Project description:Bathymodiolus mussels inhabiting deep-sea hydrothermal vents harbor bacterial symbionts in their gills, which support the animals’ diet. While the basic mechanisms of energy generation and CO2 fixation that drive these symbioses are largely established, details of molecular interactions between the symbiotic partners and adaptations to their respective habitats remain unknown. In this study, we therefore comparatively examined the genomes and proteomes of two Bathymodiolus hosts and their respective symbionts from different geographical locations. Two mussel species were proteomically compared: i) B. thermophilus mussel containing sulfur-oxidizing symbiont from the east pacific rise. thermophilus and ii) B. azoricus containing thiotrophic and methanotrophic symbionts from the mid-atlantic ridge. Symbionts (for both species) and host components (for B. azoricus) were selectively enriched using a multi-step centrifugation procedure. Enriched host and symbiont fractions along with unenriched gill foot tissue were subject to in-depth semi-quantitative proteomic analyses using the orbitrap and velos mass spectrometers. Proteins were quantified based on their spectral counts using the normalized spectral abundance factor (NSAF) method. We identified common strategies of metabolic interactions that provide mutual nutritional support between host and symbionts, such as the detoxification of ambient sulfide by the Bathymodiolus host, which provides a stable thiosulfate reservoir for the thiotrophic symbionts, and a putative amino acid cycling mechanism that could supply the host with symbiont-derived amino acids. A suite of genes and proteins putatively related to virulence or defense functions was particularly abundant in the B. thermophilus symbiont, compared to its symbiont relatives, and may pose a host species-specific adaptation. Our results reveal both, a high degree of integration between the symbiotic partners, and great potential to adapt to the prevailing environment, which facilitate the holobiont’s survival in its hydrothermal vent habitat.
Project description:Bathymodiolus azoricus is a deep-sea mussel found in the hydrothermal vent fields of the Mid-Atlantic Ridge. It lives in symbiosis with sulfur- and methane-oxidizing γ-proteobacteria within its gills. In our study, we aimed to understand the metabolic and physiological interconnections between the symbiotic partners. For this purpose, symbionts and host were physically separated using density gradient centrifugation. This procedure yielded a symbiont-enriched gradient pellet fraction and a supernatant fraction enriched in host components. The cytosolic and membrane-associated proteome of both these fractions along with whole gill and foot tissue of the mussel were then investigated through 1D-PAGE LC-MS/MS. Proteins were quantified based on their spectral counts using the NSAF method. For efficient identification, sequences from evolutionarily related endosymbiotic and free-living bacteria and from bivalve host relatives were compiled into a comprehensive protein database. A total of 3178 host and symbiont proteins were identified from all samples.
Project description:Bathymodiolus thermophilus, a mytilid mussel inhabiting the deep-sea hydrothermal vents of the East Pacific Rise, lives in symbiosis with chemosynthetic Gammaproteobacteria within its gills. The intracellular symbiont population synthesizes nutrients for the bivalve host using the reduced sulfur compounds emanating from the vents as energy source. As the symbiont is uncultured, comprehensive and detailed insights into its metabolism and its interactions with the host can only be obtained from culture-independent approaches such as genomics and proteomics. In this study, we report the first draft genome sequence of the sulfur-oxidizing symbiont of B. thermophilus, here tentatively named Candidatus Thioglobus thermophilus. The draft genome (3.1 Mb) harbors 3045 protein-coding genes. It revealed pathways for the use of sulfide and thiosulfate as energy sources and encodes the Calvin-Benson-Bassham cycle for CO2 fixation. Enzymes required for the synthesis of the tricarboxylic acid cycle intermediates oxaloacetate and succinate were absent, suggesting that these intermediates may be substituted by metabolites from external sources. We also detected a repertoire of genes associated with cell surface adhesion, bacteriotoxicity and phage immunity, which may perform symbiosis-specific roles in the B. thermophilus symbiosis.