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

High densities of mussels of the genus Bathymodiolus 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 5
on Bathymodiolus azoricus 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 H13CO_3 in the presence of sulphide, 10
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 ac- 15
tively to B. azoricus 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 B. azoricus' chemosynthetic nutrition.