Options
Molecular biology of ammonium and nitrate transporters in the symbiotic dinoflagellates of the fluted giant clam, Tridacna squamosa
Author
Pang, Caryn Zhiqin
Supervisor
Chew, Shit Fun
Abstract
Giant clams survive in the nutrient-poor waters of the tropical Indo-Pacific by maintaining a mutualistic relationship with phototrophic dinoflagellates. The Symbiodineaceae dinoflagellates that live extracellularly in the zooxanthellal tubular system, which extends into the outer mantle of the giant clam, Tridacna squamosa, includes Symbiodinium, Cladocopium, and Durusdinium. As these symbionts have no access to ambient seawater, they are nitrogen deficient and rely entirely on the host clam for the supply of nitrogen. Nitrogen is essential for synthesizing amino acids and other nitrogenous compounds. Tridacna squamosa does not normally excrete ammonia and despite the degradation of nitrogenous compounds in its body, it is still able to maintain a low level of ammonia in its hemolymph. Thus, the nitrogen deficient symbionts must be able to uptake NH4+ from the host clam through ammonium transporters (AMTs). At the same time, T. squamosa is known to absorb nitrate (NO3−) despite lacking nitrate reductase and nitrite reductase to assimilate NO3−. Thus, the NO3− could be taken up by the symbionts via nitrate transporters (NRTs) for assimilation. This study reports three complete cDNA coding sequences of AMT2 and NRT2 cloned from Symbiodinium spp. (Symb-AMT2 and Symb-NRT2), Cladocopium spp. (Clad-AMT2 and Clad-NRT2), and Durusdinium spp. (Duru-AMT2 and Duru-NRT2) in the outer mantle of T. squamosa. Through immunofluorescence staining of the outer mantle, both zooxanthellae-AMT2 (Zoox-AMT2) and zooxanthellae-NRT2 (Zoox-NRT2) were localized to the plasma membranes of the dinoflagellates. Using quantitative polymerase chain reaction, the mRNA expression of Symb-AMT2, Clad-AMT2, Duru-AMT2, Symb-NRT2, Clad-NRT2, and Duru-NRT2 were strongest in the outer mantle compared to the inner mantle, ctenidium, foot muscle, and hepatopancreas. Based on the transcript levels of form II ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcII), AMT2 and NRT2, Durusdinium spp. was the dominant genus in the outer mantle, inner mantle and foot muscle. This possibly indicate that T. squamosa acquired from Vietnam could be residing in waters of a higher temperature. The potential of the three genera of dinoflagellates residing in the outer mantle, inner mantle, and foot muscle for NH4+ and NO3− transport with reference to their phototrophic potential were evaluated based on the ratios of the transcript levels of AMT2 and NRT2 to that of rbcII. Results in this study demonstrated that Durusdinium spp. in the outer mantle had the greatest potential for NH4+ transport while Symbiodinium spp. had the greatest potential for NO3− transport in all three organs. As the outer mantle contains majority of symbiotic dinoflagellates and is thus the main site for photosynthesis, there is a possible functional relationship between NH4+ and NO3− uptake and photosynthetic carbon-fixation. Indeed, there was a ∼4.1-fold increase in the relative protein abundance of Zoox-AMT2 in the outer mantle after 12 h of light exposure suggesting that light-enhanced transport of NH4+ occurred in the photosynthesizing dinoflagellates. However, there was no significant difference in the relative protein abundance of Zoox-NRT2 in the outer mantle after exposure to 12 h light. Thus, Zoox-NRT2 might be controlled by post-translational modifications instead.
Date Issued
2022
Call Number
QL430.7.T7 Pan
Date Submitted
2022