Characterization of chemical energy resources used by chemosynthetic bivalves in present and fossil hydrothermal systems using stable isotope signature of shells

Lartaud F.1, Nedoncelle K. 1, de Rafelis M.2, Le Bris N. 1

1 UPMC Univ Paris 06, CNRS UMR 8222, LECOB, Observatoire Océanologique, F-66650 Banyuls-sur mer, France.

2 UPMC Univ Paris 06, CNRS UMR 7193, ISTeP, Laboratoire Biominéralsations et Environnements sédimentaires, F-75252 Paris cedex 05, France.

Hydrothermal vents host a large variety of chemosynthetic ecosystems. Particularly on slow spreading ridges where basement rocks and tectonic environments result in three classes of vent systems: high-temperature vents hosted on basaltic crust (i.e., Lucky Strike), high-temperature vents hosted on gabbroic and ultramafic material (i.e., Rainbow and Logatchev), and low-temperature vents predominantly influenced by mantle exothermic serpentinization reactions from ultramafic rocks (i.e., Lost City). This tectonic and rock diversity is reflected in differing end-member fluids and provides access to a wide range of geochemical energy sources for chemosynthetic organisms, like Bathymodiolus mussels. The flexibility of symbiotic associations is a key to the success of some species (i.e., multiple symbionts with various abundances reflecting the availability of chemical substrates).

The isotopic composition (carbon and nitrogen) of Bathymodiolus soft tissues is widely used to reflect the energy resources. But the organism tissues only reflect an isotopic signal from the last days of life. Other proxies have to be used to identify chemosynthetic pathways from dead/fossil shells or investigate potential metabolic changes over the lifetime of an individual.

This study investigated the carbon and oxygen isotope signature of living and fossil bivalve shells from different hydrothermal sites to trace chemical energy sources and their variability over spatial and temporal scales.

Based on the isotopic composition of the shells, we identified three types of different habitats in the Rainbow area (in a radius below 1.5 km): the present high-temperature Rainbow environment dominated by Bathymodiolus, a fossil (~25 kyr) sedimented vent system on the summit of the ridge colonized by opportunistic chemosynthetic genera (Vesicomyidae and Thyasirid), and a fossil (~110 kyr) Lost-City like habitat supporting high biomass mussel-dominated chemosynthetic communitites. Additionally to this large scale fragmentation of the habitats, micro-habitat fluctuations have been identified using the d13C of living Bathymodiolus shells reflecting modifications in the symbiont activity (methanotrophic or thiotrophic dominance).

Those results highlight the great potential in using stable isotopes from bivalve shells for a better characterization of past and present hydrothermal ecosystems.