterized in corals. In the coral Acropora yongei, this SLC9 protein is found to share the strongest homology with the Na+/H+ exchangers (NHEs) 1-5 and potentially experiences post-translational modifications, such as glycosylation and phosphorylation. Specific anti-AcroporaNHE antibodies were developed and validated, which allowed investigation of protein abundance and cellular localization in coral tissues. Immunolocalization analysis of AcroporaNHE revealed it to be highly abundant in the calicoblastic cells that secrete skeleton. AcroporaNHE also localizes to desmocytes, intracellular structures within symbiocyte cells, and the apical membrane of oral ectodermal cells. The presence of NHE in the calicoblastic cells suggests its involvement in intracellular pH regulation, as well as in removing H+ from the SCM. Future work is needed to confirm the physiological functions of AcroporaNHE.