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Hentschel, U, Felbeck H.  1993.  Nitrate respiration in the hydrothermal vent tubeworm Riftia pachyptila. Nature. 366:338-340.   10.1038/366338a0   AbstractWebsite

THE vestimentiferan tubeworm Riftia packyptila is found around hydrothermal vent areas in the deep sea. Intracellular bacterial chemoautotrophic symbionts use the oxidation of sulphide from the effluent of the vents as an energy source for CO2 fixation. They apparently provide most or all of the nutritional requirements for their gutless hosts1-5. This kind of symbiosis has since been found in many other species from various other phyla from other habitats6-9. Here we present results that the bacteria of R. pachyptila may cover a significant fraction of their respiratory needs by the use of nitrate in addition to oxygen. Nitrate is reduced to nitrite, which may be the end product (nitrate respiration)10 or it may be further reduced to nitrogen gas (denitrification)11. This metabolic trait may have an important role in the colonization of hypoxic habitats in general by animals with this kind of symbiosis.

Hentschel, U, Hand SC, Felbeck H.  1996.  The contribution of nitrate respiration to the energy budget of the symbiont-containing clam Lucinoma aequizonata: A calorimetric study. Journal of Experimental Biology. 199:427-433. AbstractWebsite

Heat production and nitrate respiration rates were measured simultaneously in the gill tissue of Lucinoma aequizonata. This marine bivalve contains chemoautotrophic, intracellular, bacterial symbionts in its gill tissue. The symbionts show constitutive anaerobic respiration, using nitrate instead of oxygen as a terminal electron acceptor. An immediate increase in heat production was observed after the addition of nitrate to the perfusion medium of the calorimeter and this was accompanied by the appearance of nitrite in the effluent sea water. The nitrate-stimulated heat output was similar under aerobic and anaerobic conditions, which is consistent with the constitutive nature of nitrate respiration. The amount of heat released was dependent on the concentration of nitrate in the perfusion medium. At nitrate concentrations between 0.5 and 5 mmoll(-1), the total heat production was increased over twofold relative to baseline values. A mean (+/-S.E.M.) enthalpy of -130+/-22.6 kJ mol(-1) nitrite (N=13) was measured for this concentration range.

Markert, S, Gardebrecht A, Felbeck H, Sievert SM, Klose J, Becher D, Albrecht D, Thurmer A, Daniel R, Kleiner M, Hecker M, Schweder T.  2011.  Status quo in physiological proteomics of the uncultured Riftia pachyptila endosymbiont. Proteomics. 11:3106-3117.   10.1002/pmic.201100059   AbstractWebsite

Riftia pachyptila, the giant deep-sea tube worm, inhabits hydrothermal vents in the Eastern Pacific ocean. The worms are nourished by a dense population of chemoautotrophic bacterial endosymbionts. Using the energy derived from sulfide oxidation, the symbionts fix CO(2) and produce organic carbon, which provides the nutrition of the host. Although the endosymbionts have never been cultured, cultivation-independent techniques based on density gradient centrifugation and the sequencing of their (meta-) genome enabled a detailed physiological examination on the proteomic level. In this study, the Riftia symbionts' soluble proteome map was extended to a total of 493 identified proteins, which allowed for an explicit description of vital metabolic processes such as the energy-generating sulfide oxidation pathway or the Calvin cycle, which seems to involve a reversible pyrophosphate-dependent phosphofructokinase. Furthermore, the proteomic view supports the hypothesis that the symbiont uses nitrate as an alternative electron acceptor. Finally, the membrane-associated proteome of the Riftia symbiont was selectively enriched and analyzed. As a result, 275 additional proteins were identified, most of which have putative functions in electron transfer, transport processes, secretion, signal transduction and other cell surface-related functions. Integrating this information into complex pathway models a comprehensive survey of the symbiotic physiology was established.