Metabolite comparisons and the identity of nutrients translocated from symbiotic algae to an animal host

Dinoflagellate algae of the genus Symbiodinium in symbiosis with marine animals release much of their photosynthetic carbon to the animal host. The compounds translocated to the host ('mobile compounds') were investigated by metabolite comparison as follows: a substrate was identified as a candidate mobile compound when comparable profiles of metabolites were generated from host metabolism of this substrate (supplied exogenously) and the endogenous mobile compounds. When the sea anemone Anemonia viridis was incubated with NaH¹⁴CO₂ under photosynthesizing conditions, most of the radioactivity in the animal tissue was recovered from the low-molecular-mass fraction and distributed in the ratio 1:2:1 between the neutral, acidic and basic subfractions. Prominent ¹⁴C-labelled compounds included glucose, malate and glucose-6-phosphate. When the symbiosis was incubated with ¹⁴C-labelled glucose plus succinate or fumarate (but none of eight other substrate combinations tested), the ¹⁴C-labelled metabolites closely matched those obtained with NaH¹⁴CO₂. These data suggest that glucose and succinate/fumarate (or metabolically allied compounds) may be important photosynthetic compounds transferred from the Symbiodinium cells to the tissues of A. viridis. Metabolite comparisons can be applied to study nutritional interactions in symbioses involving photosynthetic algae and, with appropriate modification, other associations between microorganisms and plants or animals.