The fluted giant clam (Tridacna squamosa) increases nitrate absorption and upregulates the expression of a homolog of SIALIN (H+:2NO3− cotransporter) in the ctenidium during light exposure

Abstract

Giant clams flourish in nutrient-poor waters of tropical Indo-Pacific because they live in symbiosis with extracellular dinoflagellates (zooxanthellae) and receive photosynthates from them. Zooxanthellae have no access to the ambient seawater and are nitrogen-deficient; hence, they need to obtain nitrogen from the host clam. Unlike algae and plants, aquatic animals generally absorb little nitrate (NO3−) from the environment. Here, we report for the first time that the fluted giant clam, Tridacna squamosa, absorbed NO3− from the external seawater at a rate faster in light than in darkness. In addition, its ctenidium (gill) expressed a homolog of SIALIN (SIALIN-like), which is known to function as an electrogenic H+:2NO3− cotransporter and facilitate NO3− excretion in acinar cells of human salivary glands. The complete cDNA coding sequence of SIALIN-like of T. squamosa, which was derived from the host clam, consisted of 1905 bp and encoded for 634 amino acids of 69.6 kDa. It had the strongest expression in the ctenidium and weak expression in the colorful outer mantle and hepatopancreas. Being localized in the apical membrane of the epithelial cells at the tips of ctenidial filaments of T. squamosa, SIALIN-like was well positioned to absorb NO3− from the ambient seawater. Furthermore, the transcript level and protein abundance of SIALIN-like/SIALIN-like increased significantly in the ctenidium during 12 h of light exposure, denoting its possible role in light-enhanced NO3− absorption in T. squamosa. While scleractinian corals are known to absorb exogenous NO3− to benefit their intracellular zooxanthellae, they display light-independent NO3−absorption. Hence, the ability of T. squamosa to conduct light-enhanced NO3− absorption could be related to the extracellular location of its zooxanthellae.