Riftia pachyptila is a dominant member of the biota at
hydrothermal vent sites along the East pacific rise
(Jones 1981, Hessler 1984). This vestimentiferan
tubeworm lives its adult life without a mouth or
digestive tract. Rather, it relies on chemoautotrophic
symbionts that are housed in host bacteriocytes deep
within the worm. Riftia pachyptila is believed to be one
of the fastest growing metazoans (Lutz et al, 1994), a
feat accomplished without significant heterotrophic
nutrient acquisition. The observed tremendous growth
rates suggest that the symbionts have high rates of
chemoautotrophic metabolism and require that the
host maintain sufficient influx of metabolites and efflux
of end products to sustain chemoautotrophic
metabolism. The studies presented here have shown
that Riftia pachyptila can acquire carbon dioxide from
the environment at rates that can support very high
growth rates, up to 7.9% total body carbon per day. The
oxidation of sulfide by the symbionts required oxygen,
not nitrate, and is responsible for the production of
protons. Because Riftia pachyptila relies on a pH
gradient between the internal milieu and the
environment to sustain inorganic carbon acquisition for
chemoautotrophic carbon fixation, the rapid elimination
of protons is critical to sustaining carbon fixation. The
studies presented here have shown that proton
elimination by Riftia pachyptila sustains inorganic
carbon uptake, and that Riftia pachyptila proton
elimination rates are the highest recorded to date.
Nitrate acquisition by Riftia pachyptila and the symbiotic
reduction to ammonia is the source of nitrogen for this
association, and is sufficient to sustain the observed
growth rates of Riftia pachyptila. In concert, these
findings demonstrate that the tight physiological
coupling between the Riftia pachyptila host and
symbionts supports the very high rates of
chemoautotrophic function. Because metabolite
availability in the vent environment is temporally
variable, the ability of Riftia pachyptila to rapidly acquire
the necessary metabolites when available (as well
eliminate the chemoautotrophic end products), is
critical to maintaining the observed high growth rates.
The dominance of Riftia pachyptila at hydrothermal
vents is attributable to this association’s ability to
provide ample metabolites to its symbionts and sustain
high rates of carbon fixation and growth.