Characterization of symbiont populations in life-history stages of mussels from chemosynthetic environments

The densities of chemoautotrophic and methanotrophic symbiont morphotypes were determined in life-history stages (post-larvae, juveniles, adults) of two species of mussels (Bathymodiolus azoricus and B. heckerae) from deep-sea chemosynthetic environments (the Lucky Strike hydrothermal vent and the Blake Ridge cold seep) in the Atlantic Ocean. Both symbiont morphotypes were observed in all specimens and in the same relative proportions, regardless of life-history stage. The relative abundance of symbiont morphotypes, determined by transmission electron microscopy, was different in the two species: chemoautotrophs were dominant (13:1-18:1) in B. azoricus from the vent site; methanotrophs were dominant (2:1-3:1) in B. heckerae from the seep site. The ratio of CH₄:H₂S is proposed as a determinant of the relative abundance of symbiont types: where CH₄:H₂S is less than 1, as at the Lucky Strike site, chemoautotrophic symbionts dominate; where CH₄:H₂S is greater than 2, as at the seep site, methanotrophs dominate. Organic carbon and nitrogen isotopic compositions of B. azoricus (δ¹³C = -30‰; δ¹⁵N = -9‰) and B. heckerae (δ¹³C = -56‰; δ¹⁵N = -2‰) varied little among life-history stages and provided no record of a larval diet of photosynthetically derived organic material in the post-larval and juvenile stages.