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 delta(13)C values of chemoautotrophs at deep-sea hydrothermal vents, which cluster in two groups around -11parts per thousand and -30parts per thousand, are due to variation in isotopic discrimination by different forms of RubisCO. The most enriched 5130 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 delta(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 mumol L(-1), whereas the CO(2)/O(2) specificity factor (Omega) is low, at 8.6. The epsilon value, which is proportional to the degree of isotopic discrimination by the enzyme, was determined to be 19.5parts per thousand, lower than that for form I RubisCO. This low epsilon value supports the hypothesis that the degree of isotopic fractionation during CO(2) fixation appears to be an important influence on the delta(13)C values of R. pachyptila biomass. Our results indicate that it is necessary to consider RubisCO epsilon values in interpreting delta(13)C values from autotrophs.