Non-equilibrium fractionation of stable carbon isotopes in chemosynthetic mussels

•We measured δ13Cshell of mussel inhabiting contrasted hydrothermal vent fields.•The 13C fractionation is attributed to chemosynthetic metabolism.•δ13Cshell is higher when a thiotrophic carbon-fixation pathway is favored.•Results also suggest differences in the symbiosis activity at micro-habitat scale.

Chemosynthetic bivalves from deep-sea hydrothermal vents exploit the energy derived from chemical compounds, such as methane, sulfide or hydrogen, using symbiotic bacteria that are able to fix inorganic carbon. Available chemical resources in their habitat vary widely at various scales, from the vent field scale to the micro-habitat scale. Parallel to this environmental heterogeneity, Bathymodiolus species are considered to be flexible in their energy acquisition pathways.The goal of this study was to determine whether the isotopic compositions archived in the shells of hydrothermal vent mussels could trace chemical energy sources and their variability over spatial and temporal scales. Two different species (Bathymodiolus azoricus and Bathymodiolus thermophilus) inhabiting three vent fields with contrasted geochemical features on the Mid Atlantic Ridge (MAR; Rainbow and Menez Gwen) and the East Pacific Rise (EPR; 9°47′N), were considered for carbon isotopes and growth rate variation along the shell length. The study revealed that 13C fractionation between shells and seawater is higher than expected from calcite-bicarbonate equilibrium fractionation, suggesting a significant influence of the chemosynthetic pathway on the shell composition. Furthermore, significant differences in δ13Cshell fractionation with respect to seawater are observed between sites and habitats of the two MAR vent fields, suggesting that different chemosynthetic pathway (e.g. methanotrophic and thiotrophic) could lead to variable enrichments of the shell in 13C. Mussels supposed to rely more largely on methanotrophy (at Rainbow where free sulfide is unavailable) display a lower δ13Cshell values than mussels relying also on sulfide-oxidizing symbiosis (at Menez Gwen). Variability in δ13Cshell between habitats, or between individuals within the same assemblage, could thus reflect differences in the symbiosis activity at a micro-habitat scale. These isotopic signatures could provide useful information on the relationships between micro-habitat properties, symbiont activity and shell mineralization.