Symbiotic 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 illumination

Giant clams are invertebrates that form mutualistic associations with Symbiodiniaceae dinoflagellates. Despite their ecological significance, gaps persist regarding our understanding of their trophic ecology. Specifically, it is unknown whether Symbiodiniaceae‐derived photosynthates are metabolized differently according to species and organ. Therefore, we maintained Tridacna derasa and T. noae for 3 months in a well‐lit recirculated system without food supply. Samples were taken from eight organs and underwent lipid extraction and fatty acid esterification before analysis of three symbiont‐derived fatty acids (stearidonic acid, docosapentaenoic acid, and docosahexaenoic acid—SDA, DPA, and DHA, respectively) using gas chromatography. Results show considerable variation in fatty acids concentration among species and organs. SDA was found in higher concentrations in T. noae, especially in the adductor muscle. DPA was detected in low concentrations across T. noae organs and absent for T. derasa. DHA did not vary significantly among species and organs. Our findings indicate that Symbiodiniaceae supply clams with fatty acids, which are stored differentially according to species and organs. This demonstrates that these compounds are translocated to multiple organs throughout the complex giant clam anatomical system, in contrast to simpler hosts like corals. These results advance our understanding of the physiological dynamics of the mollusk‐algae association.

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The genome of a giant clam zooxanthella (Cladocopium infistulum) offers few clues to adaptation as an extracellular symbiont with high thermotolerance

González-Pech,

Shepherd,

Fuller

et al. 2024

BMC Genomics

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Background Cladocopium infistulum (Symbiodiniaceae) is a dinoflagellate specialized to live in symbiosis with western Pacific giant clams (Tridacnidae). Unlike coral-associated symbionts, which reside within the host cells, C. infistulum inhabits the extracellular spaces of the clam’s digestive diverticula. It is phylogenetically basal to a large species complex of stress-tolerant Cladocopium , many of which are associated with important reef-building corals in the genus Porites . This close phylogenetic relationship may explain why C. infistulum exhibits high thermotolerance relative to other tridacnid symbionts. Moreover, past analyses of microsatellite loci indicated that Cladocopium underwent whole-genome duplication prior to the adaptive radiations that led to its present diversity. Results A draft genome assembly of C. infistulum was produced using long- and short-read sequences to explore the genomic basis for adaptations underlying thermotolerance and extracellular symbiosis among dinoflagellates and to look for evidence of genome duplication. Comparison to three other Cladocopium genomes revealed no obvious over-representation of gene groups or families whose functions would be important for maintaining C. infistulum ’s unique physiological and ecological properties. Preliminary analyses support the existence of partial or whole-genome duplication among Cladocopium , but additional high-quality genomes are required to substantiate these findings. Conclusion Although this investigation of Cladocopium infistulum revealed no patterns diagnostic of heat tolerance or extracellular symbiosis in terms of overrepresentation of gene functions or genes under selection, it provided a valuable genomic resource for comparative analyses. It also indicates that ecological divergence among Cladocopium species, and potentially among other dinoflagellates, is partially governed by mechanisms other than gene content. Thus, additional high-quality, multiomic data are needed to explore the molecular basis of key phenotypes among symbiotic microalgae. Supplementary Information The online version contains supplementary material available at 10.1186/s12864-024-10822-0.

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Symbiotic 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 illumination

Cited by 4 publications

References 64 publications

Molecular characterization and light-dependent expression of glycerol facilitator (GlpF) in coccoid Symbiodiniaceae dinoflagellates of the giant clam Tridacna squamosa

Molecular characterization and light-dependent expression of glycerol facilitator (GlpF) in coccoid Symbiodiniaceae dinoflagellates of the giant clam Tridacna squamosa

Symbiodiniaceae‐Derived Fatty Acids Are Stored Differentially Across Giant Clam Species and Organs

The genome of a giant clam zooxanthella (Cladocopium infistulum) offers few clues to adaptation as an extracellular symbiont with high thermotolerance

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