Symbiosis of the acoel flatworm Convoluta roscoffensis with the alga Platymonas convolutae

Convoluta roscoffensis can carry out photosynthetic carbon fixation at rates comparable to free-living algae. The main product of fixation is mannitol, previously shown to be the main product accumulating in the isolated algal symbiont in pure culture. Since both eggs and mucus of Convoluta became radioactive when incubated in NaH14CO3 in the light, photosynthetically fixed carbon can evidently move from alga to animal. Available evidence indicates that it is amino acids, not mannitol, which are released from the algae in Convoluta. The difficulty of separating algae from animal tissue prevented direct estimates of the amount of carbon moving, but indirect evidence indicated it may not be as high as in other autotroph-heterotroph associations. When Convoluta is incubated in the light in seawater containing NaH14CO3 and certain amino acids (especially alanine), fixed 14C is released to the medium as amino acids. Up to 8% of the total fixed carbon may be released in this way, and the effect is believed to be due to the external amino acids being able to penetrate the tissues and exchange with the radioactive amino acids as they move through the animal tissues. Pyruvic acid was also effective in causing the release of fixed 14C.