EVOLUTION OF CARBON FIXATION IN CYANOBACTERIA DURING PRECAMBRIAN TIME

It has long been appreciated that patterns observed in carbon isotope ratios preserved in different materials in the rock record reflect both environmental and evolutionary changes, but it has been difficult to deconvolve the two. The enzyme Rubisco (ribulose-1,5,-bisphosphate carboxylase/oxygenase) in the Calvin cycle is thought to be largely responsible for the large carbon isotope fractionation between inorganic and organic carbon pools in the modern, but Rubisco and the CBB cycle has also co-evolved in cell and environments in which the abundances of carbon dioxide and dioxygen have changed dramatically over time. To understand better understand how carboxylation has evolved in Cyanobacteria we generated a strain of Cyanobacteria in the Synechococcus elongatus PCC 7942 background in which its native Rubisco was swapped out for one we inferred and synthesized as an ancestral form 1B Rubisco (approximate antiquity >> 1 Ga). Remarkably this strain grows well across a range of carbon dioxide concentrations, including in ambient. From observations of the carbon isotope fractionation of the pure enzymes in vitro and in vivo in whole cells under a range of different growth conditions, we developed the hypothesis that patterns seen in the Proterozoic carbon isotope record might provide a measure of the evolution and efficiency of carboxylation process over time as much as it might inform changes in ancient atmospheric carbon dioxide and dioxygen. Moreover, this study provide an example of how we are beginning to access tools that allow one to study—experimentally—how evolution has honed metabolic processes like photosynthesis over large swaths of geological time.