Unusual carbon dioxide-combining properties of body fluids in the hydrothermal vent tubeworm Riftia pachyptila

Abstract

Total CO2 (ΣCO2) up to 50 mmol l−1, corresponding to internal CO2 partial pressures (PCO2) up to 6 kPa (1 kPa ≈ 7.5 mm Hg or Torr), have been measured in the body fluids of the hydrothermal vent tubeworm Riftia pachyptila (Jones) sampled at 13°N on the East Pacific Rise. At physiological pH values, such high ΣCO2 and PCO2 are quite unusual in water-breathing animals. In Riftia, both blood and coelomic fluid contain extracellular haemoglobins at fairly high concentrations, but their titration by a strong acid as well as CO2 equilibration experiments on dialysed and ultra-filtered fluids showed that the presence of these proteins account for only one-tenth to one-third of the observed ΣCO2. Gel filtration analysis of the fluids revealed that the protein-free fractions retained most of the CO2: about 64% in blood and 80% in coelomic fluid. This corresponds to a base excess of unknowm nature and origin at a concentration up to 30 meq l−1. The nutritional needs of the mouthless and gutless Riftia are totally derived from the metabolic activity of sulphide-oxidizing, chemolithoautotrophic bacterial symbionts able to fix inorganic carbon into organic molecules. In an external environment in which PCO2 varies widely due to the turbulent mixing of the hot, sulphide- and CO2-rich vebt water with the cold, CO2-poor deep sea water, we propose that the base excess has three main functions: first, to retain CO2 in the body fluids when the external PCO2 is low; second, to act as a buffer when external PCO2 is high; third, to allow the large coelomic compartment to act as a CO2 store, thus permitting the symbiotic bacteria to fix carbon continuously even when the external supply of carbon dioxide fluctuates.