Fate of Nitrate Acquired by the Tubeworm Riftia pachyptila

The hydrothermal vent tubeworm Riftia pachyptila lacks a mouth and gut and lives in association with intracellular, sulfide-oxidizing chemoautotrophic bacteria. Growth of this tubeworm requires an exogenous source of nitrogen for biosynthesis, and, as determined in previous studies, environmental ammonia and free amino acids appear to be unlikely sources of nitrogen. Nitrate, however, is present in situ (K. Johnson, J. Childress, R. Hessler, C. Sakamoto-Arnold, and C. Beehler, Deep-Sea Res. 35:1723–1744, 1988), is taken up by the host, and can be chemically reduced by the symbionts (U. Hentschel and H. Felbeck, Nature 366:338–340, 1993). Here we report that at an in situ concentration of 40 μM, nitrate is acquired by R. pachyptila at a rate of 3.54 μmol g⁻¹ h⁻¹, while elimination of nitrite and elimination of ammonia occur at much lower rates (0.017 and 0.21 μmol g⁻¹ h⁻¹, respectively). We also observed reduction of nitrite (and accordingly nitrate) to ammonia in the trophosome tissue. When R. pachyptila tubeworms are exposed to constant in situ conditions for 60 h, there is a difference between the amount of nitrogen acquired via nitrate uptake and the amount of nitrogen lost via nitrite and ammonia elimination, which indicates that there is a nitrogen “sink.” Our results demonstrate that storage of nitrate does not account for the observed stoichiometric differences in the amounts of nitrogen. Nitrate uptake was not correlated with sulfide or inorganic carbon flux, suggesting that nitrate is probably not an important oxidant in metabolism of the symbionts. Accordingly, we describe a nitrogen flux model for this association, in which the product of symbiont nitrate reduction, ammonia, is the primary source of nitrogen for the host and the symbionts and fulfills the association's nitrogen needs via incorporation of ammonia into amino acids.