Isotope Discrimination During Photosynthesis Remained Constant Across the Phanerozoic

Photosynthesis is among the most important evolutionary innovations in Earth history. It has formed the foundation for most life on Earth today, oxygenated our planet, revolutionised the carbon cycle, and enabled aerobic respiration that eventually led to the evolution of large complex organisms. The most widespread oxygenic photosynthesis pathway involves fixing carbon through the enzyme Ribulose 1,5-bisphosphate carboxylase-oxygenase (RuBisCO) that converts CO2 into sucrose via the Calvin-Benson cycle. During this process, RuBisCO preferentially uptakes¹²CO2 over¹³CO2, resulting in stable carbon isotopic fractionation of up to 25-28 %. Despite its fundamental importance to the operation of our planet, it is not clear whether RuBisCO has always operated the same; in other words, has photosynthetic carbon isotope discrimination undergone major evolutionary changes or remained constant across the geologic record? Supporting the argument for change, marine photoautotrophs have experienced multiple step-changes and long-term evolutionary restructurings, atmospheric CO2/O2 ratios have fluctuated dramatically, and the carbon cycle has been radically transformed (e.g., by the evolution of land plants) across the Phanerozoic (the last 541 million years), all could theoretically impact the stable carbon isotopic fractionation associated with photosynthesis by RuBisCO. Supporting the argument for no change, the carbon isotopic composition (δ¹³C) of marine bulk organic matter has remained remarkably consistent over geologic time (e.g., Garcia et al., 2021) and the reconstruction of a Precambrian-age, ancestral RuBisCO gene resurrected in modern cyanobacteria shows isotopic discrimination within modern δ¹³C range (Kędzior et al., 2022). However, the δ¹³C of bulk organic matter may be influenced by source heterogeneity and differential preservation and is thus too crude to robustly infer changes in the operation of RuBisCO and photosynthetic pathways. Here, we resolve the concerns regarding bulk δ¹³C data by applying compound specific isotope analyses of the organic compounds pristane and phytane to determine whether the photosynthetic carbon isotope discrimination has remained constant across the Phanerozoic.