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

2003; Wiley; Volume: 48; Issue: 1 Linguagem: Inglês

10.4319/lo.2003.48.1.0048

1939-5604

J. J. Robinson, Kathleen M. Scott, S. Todd Swanson, Marion H. O’Leary, Kempton M. Horken, F. Robert Tabita, Colleen M. Cavanaugh,

Marine and coastal ecosystems

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.