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

• DOI: 10.1073/pnas.2300466120
• Publication Date: 2023-05-08T19:17:50Z
• Authors: Renée Z. Wang; Robert J. Nichols; Albert K. Liu; Avi I. Flamholz; Juliana Artier; Doug M. Banda; David F. Savage; John M. Eiler; Patrick M. Shih; Woodward W. Fischer
• Keywords: Carbon fibers; Carbon-14; Geologic time scale

The history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution photosynthesis. Fortunately, key parts have been recorded isotope ratios sedimentary rocks. dominant model used to interpret this record as a proxy for ancient CO 2 is based on fractionations modern photoautotrophs, and longstanding questions remain about how their might impacted record. Therefore, we measured both biomass (ε p ) enzymatic Rubisco cyanobacterial strain ( Synechococcus elongatus PCC 7942) solely expressing putative ancestral Form 1B rubisco dating ≫1 Ga. This strain, nicknamed ANC, grows ambient pCO displays larger ε values than WT, despite having much smaller (17.23 ± 0.61‰ vs. 25.18 0.31‰, respectively). Surprisingly, ANC exceeded all conditions tested, contradicting prevailing models fractionation. Such can be rectified by introducing additional isotopic fractionation associated powered inorganic uptake mechanisms present Cyanobacteria, but amendment hinders ability accurately estimate historical from geological data. Understanding concentrating mechanism therefore critical interpreting record, fluctuations may reflect evolving efficiency fixing metabolisms addition changes .