The influence of symbiont identity and nitrogen source on the thermal disruption of carbon and nitrogen cycling in a model cnidarian-dinoflagellate symbiosis

Abstract This study examined how carbon and nitrogen assimilation and exchange shift under normal and elevated temperatures in three cnidarian–dinoflagellate symbioses involving the model host sea anemone Exaiptasia diaphana (Aiptasia) and either the native symbiont Breviolum minutum or the non-native symbionts Breviolum psygmophilum or Durusdinium trenchii . At control temperature, symbionts differed in carbon and nitrogen use— B. minutum achieved the highest total fixation, D. trenchii showed higher per -cell photosynthesis but low population-level production due to its low density, while B. psygmophilum had a similar per -cell photosynthesis to B. minutum , but a lower population-level production. Heterotrophic feeding consistently boosted photosynthetic carbon fixation. Across the different host-symbiont pairings, photosynthesis unexpectedly increased at higher temperature, while nitrogen pathways shifted: for the hosts and their symbionts, ammonium assimilation rose substantially and dissolved free amino acid uptake stayed stable or declined, while the hosts relied more on heterotrophic nitrogen. Elevated temperature also altered symbiont carbon translocation and nitrogen balance in species-specific ways: B. minutum became more nitrogen-limited, B. psygmophilum lost assimilatory capacity as densities fell, and D. trenchii maintained nitrogen balance, likely via the acquisition of heterotrophic nitrogen. These results highlight how thermal stress reshapes nutrient fluxes and may influence long-term host–symbiont compatibility.