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.

中文翻译：

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有凹槽的巨蛤（Tridacna squamosa）在光照期间增加硝酸盐吸收并上调 SIALIN（H+:2NO3− 协同转运蛋白）同系物在 ctenidium 中的表达

巨蛤在热带印度洋-太平洋地区营养贫乏的水域繁衍生息，因为它们与细胞外甲藻（虫黄藻）共生并从它们那里获得光合作用。虫黄藻无法进入周围的海水，并且缺氮；因此，它们需要从寄主蛤中获取氮。与藻类和植物不同，水生动物通常从环境中吸收很少的硝酸盐 (NO3−)。在这里，我们首次报道了有凹槽的巨型蛤蜊 Tridacna squamosa 在光下比在黑暗中吸收外部海水中的 NO3− 的速度更快。此外，它的 ctenidium（鳃）表达 SIALIN（SIALIN 样）的同源物，已知其作为生电 H+:2NO3− 协同转运蛋白起作用，并促进人唾液腺腺泡细胞中的 NO3− 排泄。来自寄主蛤的鳞翅目甲壳虫SIALIN-like的完整cDNA编码序列由1905 bp组成，编码634个氨基酸，69.6 kDa。它在栉水母中表达最强，在多彩的外地幔和肝胰腺中表达较弱。SIALIN 样位于 T.squamosa 的 ctenidial 细丝尖端的上皮细胞的顶膜中，可以很好地吸收周围海水中的 NO3-。此外，在光照射 12 小时期间，在 ctenidium 中 SIALIN 样/SIALIN 样的转录水平和蛋白质丰度显着增加，表明其可能在 T.squamosa 中光增强 NO3− 吸收中发挥作用。虽然已知石珊瑚会吸收外源性 NO3− 以有益于它们的细胞内虫黄藻，它们显示出与光无关的 NO3-吸收。因此，T.squamosa 进行光增强的 NO3− 吸收的能力可能与其虫黄藻的细胞外位置有关。