Transepithelial absorption of exogenous inorganic carbon in the ctenidium of the giant clam, Tridacna squamosa involves a basolateral electrogenic Na+-HCO3- cotransporter 1 that displays light-enhanced gene and protein expression levels

Giant clams live in symbiosis with phototrophic dinoflagellates. They need to increase the uptake of inorganic carbon (C-i) from the ambient seawater to support light-enhanced shell formation in the host and photosynthesis in the symbionts during illumination. The ctenidium is the major site of light-enhanced C-i absorption in the fluted giant clam, Tridacna squamosa. Catalyzed by dual-domain carbonic anhydrase, exogenous HCO3- is dehydrated to CO2, which permeates the apical membrane of the ctenidial epithelium and is rehydrated back to HCO3- in the cytoplasm. However, the molecular mechanism that transports cytoplasmic HCO3- through the basolateral membrane to the hemolymph has not been elucidated. We have obtained from the ctenidium of T. squamosa the complete cDNA coding sequence of a homolog of electrogenic Na+-HCO(3)(-)cotransporter 1 (NBCe1-like), which comprised 3450 bp, encoding a protein (NBCe1-like) of 1142 amino acids and 128.9 kDa. NBCe1-like had a basolateral localization in epithelial cells covering the ctenidial filament and those surrounding the tertiary water channels. Light exposure led to significant increases in the transcript and protein levels of NBCe1-like/NBCe1-like in the ctenidium of T. squamosa, indicating that NBCe1-like could be involved in the increased transport of cytoplasmic HCO3- across the basolateral membrane into the hemolymph during illumination. Additionally, NBCe1-like might also participate in light-enhanced NO3- absorption in T. squamosa, due to the replacement of aspartate (residue 585) with glutamine. Exogenous NO3- could be absorbed by the ctenidial epithelial cells through the apical H+:NO3- cotransporter (SIALIN) and then transported through the basolateral NBCe1-like to the hemolymph.

Automated summary

Giant clams living in symbiosis with phototrophic dinoflagellates increase inorganic carbon uptake in order to support shell formation and photosynthesis. The Na+-HCO(3)(-) cotransporter may play a role in transporting cytoplasmic HCO3- across the basolateral membrane into the hemolymph.