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

RubisCO, the CO2 fixing enzyme of the Calvin-Benson-Bassham (CBB) cycle, is responsible for the majority of carbon fixation on Earth. RubisCO fixes 12 CO2 faster than 13 CO2 resulting in 13 C-depleted biomass, enabling the use of δ13 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 δ13 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 δ13 C values in nature.