Abstract
Giant clams live in symbiosis with extracellular zooxanthellae and display high rates of growth and shell formation (calcification) in light. Light-enhanced calcification requires an increase in the supply of Ca2+ to, and simultaneously an augmented removal of H+ from, the extrapallial fluid where shell formation occurs. We have obtained the complete coding cDNA sequence of Plasma Membrane Ca2+-ATPase (PMCA) from the thin and whitish inner mantle, which is in touch with the extrapallial fluid, of the giant clam Tridacna squamosa. The deduced PMCA sequence consisted of an apical targeting element. Immunofluorescence microscopy confirmed that PMCA had an apical localization in the shell-facing epithelium of the inner mantle, whereby it can actively secrete Ca2+ in exchange for H+. More importantly, the apical PMCA-immunofluorescence of the shell-facing epithelium of the inner mantle increased significantly after 12 h of exposure to light. The transcript and protein levels of PMCA/PMCA also increased significantly in the inner mantle after 6 or 12 h of light exposure. These results offer insights into a light-dependable mechanism of shell formation in T. squamosa and a novel explanation of light-enhanced calcification in general. As the inner mantle normally lacks light sensitive pigments, our results support a previous proposition that symbiotic zooxanthellae, particularly those in the colorful and extensible outer mantle, may act as light-sensing elements for the host clam.
Highlights
There is a general theme of metabolic interaction in alga–invertebrate symbioses
At the splice site A, PMCA2w/b of H. sapiens had an insert of 45 amino acids (Gly303-Lys347) while the Plasma Membrane Ca2+-ATPase (PMCA) of T. squamosa had an insert of 36 amino acids (Asp285-Asn320), as compared to the PMCA2z/b of H. sapiens (Figure 2)
The physiological functions of PMCA isoforms are defined by their localization to specific membrane compartments, and the subcellular localizations of a specific PMCA isoform is dependent on the amino acid residues in the splice site A as well as some other factors
Summary
There is a general theme of metabolic interaction in alga–invertebrate symbioses. In general, the heterotrophic animal host supplies inorganic nutrients to the photoautotrophic symbiotic zooxanthellae, which in turn donate photosynthate to the host to support its energy and nutrient demands. Class: Bivalvia, Order: Veneroida, Family: Cardiidae, and Subfamily Tridacninae They live in symbiosis with zooxanthellae (Symbiodinium; Clade A, C, and D), and can be found along coral reefs in the tropical Indo-Pacific. Microscopic tertiary tubules are located mainly in the surface tissue of the fleshy and colorful outer mantle, which is extensible and retractable (Figure 1). The symbiotic zooxanthellae residing in these tertiary tubules are well-positioned to receive sufficient light for photosynthesis. With contributions from the symbiotic zooxanthellae, giant clams can grow at high rates in nutrient deficient tropical seawater, the rates of shell formation (Klumpp and Griffith, 1994; Watanabe and Oba, 1999) and growth (Lucas et al, 1989) are critically dependent on the availability of light. The Sr/Ca ratio in the shell of the giant clam, Tridacna derasa, exhibits striking diurnal variations attributable to the daily cycle of light-enhanced calcification (Sano et al, 2012)
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