Journal
FRONTIERS IN MARINE SCIENCE
Volume 8, Issue -, Pages -Publisher
FRONTIERS MEDIA SA
DOI: 10.3389/fmars.2021.655714
Keywords
photosynthesis; shell formation; Symbiodiniaceae; symbiosis; zooxanthellae
Funding
- Singapore Ministry of Education [R-154-000-B69-114]
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The study reveals that PiT1-like in the kidney of giant clams can absorb and remove P-i taken up during illumination, while PiT1-like in the ctenidial epithelial cells and outer mantle may play a role in P-i sensing and translocation.
Giant clams display light-enhanced inorganic phosphate (P-i) absorption, but how the absorbed P-i is translocated to the symbiotic dinoflagellates living extracellularly in a tubular system is unknown. They can accumulate P-i 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 P-i 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 P-i from the hemolymph during light-enhanced P-i uptake. In the ctenidial epithelial cells, PiT1-like had a basolateral localization and its expression was also light-dependent. It might function in P-i sensing and the absorption of P-i from the hemolymph when P-i 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 P-i from the hemolymph into the tubular epithelial cells and subsequently into the luminal fluid to support increased P-i metabolism in the photosynthesizing dinoflagellates. Taken together, the accumulation of P-i in the kidney of giant clams might be unrelated to limiting the availability of P-i to the symbionts to regulate their population.
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