Browsing by Author "Ng, Siow Y."
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- PublicationMetadata onlyMolecular characterization, cellular localization and light-dependent expression of dinoflagellate vacuolar-type h+-atpase (VHA) subunit B in the colourful outer mantle of the giant clam, tridacna squamosa, indicate the involvement of VHA in CO2 uptake in the photosynthesizing symbionts(Elsevier, 2021)
;Raagavi Mani ;Boo, Mel Veen ;Poo, Jeslyn Shi Ting ;Ng, Siow Y.; Ip, Yuen KwongGiant clams are animal-dinoflagellate associations found in Indo-Pacific reef ecosystems. The clam host obtains organic nutrients from phototrophic dinoflagellates of genera Symbiodinium, Cladocopium, and Durusdinium, which reside extracellularly as symbionts (alias zooxanthellae) in the luminal fluid of zooxanthellal tubules located mainly in the colourful outer mantle. The host also needs to supply the symbionts with inorganic carbon for photosynthesis. Symbiont photosynthesis can be impeded by inhibitors of vacuolar H+-ATPase (VHA) because the host possesses a carbon concentration mechanism consisting of VHA to facilitate the supply of CO2(aq) to the symbionts. Here, we report that VHA was also expressed in dinoflagellates residing in the outer mantle of the fluted giant clam, Tridacna squamosa. Three complete cDNA coding sequences of VHA subunit B (VHA-B), one for each genus of dinoflagellate, had been obtained, and each sequence comprised 1482 bp, encoding a protein of 493 amino acids (~55 kDa). As these three sequences were highly similar, we could only design real-time PCR primers to quantify comprehensively zooxanthellae-VHA-B (Zoox-VHA-B) that represented VHA-B of all three genera of dinoflagellates. The outer mantle had the highest transcript level of Zoox-VHA-B among the three organs studied, and illumination led to a significant increase in the protein abundance of Zoox-VHA-B therein. Zoox-VHA-B was immunolocalized to intracellular vesicles, which could apparently align and fuse with the plasma membrane, in the symbiotic dinoflagellates. Overall, these results indicate that photosynthesizing symbionts could increase the capacity of H+ secretion through VHA-containing vesicles to promote the dehydration of luminal HCO3− and the absorption of CO2(aq) during illumination.WOS© Citations 2Scopus© Citations 3 80 - PublicationOpen AccessSymbiotic dinoflagellates of the giant clam, tridacna squamosa, express an extracellular Alpha carbonic anhydrase associated with the plasma membrane to promote HCO3− dehydration and CO2 uptake during illuminationGiant clams generally harbor phototrophic Symbiodiniaceae dinoflagellates of genera Symbiodinium, Cladocopium, and Durusdinium. The coccoid symbiotic dinoflagellates (zooxanthellae) reside extracellularly inside the lumen of zooxanthellal tubules in the colorful outer mantle. They obtain from the host inorganic carbon (Ci) for photosynthesis and supply photosynthate to the host. The outer mantle has a host-derived carbon concentration mechanism (CCM) to facilitate the transport of Ci from the hemolymph into the luminal fluid. To regulate Ci uptake, the symbionts probably possess their own CCMs that comprise an extracellular alpha carbonic anhydrase (αCA) and a proton transporter. Indeed, we obtained from the outer mantle of the giant clam, Tridacna squamosa, three complete cDNA coding sequences of a membrane-associated αCA derived from Symbiodinium (Symb-αCA), Cladocopium (Clad-αCA), and Durusdinium (Duru-αCA), which consisted of 2808, 2847, and 2829 bp, respectively. The respective encoded proteins had 935 (104.7 kDa), 948 (106.1 kDa), and 942 (105 kDa) amino acids, each containing a transmembrane domain. The outer mantle had the highest level of Duru-αCA transcripts. Phenogramic analyses denoted Duru-αCA as an extracellular CA closely associated with human CA4 and had a dinoflagellate-origin. Using an antibody that could react comprehensively with zooxanthellae-αCAs (Zoox-αCA) derived from all three genera of dinoflagellate, immunofluorescence microscopy revealed immuno-labeling at the dinoflagellate’s plasma membrane. As Symb-αCA, Clad-αCA, and Duru-αCA possessed extracellular catalytic domains, they could catalyze the dehydration of HCO3− to CO2 in the luminal fluid. Importantly, illumination led to significant increases in the gene and protein expression levels of Zoox-αCA/Zoox-αCA in the outer mantle of T. squamosa. Taken together, Zoox-αCA could be part of the symbiont’s light-enhanced external CCM to promote and regulate the acquisition of Ci from the host for photosynthesis. Our results also suggested that the potentials of generating CO2 adjacent to the symbionts’ plasma membrane could differ among different phylotypes of Symbiodinium and Cladocopium.
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