Abstract
Giant clams display light-enhanced inorganic phosphate (Pi) absorption, but how the absorbed Pi is translocated to the symbiotic dinoflagellates living extracellularly in a tubular system is unknown. They can accumulate Pi in the kidney, but the transport mechanism remains enigmatic. This study aimed to elucidate the possible functions of sodium-dependent phosphate transporter protein 1-homolog (PiT1-like), which co-transport Na+ and H2PO4–, in these two processes. The complete cDNA coding sequence of PiT1-like, which comprised 1,665 bp and encoded 553 amino acids (59.3 kDa), was obtained from the fluted giant clam, Tridacna squamosa. In the kidney, PiT1-like was localized in the plasma membrane of nephrocytes, and could therefore absorb Pi from the hemolymph. As the gene and protein expression levels of PiT1-like were up-regulated in the kidney during illumination, PiT1-like could probably increase the removal of Pi from the hemolymph during light-enhanced Pi uptake. In the ctenidial epithelial cells, PiT1-like had a basolateral localization and its expression was also light-dependent. It might function in Pi sensing and the absorption of Pi from the hemolymph when Pi was limiting. In the outer mantle, PiT1-like was localized in the basolateral membrane of epithelial cells forming the tertiary tubules. It displayed light-enhanced expression levels, indicating that the host could increase the translocation of Pi from the hemolymph into the tubular epithelial cells and subsequently into the luminal fluid to support increased Pi metabolism in the photosynthesizing dinoflagellates. Taken together, the accumulation of Pi in the kidney of giant clams might be unrelated to limiting the availability of Pi to the symbionts to regulate their population.
Highlights
Tropical waters are low in nutrients, but coral reefs, which are hubs of a wide variety of marine organisms including scleractinian corals and giant clams, can be found in shallow tropical waters with adequate irradiance
Giant clams are inhabitants of the Indo-Pacific reef ecosystems. They can grow rapidly despite the shortage of nutrients and are the largest of all bivalves, because they live in symbiosis with mainly three genera (Symbiodinium, Cladocopium, and Durusdinium; LaJeunesse et al, 2018) of phototrophic dinoflagellates belonging to the family Symbiodiniaceae (Hernawan, 2008; Weber, 2009; DeBoer et al, 2012; Ikeda et al, 2017; Lim et al, 2019)
The complete cDNA coding sequence of PiT1-like obtained from the kidney of T. squamosa (GenBank accession number MN256447) comprised 1,665 bp, and the deduced amino acid sequence had 553 amino acids with an estimated molecular mass of 59.3 kDa
Summary
Tropical waters are low in nutrients, but coral reefs, which are hubs of a wide variety of marine organisms including scleractinian corals and giant clams, can be found in shallow tropical waters with adequate irradiance. Giant clams (genus: Tridacna or Hippopus) are inhabitants of the Indo-Pacific reef ecosystems They can grow rapidly despite the shortage of nutrients and are the largest of all bivalves, because they live in symbiosis with mainly three genera (Symbiodinium, Cladocopium, and Durusdinium; LaJeunesse et al, 2018) of phototrophic dinoflagellates belonging to the family Symbiodiniaceae (Hernawan, 2008; Weber, 2009; DeBoer et al, 2012; Ikeda et al, 2017; Lim et al, 2019). Symbiotic dinoflagellates (zooxanthellae) reside extracellularly in a branched tubular system that originates from the digestive tract of the host (Norton and Jones, 1992) They are located mainly inside numerous tertiary tubules in the colorful and extensible outer mantle that contains pigments and iridocytes (Norton and Jones, 1992). Dissolved Pi comprises dihydrogen phosphate (H2PO4−), hydrogen phosphate (HPO42−), and phosphate (PO43−), with HPO42− and H2PO4− being the major components in water at pH 7
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