Using glutamine synthetase 1 to evaluate the symbionts' potential of ammonia assimilation and their responses to illumination in five organs of the giant clam, Tridacna squamosa

Nitrogen-deficient symbiotic dinoflagellates (zooxanthellae) living inside the fluted giant clam, Tridacna squamosa, need to obtain nitrogen from the host. Glutamine synthetase 1 (GS1) is a cytosolic enzyme that assimilates ammonia into glutamine. We determined the transcript levels of zooxanthellal GS1 (Zoox-GS1), which represented comprehensively GS1 transcripts of Symbiodinium, Cladocopium and Durusdinium, in five organs of T. squamosa. The outer mantle had significantly higher transcript level of Zoox-GS1 than the inner mantle, foot muscle, hepatopancreas and ctenidium, but the transcript ratios of Zoox-GS1 to zooxanthellal form II ribulose-1,5-bisphosphate carboxylase/oxygenase (Zoox-rbcII), which represented the potential of ammonia assimilation relative to the phototrophic potential, were comparable among these five organs. Based on transcript ratios of Zoox-GS1 to zooxanthellal Urease (Zoox-URE), the outer mantle had the highest potential of urea degradation relative to ammonia assimilation among the five organs, probably because urea degradation could furnish CO2 and NH3 for photosynthesis and amino acid synthesis, respectively, in the symbionts therein. The protein abundance of Zoox-GS1 was upregulated in the outer mantle and the inner mantle during illumination. Zoox-GS1 could catalyze light-enhanced glutamine formation using ammonia absorbed from the host or ammonia released through urea degradation in the cytoplasm. The glutamine produced could be used to synthesize other nitrogenous compounds, including amino acids in the cytoplasm or in the plastid of the dinoflagellates. Some of the amino acids synthesized by the symbionts in the inner mantle and foot muscle could be donated to the host to support shell organic matrix formation and muscle production, respectively.

Automated summary

In the giant clam Tridacna squamosa, the outer mantle exhibited the highest transcript levels of symbiotic zooxanthellal GS1, indicating a high potential for urea degradation, which could provide essential substances for photosynthesis and amino acid synthesis in the symbionts. Light enhanced the protein abundance of Zoox-GS1 in both the outer and inner mantle, facilitating glutamine formation.