Influence of chemosynthetic substrates availability on symbiont densities, carbon assimilation and transfer in the dual symbiotic vent mussel <I>Bathymodiolus azoricus</I>

High densities of mussels of the genus <I>Bathymodiolus</I> are present at hydrothermal vents of the Mid-Atlantic Ridge. It was already proposed that the chemistry at vent sites would affect their sulphide- and methane-oxidizing endosymbionts' abundance. In this study, we confirmed the latter assumption using fluorescence in situ hybridization on <I>Bathymodiolus azoricus</I> specimens maintained in a controlled laboratory environment at atmospheric pressure with one, both or none of the chemical substrates. A high level of symbiosis plasticity was observed, methane-oxidizers occupying between 4 and 39% of total bacterial area and both symbionts developing accordingly to the presence or absence of their substrates. Using H¹³CO₃^- in the presence of sulphide, ¹³CH₄ or ¹³CH₃OH, we monitored carbon assimilation by the endosymbionts and its translocation to symbiont-free mussel tissues. Although no significant carbon assimilation could be evidenced with methanol, carbon was incorporated from methane and sulphide-oxidized inorganic carbon at rates 3 to 10 times slower in the host muscle tissue than in the symbiont-containing gill tissue. Both symbionts thus contribute actively to <I>B. azoricus</I> nutrition and adapt to the availability of their substrates. Further experiments with varying substrate concentrations using the same set-up should provide useful tools to study and even model the effects of changes in hydrothermal fluids on <I>B. azoricus</I>' chemosynthetic nutrition.