Differential localization of ion transporters suggests distinct cellular mechanisms for calcification and photosynthesis between two coral species.

Ion transport is fundamental for multiple physiological processes, including but not limited to pH regulation, calcification, and photosynthesis. Here, we investigated ion-transporting processes in tissues from the corals Acropora yongei and Stylophora pistillata, representatives of the complex and robust clades that diverged over 250 million years ago. Antibodies against complex IV revealed that mitochondria, an essential source of ATP for energetically costly ion transporters, were abundant throughout the tissues of A. yongei. Additionally, transmission electron microscopy revealed septate junctions in all cell layers of A. yongei, as previously reported for S. pistillata, as well as evidence for transcellular vesicular transport in calicoblastic cells. Antibodies against the alpha subunit of Na(+)/K(+)-ATPase (NKA) and plasma membrane Ca(2+)-ATPase (PMCA) immunolabeled cells in the calicoblastic epithelium of both species, suggesting conserved roles in calcification. However, NKA was abundant in the apical membrane of the oral epithelium in A. yongei but not S. pistillata, while PMCA was abundant in the gastroderm of S. pistillata but not A. yongei. These differences indicate that these two coral species utilize distinct pathways to deliver ions to the sites of calcification and photosynthesis. Finally, antibodies against mammalian sodium bicarbonate cotransporters (NBC; SLC4 family) resulted in strong immunostaining in the apical membrane of oral epithelial cells and in calicoblastic cells in A. yongei, a pattern identical to NKA. Characterization of ion transport mechanisms is an essential step toward understanding the cellular mechanisms of coral physiology and will help predict how different coral species respond to environmental stress.