All organisms inhabiting high-sulphide environments have to protect themselves from sulphide toxicity. Species with sulphide-oxidizing symbionts must furthermore supply their symbionts with sulphide and have evolved specialized mechanisms for sulphide metabolism (for review see Fisher 1995): vesicomyides and vestimentiferans possess specialized blood components for the uptake and transport of sulphide. It was suggested that sulphur-amino compounds could be involved in sulphide metabolism in thiotrophic symbiosis. To our knowledge, large amounts of free sulphur-amino compounds were found in all the symbiont-containing species studied: the vestimentiferan Riftia pachyptila, the vesicomyide Calyptogena phaseoliformis and several mytilides Bathymodiolus brevior, B. elongatus (Pranal & al. 1995), B. thermophilus, B. puteoserpentis and the seep Mytilid Ia (Pruski et al. submitted, Pranal 1995). The three main sulphur-amino compounds found are taurine and its sulphinic and thiosulphonic analogues, hypotaurine and thiotaurine. The free sulphur-amino compound composition differs according to the species and the tissues, and could be related to the metabolism of the symbionts. Thus, we previously shown that thiotaurine was only found in species harbouring sulphur-oxidizing symbionts. High concentrations of thiotaurine are furthermore restricted to the symbiont-containing tissues (the gill of the bivalves and the trophosome of the vestimentiferans). Experiments realized on crushed symbiont-containing tissues of several deep-sea symbiotic species (1) have demonstrated that sulphide stimulated thiotaurine biosynthesis, (2) have confirmed that hypotaurine is the substrate for thiotaurine synthesis, (3) have evidenced the incorporation of sulphide into thiotaurine. These results confirm the involvement of sulphur-amino compounds in sulphide metabolism. Thiotaurine biosynthesis could serve (1) as a trap for free sulphide, thus protecting the cell from sulphide toxicity, (2) as a reserve of reduced sulphur for further utilisation by the symbionts.
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