Carbon isotope fractionation by an ancestral rubisco suggests that biological proxies for CO ₂ through geologic time should be reevaluated

Abstract

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 CO 2 is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. Therefore, we measured both biomass (ε p ) and enzymatic (ε 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 pCO 2 and displays larger ε p values than WT, despite having a much smaller ε Rubisco (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Surprisingly, ANC ε p exceeded ANC ε Rubisco in all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. Such models can be rectified by introducing additional isotopic fractionation associated with powered inorganic carbon uptake mechanisms present in Cyanobacteria, but this amendment hinders the ability to accurately estimate historical pCO 2 from geological data. Understanding the evolution of rubisco and the CO 2 concentrating mechanism is therefore critical for interpreting the carbon isotope record, and fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO 2 .

The Carbon Isotopic Composition of Archean Kerogen and Its Resilience Through the Rock Cycle

“…Metabolic pathways that produce biomass impart characteristic and complex effects on the isotopic composition of organic matter (and nitrogen, hydrogen, sulfur, and oxygen; Berg et al, 2010;Blaser et al, 2013). Although the dominant form of carbon fixation (Fuchs, 2011) and the respective carbon isotope fractionation in the Archean remains unknown (Flamholz et al, 2022;Wang et al, 2023), biomass fixation processes produce non-stochastic distributions of isotopes at both the site-and compound-specific level (See Supplementary Text for discussion of abiotic or-…”