| R. pachyptila, a vestimentiferan tubeworm found at deep sea hydrothermal vents, must overcome many biochemical demands associated with being a host to carbon-fixing sulfide-oxidizing symbionts. To support its large size and high growth rates, this invertebrate must supply inorganic carbon (C{dollar}rmsb{lcub}i{rcub}{dollar}) and sulfide to the bacteria, which are far removed from the external environment. These studies show that R. pachyptila acquires C{dollar}rmsb{lcub}i{rcub}{dollar} via diffusion of CO{dollar}sb2{dollar}, rather than mediated transport of HCO{dollar}sb3{dollar}-, and is able to support its symbionts' large demand for C{dollar}rmsb{lcub}i{rcub}{dollar} due to the elevated P{dollar}rmsb{lcub}COsb2{rcub}{dollar} in the vent environment and the ability to maintain an alkaline internal pH. R. pachyptila must also supply sulfide to the bacteria, despite the fact that it is lethal to both partners. The data show that uptake of HS{dollar}sp-{dollar}, rather than H{dollar}sb2{dollar}S, is the primary mechanism used by R. pachyptila to obtain sulfide, which can reach very high levels internally (2-12 mM). This is a novel strategy for sulfide acquisition and may have evolved in R. pachyptila because sulfide accumulation can be controlled by the worm, protecting both the animal and symbionts from sulfide toxicity. R. pachyptila must also effectively eliminate protons and sulfate ions, which result from bacterial processes. The maintenance of an alkaline internal pH is important in that it provides an optimal pH at which a variety of mechanisms function. Based on studies with proton transport inhibitors, these worms appear to control their internal pH via high concentrations of H{dollar}sp+{dollar}-ATPases, which allows for unprecedented pH regulation. In addition, R. pachyptila has a high growth potential based entirely on the bacterial oxidation of hydrogen sulfide, which results in sulfate production. It was shown that R. pachyptila does not have unusually high internal concentrations of sulfate (19-23 mM) and thus, must be using some, currently unknown, mechanism for the elimination of sulfate ions. These findings show that, in order to overcome the demands of being a host to chemoautotrophic bacteria, R. pachyptila has evolved pH dependent acquisition of CO{dollar}sb2{dollar}, mediated uptake of HS{dollar}sp-{dollar}, extremely efficient pH regulation, and perhaps a novel mechanism for sulfate elimination. All of these mechanisms illustrate the great autotrophic potential of this symbiosis, which enables the worm to dominate newly established vents along the East Pacific Rise. |
