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.