Light-enhanced phosphate absorption in the fluted giant clam, Tridacna squamosa, entails an increase in the expression of sodium-dependent phosphate transporter 2a in its colourful outer mantle

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 (P i ). We report for the first time that the fluted giant clam, Tridacna squamosa , could absorb P i 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 P i through the apical NPT2a-like. Nonetheless, the outer mantle could be a major site of light-enhanced P i 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 P i , and the outer mantle could deliver the absorbed P i directly to the zooxanthellae therein, reducing their need to compete with other host organs for the P i absorbed by the ctenidium and circulating in the hemolymph.