page_title: Symsagittifera roscoffensis
section_title: Modus vivendi: a highly integrated partnership—not only additive
hierarchical_section_title: Symsagittifera roscoffensis / The photosynthetic partner and the modus vivendi between the animal and the micro-algae / Modus vivendi: a highly integrated partnership—not only additive
caption_attribution_description: Français&#160;: juvénile symbiotique
context_page_description: Symsagittifera roscoffensis, also called the Roscoff worm, the mint-sauce worm, or the shilly-shally worm, is a marine flatworm belonging to the phylum of Xenacoelomorpha. The origin and nature of the green color of this worm stimulated the curiosity of zoologists early on. It is due to the partnership between the animal and a green micro-algae, the species Tetraselmis convolutae, hosted under its epidermis. It is the photosynthetic activity of the micro-algae in hospite that provides the essential nutrients for the worm. This partnership is called photosymbiosis, from "photo", "light", and symbiosis "who lives with". These photosynthetic marine animals live in colonies on the tidal zone.
context_section_description: Few works describe the trophic nature of the exchanges between the animal and its photosynthetic partners. The photosynthetic activity provides, in addition to oxygen, various organic compounds: amino acids, proteins, polysaccharides and fatty acids.
  Micro-algae recycle uric acid (coming from the worm's nitrogen metabolism) for the synthesis of its amino acids / proteins. The assimilation of uric acid by S. roscoffensis has attracted the attention of early experimenters, including Louis Destouches aka Louis Ferdinand Céline who carried out work on the physiology of these animals and who concluded in 1920 "that it is therefore very likely that symbiotic exchanges lead to the transformation of uric acid into nitrogenous food for zoochlorella". Douglas has formally shown that endogenous uric acid from the worm (derived from its metabolism) is a source of nitrogen for micro-algae and that non-symbiotic juveniles contain uric acid crystals that disappear 15 to 20 days after the establishment of symbiosis. However, the author proposes that if exogenous uric acid is not used by the worm and that if its concentration in the medium culture decreases over time it is because of a bacterial activity associated with the animals.
  Near resurgences rich in nitrate, in intertidal areas where live S. roscoffensis, worms are able to assimilate significant amounts of nitrate depending on the exposure and intensity of light. This quantity is ten times greater than that absorbed by the alga in the free living state. So, S. roscoffensis could be a major nitrate interceptor.
  In the tissues of the animal, the micro-algae produces a sulfur compound, DMSP (dimethylsulfoniopropionate), which also diffuses into the environment. This compound is generally degraded enzymatically by DMSP-lyases to acrylic acid and DMS (Dimethyl sulfide). However, no DMSP-lyase activity was measured in cultures of Tetraselmsis convolutae alone. Bacterial DMSP-lyase activity has been hypothesized to explain the presence of DMS and acrylic acid in colonies of S. roscoffensis. It is likely that S. roscoffensis mucus (such as the mucus of other photosymbiotic animals such as corals) harbor specific bacterial populations expressing DMSP-lyases. DMSP is a sulfur compound that exhibit, among other things, a repellent property that could confer a chemical barrier explaining the absence of known or observed predators and as a consequence the abundance of worms within colonies.
caption: Juvenile Symsagittifera roscoffensis (about 1/2 millimeter long) at an early stage of symbiosis
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