Giant clams live in symbiosis with extracellular dinoflagellates (zooxanthellae) in oligotrophic tropical waters of the Indo-Pacific. Zooxanthellae are found mainly in the colourful outer mantle. They donate photosynthates to the host during insolation, while the host absorbs and supplies them with exogenous nutrients, including inorganic phosphate (Pi). We report for the first time that the fluted giant clam, Tridacna squamosa, could absorb Pi at a higher rate in light than in darkness. We had also obtained the complete coding cDNA sequence of a homolog of sodium-dependent phosphate transport protein 2a (NPT2a-like) from its ctenidium, which comprised 1740 bp, coding for 580 amino acids of 63.8 kDa. The gene, NPT2a-like, was expressed strongly in the colorful outer mantle, the whitish inner mantle and the ctenidium, but weakly in all other organs examined including the kidney. The protein, NPT2a-like, was localized apically in the epithelium covering the ctenidial filaments, the upper epithelium of the outer mantle and the seawater-facing epithelium of the inner mantle. As these epithelia are in contact with seawater, all three organs could probably absorb Pi through the apical NPT2a-like. Nonetheless, the outer mantle could be a major site of light-enhanced Pi absorption, as the protein abundance of its NPT2a-like, but not those in the other two organs, was up-regulated by illumination. Photosynthesizing zooxanthellae need Pi, and the outer mantle could deliver the absorbed Pi directly to the zooxanthellae therein, reducing their need to compete with other host organs for the Pi absorbed by the ctenidium and circulating in the hemolymph.

中文翻译：

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有凹槽的巨型蛤蜊中的光增强磷酸盐吸收，Tridacna squamosa，导致其彩色外地幔中钠依赖性磷酸盐转运蛋白 2a 的表达增加

巨蛤与细胞外鞭毛藻（虫黄藻）共生生活在印度太平洋的贫营养热带水域。虫黄藻主要存在于五颜六色的外地幔中。它们在日晒期间向宿主提供光合作用，而宿主则吸收并提供外源性营养物质，包括无机磷酸盐 (Pi)。我们第一次报告了有凹槽的巨蛤，Tridacna squamosa，在光下比在黑暗中吸收 Pi 的速率更高。我们还从其鲎中获得了钠依赖性磷酸盐转运蛋白 2a（NPT2a 样）同源物的完整编码 cDNA 序列，该序列包含 1740 bp，编码 580 个氨基酸，63.8 kDa。NPT2a 样基因在五颜六色的外地幔、白色的内地幔和 ctenidium 中强烈表达，但在包括肾脏在内的所有其他器官检查中都很弱。蛋白质，NPT2a 样，位于顶端上皮覆盖 ctenidial 细丝，外地幔的上皮细胞和内地幔的面向海水的上皮细胞。由于这些上皮细胞与海水接触，所有三个器官都可能通过顶端 NPT2a 样吸收 Pi。尽管如此，外地幔可能是光增强 Pi 吸收的主要部位，因为其 NPT2a 样蛋白丰度，而不是其他两个器官中的蛋白丰度，被光照上调。进行光合作用的虫黄藻需要磷，外地幔可以将吸收的磷直接输送给其中的虫黄藻，从而减少它们与其他宿主器官竞争被蠊吸收并在血淋巴中循环的磷的需要。