INORGANIC CARBON UPTAKE IN HYDROTHERMAL VENT TUBEWORMS FACILITATED BY HIGH ENVIRONMENTAL PCO2

THE marine invertebrate Riftia pachyptila has a remarkable symbiosis with intracellular carbon-fixing sulphide-oxidizing bacteria which was first discovered at 2,450 m depth on the Galapagos Rift1-4. Such symbiotic arrangements have since been found in a variety of invertebrate taxa and habitat5,6. Studies of these symbioses have focused on temperature, sulphide and oxygen as critical environmental parameters5,7-9. As Riftia has a high growth rate and its symbionts are far removed from the host surface10,11, inorganic carbon supply to the symbionts has been recognized as a problem and host mechanisms to concentrate inorganic carbo have been posited12,13. Increased environmental CO2 partial pressure (pCO2) has not seriously been considered as a critical environmental parameter7,14. Here we report that elevated pCO2 (2.9 kPa) in the worms' environment is a determinant of internal total CO2 (SIGMACO2) and pCO2, facilitating CO2 transport and diffusion to the symbionts. We propose that elevated pCO2 is a potentially critical environmental factor for this species as well as for other chemoautotrophic symbioses.