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

Abstract

It has been hypothesized that the ␦ 13 C values of chemoautotrophs at deep-sea hydrothermal vents, which cluster in two groups around Ϫ11‰ and Ϫ30‰, are due to variation in isotopic discrimination by different forms of RubisCO. The most enriched ␦ 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 ␦ 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 is high, at 240 mol L Ϫ1 , whereas the CO 2 /O 2 specificity factor (⍀) is low, at 8.6. The CO2 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 value supports the hypothesis that the degree of isotopic fractionation during CO 2 fixation appears to be an important influence on the ␦ 13C values of R. pachyptila biomass. Our results indicate that it is necessary to consider RubisCO values in interpreting ␦ 13 C values from autotrophs.At deep-sea hydrothermal vents, invertebrates with chemoautotrophic bacterial symbionts provide a substantial por-1 Present address: E&J Gallo Winery,

Stable isotopes reveal limitations in C and N assimilation in the Caribbean reef corals Madracis auretenra, M. carmabi and M. formosa

“…This indicates that seawater DIN supply would be sufficient at, or below, this depth to meet levels of the actual photosynthetic capacity of the Madracis species investigated. The δ 13 C values of POM -as an allochthonous food source -are similar to δ 13 C values expected for enzymatically driven photosynthetic isotope fractionation by RubisCO, form II, known for dinoflagellates (Leggat et al 1999, Robinson et al 2003. This ambiguity limits our ability to further interpret the trophic state of deep-water specimens by means of stable isotopes.…”

Kinetic isotope effect and biochemical characterization of form IA RubisCO from the marine cyanobacterium Prochlorococcus marinus MIT9313

“…For enzyme from R. rubrum, three separate investigations with high-precision methods are in agreement and have determined a fractionation factor of between 17% and 22% for the enzyme, depending on the concentration of Mg +2 (Roeske and O'Leary 1985;Guy et al 1993;McNevin et al 2006). By use of the same methods, the form II enzyme from R. pachyptila symbionts was found to have an e value of 19.5% (Robinson et al 2003). This smaller e value in form II RubisCOs is reflected by the more isotopically enriched stable isotope compositions measured in most hydrothermal vent organisms expressing form II (Cavanaugh and Robinson 1996).…”

Trophic structure of hydrothermal vent communities at Myojin Knoll and Nikko Seamount in the northwestern Pacific: Implications for photosynthesis-derived food supply

“…In chemosynthesis-based ecosystems, bacterial δ 13 C values reflect the carbon fixation pathway: The rTCA cycle and the CBB cycle using form II RuBisCo result in 2-13 and 9-15 depletion, respectively; while the CBB cycle using form I RuBisCo results in 27-35 depletion (House et al 2003;Robinson et al 2003). Form II RuBisCo requires less O 2 than form I RuBisCo (Paoli & Tabita 1998).…”