Influence of Symbiont Species on the Glycerol and Glucose Pools in a Model Cnidarian-Dinoflagellate Symbiosis

Abstract

AbstractSymbiotic dinoflagellates in the family Symbiodiniaceae release mobile compounds (e.g., glucose, glycerol, amino acids, and lipids) to their host's tissues. Little is known about how different symbionts affect quantitative and qualitative differences in these compounds. We tested how symbiont identity affects glycerol and glucose pools in the tissues of the sea anemone Exaiptasia pallida ("Aiptasia"). We infected symbiont-free anemones with three different symbiotic dinoflagellates: Breviolum minutum isolated from our Aiptasia stock, B. minutum isolated from a different Aiptasia population, and the free-living Effrenium voratum. We measured free glycerol and glucose levels in host tissues under photosynthetic conditions, as well as metabolite release by these algae when freshly isolated from Aiptasia and incubated in a host tissue homogenate. Anemone tissues containing the stock B. minutum accumulated glycerol at a higher symbiont cell-specific rate than those containing the alternative B. minutum or E. voratum (e.g., at 9 h of light incubation: stock B. minutum, 4.05 × 10^-5 nmol per algal cell; alternative B. minutum, 0.90 × 10^-5 nmol per algal cell; E. voratum: 1.14 × 10^-5 nmol per algal cell). All symbiotic hosts accumulated glucose between 1 and 12 h of light incubation. At 12 h, the symbiont cell-specific glucose content was 6-fold higher in anemone tissues that contained stock B. minutum than in those containing the alternative B. minutum (1.73 × 10^-6 vs. 0.30 × 10^-6 nmol per algal cell, respectively). All freshly isolated algae released glycerol and glucose when incubated in host homogenate except E. voratum, which did not release glycerol. These trends mirrored those in hospite. Our results suggest that, on an algal cell-specific basis, B. minutum isolated from the same Aiptasia stock provided better nutritional support to the host than did the two other algae, though this was not reflected in colonization success, highlighting the underlying complexity of host-symbiont recognition and symbiosis establishment in the cnidarian-dinoflagellate partnership.