Physiological dynamics of chemosynthetic symbionts in hydrothermal vent snails

Symbioses between invertebrate animals and chemosynthetic bacteria form the basis of hydrothermal vent ecosystems worldwide. In the Lau Basin, deep-sea vent snails of the genus Alviniconchaassociate with either Gammaproteobacteria(A. kojimai, A. strummeri) or Campylobacteria(A. boucheti) that use sulfide and/or hydrogen as energy sources. While the A. bouchetihost–symbiont combination (holobiont) dominates at vents with higher concentrations of sulfide and hydrogen, the A. kojimaiand A. strummeriholobionts are more abundant at sites with lower concentrations of these reductants. We posit that adaptive differences in symbiont physiology and gene regulation might influence the observed niche partitioning between host taxa. To test this hypothesis, we used high-pressure respirometers to measure symbiont metabolic rates and examine changes in gene expression among holobionts exposed to in situ concentrations of hydrogen (H2: ~25 µM) or hydrogen sulfide (H2S: ~120 µM). The campylobacterialsymbiont exhibited the lowest rate of H2S oxidation but the highest rate of H2oxidation, with fewer transcriptional changes and less carbon fixation relative to the gammaproteobacterialsymbionts under each experimental condition. These data reveal potential physiological adaptations among symbiont types, which may account for the observed net differences in metabolic activity and contribute to the observed niche segregation among holobionts.