Pathways of inorganic nitrogen assimilation in chemoautotrophic bacteria-marine invertebrate symbioses: Expression of host and symbiont glutamine synthetase

Symbioses between chemoautotrophic bacteria and marine invertebrates living at deep-sea hydrothermal vents and other sulfide-rich environments functio n autotrophically by oxidizing hydrogen sulfide as an energy source and fix ing carbon dioxide into organic compounds. For chemoautotrophy to support g rowth, these symbioses must be capable of inorganic nitrogen assimilation, a process that is not well understood in these or other aquatic symbioses, Pathways of inorganic nitrogen assimilation were investigated in several of these symbioses: the vent tubeworms Riftia pachyptila and Tevnia jerichona na, the vent bivalves Calyptogena magnifica and Bathymodiolus thermophilus , and the coastal bivalve Solemya velum. Nitrate reductase activity was det ected in R. pachyptila, T. jerichonana and B, thermophilus, but not in C, m agnifica and S. velum. This is evidence for nitrate utilization, either ass imilation or respiration, by some vent species and is consistent with the h igh levels of nitrate availability at vents. The ammonia assimilation enzym es glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were detecte d in all symbioses tested, indicating that ammonia resulting from nitrate r eduction or from environmental uptake can be incorporated into amino acids. A complicating factor is that GS and GDH are potentially of both host and symbiont origin, making it unclear which partner is involved in assimilatio n. GS, which is considered to be the primary ammonia-assimilating enzyme of autotrophs, was investigated further. Using a combination of molecular and biochemical approaches, host and symbiont GS were distinguished in the int act association, On the basis of Southern hybridizations, immunoreactivity, subunit size and thermal stability, symbiont GS was found to be a prokaryo te GS, Host GS was distinct from prokaryote GS, The activities of host and symbiont GS were separated by anion-exchange chromatography and quantified. Virtually all activity in symbiont-containing tissue was due to symbiont G S in R, pachyptila. C. magnifica and B, thermophilus. In contrast, no symbi ont GS activity was detected in the gill of S. velum, the predominant activ ity in this species appearing to be host GS, These findings suggest that am monia is primarily assimilated by the symbionts in vent symbioses, whereas in S, velum ammonia is first assimilated by the host. The relationship betw een varying patterns of GS expression and host-symbiont nutritional exchang e is discussed.