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 δ13C 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 δ13C values are from the vestimentiferan tubeworm Riftia pachyptila, whose bacterial endosymbionts provide essentially all of its organic carbon via CO2 fixation by a form II RubisCO. The kinetic parameters of purified R. pachyptila symbiont RubisCO were determined to assess the degree to which the δ13C values of tubeworm biomass are due to isotopic fractionation during CO2 fixation by this enzyme. Like most form II enzymes, the KCO2 is high, at 240 mmol L−1, whereas the CO2/O2 specificity factor (Ω) is low, at 8.6. The e 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 e value supports the hypothesis that the degree of isotopic fractionation during CO2 fixation appears to be an important influence on the δ13C values of R. pachyptila biomass. Our results indicate that it is necessary to consider RubisCO e values in interpreting δ13C values from autotrophs.