Carbon isotope fractionation by an ancestral rubisco suggests biological proxies for CO2 through geologic time should be re-evaluated

The history of Earth's carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO2 is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. We tested the intersection of environment and evolution by measuring both biomass (?p) and enzymatic (?Rubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to ?1 Ga. This strain, nicknamed ANC, grows in ambient pCO2 and displays larger ?p values than WT, despite having a much smaller ?Rubisco (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Measuring both enzymatic and biomass fractionation revealed a surprising result -- ANC ?p exceeded ANC ?Rubisco in all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. However, these models were corrected by accounting for cyanobacterial physiology, notably the CO2 concentrating mechanism (CCM). Our model suggested that additional fractionating processes like powered inorganic carbon uptake systems contribute to ?p, and this effect is exacerbated in ANC. Understanding the evolution of rubisco and the CCM is therefore critical for interpreting the carbon isotope record. Large fluctuations in that record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO2.