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CO2 Fixation in the Hydrothermal Vent Tube Worm Riftia pachyptila (Jones)

Specimens of the large pogonophoran tube worm Riftia pachyptila were collected at the hydrothermal vents at 21°N on the East Pacific Rise (20°50′ N, 109°06′ W). Incubations of live animals in the presence of radiolabeled CO2 showed an incorporation of radioactivity in the obturacular plume into malate and succinate. It is proposed that these substances are then transported in the circulatory system of the animals to the bacteria in the trophosome, where malate is decarboxylated and the CO2 released is incorporated in the bacteria. A CO2 fixation rate of up to 16 μmol/h−1/g fresh wt−1 could be observed in the obturacular plume. Several carboxylating enzymes were tested; pyruvate carboxylase and malic enzyme (NADP) had an activity of 0.04 U/g fresh wt−1, and phosphoenolpyruvate carboxykinase (GDP) 0.01 U/g fresh wt−1; acetyl CoA carboxylase, malic enzyme (NAD), phosphoenolpyruvate carboxykinase (ADP), and phosphoenolpyruvate carboxylase could not be detected. The quantitative importance of CO2 fixation by symbiotic bacteria for the worms' metabolic needs is discussed.