Stable carbon isotope discrimination by rubisco enzymes relevant to the global carbon cycle

Five different forms of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO; IA, IB, IC, ID, II), the carboxylase of the Calvin-Benson-Bassham cycle (CBB), are utilized by plants, algae and autotrophic bacteria for carbon fixation. Discrimination against 13C by RubisCO is a major factor dictating the stable carbon isotopic composition (δ13C = {[13C/12C sample/13C/12C standard] - 1} X 1000) of biomass. To date, isotope discrimination, expressed as ε values (={[12k/13k] - 1} X 1000; 12k and 13k = rates of 12C and 13C fixation) has been measured for form IA, IB, and II RubisCOs from only a few species, with ε values ranging from 18 to 29 /. The aim of this study was to better characterize form ID and IC RubisCO enzymes, which differ substantially in primary structure from the IB enzymes present in many cyanobacteria and organisms with green plastids, by measuring isotopic discrimination and kinetic parameters (KCO2 and Vmax). Several major oceanic primary producers, including diatoms, coccolithophores, and some dinoflagellates have form ID RubisCO, while form IC RubisCO is present in many proteobacteria of ecological interest, including marine manganese-oxidizing bacteria, some nitrifying and nitrogen-fixing bacteria, and extremely metabolically versatile organisms such as Rhodobacter sphaeroides. The ε - values of the form ID RubisCO from the coccolithophore, Emiliania huxleyi and the diatom, Skeletonema costatum (respectively 11.1 / and 18.5 /) were measured along with form IC RubisCO from Rhodobacter sphaeroides and Ralstonia eutropha (respectively 22.9 / and 19.0 /). Isotopic discrimination by these form ID/IC RubisCOs is low when compared to form IA/IB RubisCOs (22-29 /). Since the measured form ID RubisCOs are less selective against 13C, oceanic carbon cycle models based on 13C values may need to be reevaluated to accommodate lower ε values of RubisCOs found in major marine algae. Additionally, with further isotopic studies, the extent to which form IC RubisCO from soil microorganisms contributes to the terrestrial carbon sink may also be determined