Nutritional plasticity in a deep-sea dualendosymbiotic suspension-feeding mussel: Bathymodiolus azoricus from MAR hydrothermal vents

The mid-oceanic ridges running around the Earth are the theatre of intense submarine volcanic activity creating oases for a specialized fauna. Bathymodiolus mussels are dominant members of these ecosystems and live in symbiotic associations with chemosynthetic bacteria. These mussels also exhibit a potential for suspension-feeding, reflecting ability for significant nutritional plasticity. Bathymodiolus azoricus thrives at hydrothermal vents between 800 to 2400 m deep in the Azores Triple Junction of the Mid-Atlantic Ridge and hosts a dual symbiosis with methane- and sulphide-oxidising bacteria located in gill bacteriocytes. Although a suspension-feeding potential was already suggested for this species (by analogy with other bathymodiolids), it was not investigated, and the symbionts in vivo activity within this species had to be confirmed. In the present study, Bathymodiolus azoricus was submitted to different experimental conditions with carbon and nitrogen stable isotope tracers in LabHorta (Horta, Azores), a land-based aquarium facility at atmospheric pressure. The main achievements of these experiments were: i) the observation of a high plasticity in the symbionts distribution and relative abundance, related with CH4 and HS- availability. ii) the acquisition of evidence for an active contribution of the symbionts to B. azoricus nutrition and carbon transfer to non-symbiotic tissues. iii) a contribution to the biochemical description of the two symbionts and the confirmation of their metabolic potential, revealed by the tracking of CH4 and CO2 into specific bacterial fatty acid biomarkers. iv) the demonstration of the incorporation of dissolved and particulate organic matter in vent mussel soft tissues. To investigate a seasonal effect of sea-surface primary production on the mussel's nutrition, wild Bathymodiolus azoricus specimens were sampled on 4 occasions in summer and autumn 2006, and winter and spring 2007. Incidentally, this time frame coincided with the lowest primary production and shortest duration of the phytoplanktonic bloom observed over the past 10 years. Although no direct evidence was obtained for suspension-feeding on seasurface derived organic matter, a spawning event was observed in January 2007 and an increase in general mussel condition corresponded to higher surface production.