Isotope discrimination by form IC RubisCO from Ralstonia eutropha and Rhodobacter sphaeroides , metabolically versatile members of ‘ Proteobacteria ’ from aquatic and soil habitats

Summary RubisCO, the CO 2 fixing enzyme of the Calvin–Benson–Bassham (CBB) cycle, is responsible for the majority of carbon fixation on Earth. RubisCO fixes 12 CO 2 faster than 13 CO 2 resulting in 13 C‐depleted biomass, enabling the use of δ 13 C values to trace CBB activity in contemporary and ancient environments. Enzymatic fractionation is expressed as an ε value, and is routinely used in modelling, for example, the global carbon cycle and climate change, and for interpreting trophic interactions. Although values for spinach RubisCO ( ε = ~29‰) have routinely been used in such efforts, there are five different forms of RubisCO utilized by diverse photolithoautotrophs and chemolithoautotrophs and ε values, now known for four forms (IA, B, D and II), vary substantially with ε = 11‰ to 27‰. Given the importance of ε values in δ 13 C evaluation, we measured enzymatic fractionation of the fifth form, form IC RubisCO, which is found widely in aquatic and terrestrial environments. Values were determined for two model organisms, the ‘ Proteobacteria’ Ralstonia eutropha (ε = 19.0‰) and Rhodobacter sphaeroides ( ε = 22.4‰). It is apparent from these measurements that all RubisCO forms measured to date discriminate less than commonly assumed based on spinach, and that enzyme ε values must be considered when interpreting and modelling variability of δ 13 C values in nature.