Stable Carbon Isotope Discrimination by Form IC RubisCO from \u3cem\u3eRhodobacter sphaeroides\u3c/em\u3e

Variations in the relative amounts of ¹²C and ¹³C in microbial biomass can be used to infer the pathway(s) autotrophs use to fix and assimilate dissolved inorganic carbon. Discrimination against ¹³C by the enzymes catalyzing autotrophic carbon fixation is a major factor dictating the stable carbon isotopic composition (δ¹³C = {[¹³C/¹²Csample/¹³C/¹²Cstandard] - 1} X 1000) of biomass. Six different forms of ribulose 1,5-bisphosphate carboxylase/oxygenase or RubisCO (IA, IB, IC, ID, II, and III), the carboxylase of the Calvin-Benson-Bassham cycle (CBB), are utilized by algae and autotrophic bacteria that rely on the CBB cycle for carbon fixation. To date, isotope discrimination has been measured for form IA, IB, and II RubisCOs. Isotopic discrimination, expressed as ε values (={[¹²k/¹³k] - 1} X 1000; ¹²k and ¹³k = rates of ¹²C and ¹³C fixation) range from 18 to 29‰, explaining the variation in biomass δ¹³C values of autotrophs that utilize these enzymes. Isotope discrimination by form IC RubisCO has not been measured, despite the presence of this enzyme 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 purpose of this work is to determine the e value for the form IC RubisCO enzyme from R. sphaeroides. Under standard conditions (pH 7.5 and 5 mM DIC), form IC RubisCO had an ε value of 29‰. Sampling the full phylogenetic breadth of RubisCO enzymes for isotopic discrimination makes it possible to constrain the range of δ¹³C values of organisms fixing carbon via the Calvin-Benson-Bassham cycle. These results are helpful for determining the degree to which CBB cycle carbon fixation contributes to primary and secondary productivity in microbially-dominated food webs