Carbon Isotope Fractionation by Ancestral Rubisco in Cells

Rubisco (D-ribulose 1,5-bisphosphate carboxylase/oxygenase) - the enzyme that converts inorganic CO₂ to organic carbon (OC) during photosynthesis - has been the predominant mode of carbon (C) fixation over geologic time. In addition to its action as a carboxylase, Rubisco has oxygenase activity, raising questions about constraints on its evolution across Earth history as CO₂ has become more scarce and O₂ more abundant. Rubisco also facilitates a kinetic isotope effect that causes large fractionations in 13C/12C between source CO₂ and whole-cell fixed C during C fixation, which is largely responsible for the stable C isotope ratios of inorganic C and OC in the rock record. To evaluate how Rubisco and processes of carbon fixation have changed through time, we have been studying C fixation and C isotope fractionation in a Cyanobacteria mutant (Synechococcus elongatus strain PCC 7942) containing solely ancestral Rubiscos that date to >1 Ga. The data show how whole cell fractionations reflect differences between anabolic C acquisition and carboxylation rates, and suggest that ancient Cyanobacteria may have had larger C isotope fractionations than their modern counterparts.