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 am monia and free amino acids appear to be unlikely sources of nitrogen. Nitra te, however, is present in situ (K, Johnson, J. Childress, R. Hessler, C. S akamoto-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. Hentsch el and H. Felbeck, Nature 366:338-340, 1993), Here we report that at an in situ concentration of 40 mu M, nitrate is acquired by R, pachyptila at a ra te of 3.54 mu mol g(-1) h(-1), while elimination of nitrite and elimination of ammonia occur at much lower rates (0.017 and 0.21 mu mol g(-1) h(-1), r espectively), We also observed reduction of nitrite (and accordingly nitrat e) to ammonia in the trophosome tissue. When R. pachyptila tubeworms are ex posed to constant in situ conditions for 60 h, there is a difference betwee n the amount of nitrogen acquired via nitrate uptake and the amount of nitr ogen lost via nitrite and ammonia elimination, which indicates that there i s a nitrogen "sink" Our results demonstrate that storage of nitrate does no t account for the observed stoichiometric differences in the amounts of nit rogen, Nitrate uptake was not correlated with sulfide or inorganic carbon f lux, suggesting that nitrate is probably not an important oxidant in metabo lism of the symbionts, Accordingly, we describe a nitrogen flux model for t his association, in which the product of symbiont nitrate reduction, ammoni a, is the primary source of nitrogen for the host and the symbionts and ful fills the association's nitrogen needs via incorporation of ammonia into am ino acids.