Symbiodiniaceae Dinoflagellates Express Urease in Three Subcellular Compartments and Upregulate its Expression Levels in situ in Three Organs of a Giant Clam (Tridacna squamosa) During Illumination

Abstract

Giant clams harbor three genera of symbiotic dinoflagellates (Symbiodinium, Cladocopium, and Durusdinium) as extracellular symbionts (zooxanthellae). While symbiotic dinoflagellates can synthesize amino acids to benefit the host, they are nitrogen‐deficient. Hence, the host must supply them with nitrogen including urea, which can be degraded to ammonia and carbon dioxide by urease (URE). Here, we report three complete coding cDNA sequences of URE, one for each genus of dinoflagellate, obtained from the colorful outer mantle of the giant clam, Tridacna squamosa. The outer mantle had higher transcript level of Tridacna squamosa zooxanthellae URE (TSZURE) than the whitish inner mantle, foot muscle, hepatopancreas, and ctenidium. TSZURE was immunolocalized strongly and atypically in the plastid, moderately in the cytoplasm, and weakly in the cell wall and plasma membrane of symbiotic dinoflagellates. In the outer mantle, illumination upregulated the protein abundance of TSZURE, which could enhance urea degradation in photosynthesizing dinoflagellates. The urea‐nitrogen released could then augment synthesis of amino acids to be shared with the host for its general needs. Illumination also enhanced gene and protein expression levels of TSZURE/TSZURE in the inner mantle and foot muscle, which contain only small quantities of symbiotic dinoflagellate, have no iridocyte, and lack direct exposure to light. With low phototrophic potential, dinoflagellates in the inner mantle and foot muscle might need to absorb carbohydrates in order to assimilate the urea‐nitrogen into amino acids. Amino acids donated by dinoflagellates to the inner mantle and the foot muscle could be used especially for synthesis of organic matrix needed for light‐enhanced shell formation and muscle protein, respectively.

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

“…It is also supportive of a functional relationship between urea degradation and carbon-fixation in these symbionts in the outer mantle (Figure 2). Indeed, illumination up-regulates the protein abundance of Zoox-URE, despite the transcript level of Zoox-URE remains unchanged, in the outer mantle of T. squamosa (Table 1; Ip et al, 2020b), corroborating the proposition that the increased degradation of urea to release NH 3 for amino acid synthesis is synchronized with light-enhanced CO 2 fixation (Figure 2).…”

“…The absorbed urea must be delivered to the symbionts for assimilation because it cannot be metabolized by the host. Besides having the highest phototrophic potential (Poo et al, 2020), the outer mantle of T. squamosa also expresses the highest transcript level of zooxanthellal URE (Zoox-URE), which is derived comprehensively from Symbiodinium, Cladocopium, and Durusdinium, among the five organs studied (Ip et al, 2020b). This is expected as the outer mantle has the greatest quantity of symbiotic dinoflagellates and receives the greatest irradiance.…”

“…Urea is a good source of N and CO 2 for symbiotic dinoflagellates, which possess urease (URE) to catalyze the degradation of urea. URE is localized in the plastid, where it can facilitate the effective harness of urea-carbon and urea-N for photosynthesis and amino acid synthesis, respectively (Ip et al, 2020b). URE is also localized in the dinoflagellates' cytoplasm, where it can degrade some of the absorbed urea to ammonia.…”

“…Zoox-URE is strongly immunolocalized in the plastids of symbiotic dinoflagellates located in the outer mantle of T. squamosa (Ip et al, 2020b). Inside the plastid, urea-carbon and urea-N can be harnessed effectively for photosynthesis and amino acid synthesis, respectively (Figure 2).…”

“…The inner mantle lacks iridocytes and direct light exposure, but it contains symbiotic dinoflagellates concentrated in a specific area near the valve hinge (Poo et al, 2020). The transcript level and protein abundance of Zoox-URE/Zoox-ZURE in the inner mantle are upregulated during illumination (Table 1; Ip et al, 2020b). Probably, this can increase the release of urea-N for amino acid production in the symbionts, and the amino acids donated to the inner mantle cells can augment the synthesis of proteinaceous matrix for shell formation (Wilbur and Saleuddin, 1983).…”