The sea anemone, Exaiptasia diaphana, is a model of coral-dinoflagellate (Symbiodiniaceae) symbiosis. However, little is known of its potential to form symbiosis with Cladocopium—a key Indo-Pacific algal symbiont of scleractinian corals, nor the host nutritional consequences of such an association. Aposymbiotic anemones were inoculated with homologous algal symbionts, Breviolum minutum, and seven heterologous strains of Cladocopium C1acro (wild-type and heat-evolved) under ambient conditions. Despite lower initial algal cell density, Cladocopium C1acro-anemeones achieved similar cell densities as B. minutum-anemones by week 77. Wild-type and heat-evolved Cladocopium C1acro showed similar colonization patterns. Targeted LC-MS-based metabolomics revealed that almost all significantly different metabolites in the host and Symbiodiniaceae fractions were due to differences between Cladocopium C1acro and B. minutum, with little difference between heat-evolved and wild-type Cladocopium C1acro at week 9. The algal fraction of Cladocopium C1acro-anemones was enriched in metabolites related to nitrogen storage, while the host fraction of B. minutum-anemones was enriched in sugar-related metabolites. Compared to B. minutum, Cladocopium C1acro is likely slightly less nutritionally beneficial to the host under ambient conditions, but more capable of maintaining its own growth when host nitrogen supply is limited. Our findings demonstrate the value of E. diaphana to study experimentally evolved Cladocopium.

Corals and their photosynthetic endosymbiotic algae (Symbiodiniaceae) produce a strong autofluorescent signal that spans the visible to near-infrared (NIR) spectrum. However, this broad-spectrum emission hinders the use of fluorescence in situ hybridisation (FISH) for the study of bacterial heterogeneity within the different niches of corals and Symbiodiniaceae, because FISH fluorophores also fluoresce within the visible to NIR spectrum. A solution to this impediment is to use fluorescence lifetime imaging microscopy (FLIM). The ‘lifetime’ property of fluorophores is a feature that enables sample (e.g. coral/Symbiodiniaceae) autofluorescence to be distinguished from FISH-labelled bacteria. In this manner, the location of bacteria around and within Symbiodiniaceae can be quantified along with their identity and spatial distribution. Furthermore, the ‘lifetime’ of the host and associated microbe cellular autofluorescence can be analysed in terms of endogenous fluorophore composition (e.g. metabolic co-factors, aromatic amino acids) and serves as information for symbiotic versus parasitic host-microbe association.