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Ponnudurai, R, Sayavedra L, Kleiner M, Heiden SE, Thurmer A, Felbeck H, Schluter R, Sievert SM, Daniel R, Schweder T, Markert S.  2017.  Genome sequence of the sulfur-oxidizing Bathymodiolus thermophilus gill endosymbiont. Standards in Genomic Sciences. 12   10.1186/s40793-017-0266-y   AbstractWebsite

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.

Gardebrecht, A, Markert S, Sievert SM, Felbeck H, Thurmer A, Albrecht D, Wollherr A, Kabisch J, Le Bris N, Lehmann R, Daniel R, Liesegang H, Hecker M, Schweder T.  2012.  Physiological homogeneity among the endosymbionts of Riftia pachyptila and Tevnia jerichonana revealed by proteogenomics. ISME Journal. 6:766-776.   10.1038/ismej.2011.137   AbstractWebsite

The two closely related deep-sea tubeworms Riftia pachyptila and Tevnia jerichonana both rely exclusively on a single species of sulfide-oxidizing endosymbiotic bacteria for their nutrition. They do, however, thrive in markedly different geochemical conditions. A detailed proteogenomic comparison of the endosymbionts coupled with an in situ characterization of the geochemical environment was performed to investigate their roles and expression profiles in the two respective hosts. The metagenomes indicated that the endosymbionts are genotypically highly homogeneous. Gene sequences coding for enzymes of selected key metabolic functions were found to be 99.9% identical. On the proteomic level, the symbionts showed very consistent metabolic profiles, despite distinctly different geochemical conditions at the plume level of the respective hosts. Only a few minor variations were observed in the expression of symbiont enzymes involved in sulfur metabolism, carbon fixation and in the response to oxidative stress. Although these changes correspond to the prevailing environmental situation experienced by each host, our data strongly suggest that the two tubeworm species are able to effectively attenuate differences in habitat conditions, and thus to provide their symbionts with similar micro-environments. The ISME Journal (2012) 6, 766-776; doi: 10.1038/ismej.2011.137; published online 20 October 2011

Dufour, SC, Felbeck H.  2006.  Symbiont abundance in thyasirids (Bivalvia) is related to particulate food and sulphide availability. Marine Ecology-Progress Series. 320:185-194.   10.3354/meps320185   AbstractWebsite

Many bivalve species with chemoautotrophic symbionts have mixotrophic diets and some of their nutritional requirements are met by particulate feeding. The symbionts require reduced compounds (such as sulphide) for their autotrophic production. As the concentration of both particulate food and sulphide can vary in their habitat, it has been suggested that symbiont numbers may vary in response. To address whether symbiont abundance can be influenced by the external medium, we compared symbiont. abundance (1) in specimens kept with or without particulate food, and (2) in specimens kept in sediments with low or high dissolved sulphide content. The relative surface area occupied by symbionts on semi-thin gill sections was determined for Thyasira flexuosa (and, in the sulfide experiment, for Parathyasira equalis and T sarsi) kept for up to 3 wk in experimental microcosms, where the quantity of particulate food or dissolved sulphide was manipulated. Symbiont abundance declined with time; the decline was greater in particle-starved thyasirids and in specimens kept in low sulphide. In the latter conditions, T flexuosa may have also had a greater symbiont digestion rate. The observed patterns may be due to a combination of increased symbiont uptake by the bivalve host, and reduced symbiont vigour under the imposed conditions. The flexible feeding mode of chemosymbiotic thyasirids may help them to survive in fluctuating environments.

Childress, JJ, Lee RW, Sanders NK, Felbeck H, Oros DR, Toulmond A, Desbruyeres D, Kennicutt MC, Brooks J.  1993.  Inorganic carbon uptake in hydrothermal vent tubeworms facilitated by high environmental pC02. Nature. 362:147-149.   10.1038/362147a0   AbstractWebsite

THE marine invertebrate Riftia pachyptila has a remarkable symbiosis with intracellular carbon-fixing sulphide-oxidizing bacteria which was first discovered at 2,450 m depth on the Galapagos Rift1-4. Such symbiotic arrangements have since been found in a variety of invertebrate taxa and habitat5,6. Studies of these symbioses have focused on temperature, sulphide and oxygen as critical environmental parameters5,7-9. As Riftia has a high growth rate and its symbionts are far removed from the host surface10,11, inorganic carbon supply to the symbionts has been recognized as a problem and host mechanisms to concentrate inorganic carbo have been posited12,13. Increased environmental CO2 partial pressure (pCO2) has not seriously been considered as a critical environmental parameter7,14. Here we report that elevated pCO2 (2.9 kPa) in the worms' environment is a determinant of internal total CO2 (SIGMACO2) and pCO2, facilitating CO2 transport and diffusion to the symbionts. We propose that elevated pCO2 is a potentially critical environmental factor for this species as well as for other chemoautotrophic symbioses.