Using form II ribulose-1,5-bisphosphate carboxylase/oxygenase to estimate the phototrophic potentials of Symbiodinium, Cladocopium and Durusdinium in various organs of the fluted giant clam, Tridacna squamosa, and to evaluate their responses to light upon isolation from the host

Mutualistic associations with symbiotic dinoflagellates (zooxanthellae) enable invertebrate hosts to thrive in tropical waters that are shallow and oligotrophic. Giant clams can harbor multiple species of symbiotic dinoflagellates (Family: Symbiodiniaceae) from mainly three genera, Symbiodinium, Cladocopium and Durusdinium, but whether they have distinct physiological functions at the genus level in the holobiont remains unclear. As symbiotic dinoflagellates use form II ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) to fix inorganic carbon, we attempted to estimate the phototrophic potentials and thus the relative abundances of Symbiodinium, Cladocopium and Durusdinium in five organs (outer mantle, inner mantle, foot muscle, ctenidium and hepatopancreas) of the fluted giant clam Tridacna squamosa through quantitative real-time PCR (qPCR) with primers that are specific to form II RuBisCO gene sequences (rbcII) of each genus. Based on the transcript levels of rbcII derived from Symbiodinium (Symb-rbcII), Cladocopium (Clad-rbcII) and Durusdinium (Duru-rbcII), we demonstrated that the symbiont population of T. squamosa from Vietnam was dominated by Durusdinium. Furthermore, the proportion of Symb-rbcII, Clad-rbcII and Duru-rbcII, and hence the phototrophic potentials of Symbiodinium, Cladocopium and Durusdinium, varied among five organs of T. squamosa, and along the length of the outer mantle. For dinoflagellates freshly isolated from the outer mantle of T. squamosa, Symb-rbcII, Clad-rbcII and Duru-rbcII exhibited different responses to light at the transcriptional level. Importantly, these results corroborate the proposition that the association with different genera and/or species of dinoflagellates might confer distinct physiological advantages to the host clam, which differs under various environmental conditions.

Automated summary

This study investigated the phototrophic potentials and relative abundances of Symbiodinium, Cladocopium, and Durusdinium in different organs of the giant clam Tridacna squamosa, finding that the symbiont population was dominated by Durusdinium in individuals from Vietnam. The proportions and phototrophic potentials of the three dinoflagellate genera varied among organs and along the outer mantle length, indicating different physiological responses. These results suggest that the association with different genera or species of dinoflagellates may provide distinct physiological advantages to the host clam under varying environmental conditions.