Kinetic isotope effect and characterization of form II RubisCO from the chemoautotrophic endosymbionts of the hydrothermal vent tubeworm Riftia pachyptila

It has been hypothesized that the δ 13 C values of chemoautotrophs at deep‐sea hydrothermal vents, which cluster in two groups around −11‰ and −30‰, are due to variation in isotopic discrimination by different forms of RubisCO. The most enriched δ 13 C values are from the vestimentiferan tubeworm Riftia pachyptila , whose bacterial endosymbionts provide essentially all of its organic carbon via CO 2 fixation by a form II RubisCO. The kinetic parameters of purified R. pachyptila symbiont RubisCO were determined to assess the degree to which the δ 13 C values of tubeworm biomass are due to isotopic fractionation during CO 2 fixation by this enzyme. Like most form II enzymes, the K CO2 is high, at 240 mmol L −1 , whereas the CO 2 /O 2 specificity factor (Ω) is low, at 8.6. The ε value, which is proportional to the degree of isotopic discrimination by the enzyme, was determined to be 19.5‰, lower than that for form I RubisCO. This low ε value supports the hypothesis that the degree of isotopic fractionation during CO 2 fixation appears to be an important influence on the δ 13 C values of R. pachyptila biomass. Our results indicate that it is necessary to consider RubisCO ε values in interpreting δ 13 C values from autotrophs.