CARBON ISOTOPE EVIDENCE FOR THE GLOBAL PHYSIOLOGY OF PROTEROZOIC CYANOBACTERIA (Invited Presentation)

Ancestral cyanobacteria are assumed to dominate primary production after the Great Oxidation Event ≈2.4-2.0 Ga ago. However, carbon isotope fractionation by extant members of the dominant marine clade (α-cyanobacteria) is inconsistent with isotopic records of carbon fixation by primary producers in the mid‑Proterozoic Eon (1.8-1.0 Ga ago). To resolve this disagreement, we experimentally characterized carbon isotope fractionation by several strains of planktic β‑cyanobacteria and an engineered Proterozoic analog lacking a CO₂-concentrating mechanism (CCM). We also statistically characterized carbon isotope fractionation by benthic cyanobacterial communities. At relevant mid‑Proterozoic pH and pCO₂ conditions, only carbon isotope fractionation by wild-type β-cyanobacteria is consistent with the Proterozoic carbon isotope record, while carbon isotope fractionation by benthic cyanobacteria and by β-cyanobacteria lacking a CCM is not. β-Cyanobacteria with CCMs were apparently the major primary producers in the pelagic Proterozoic ocean, despite atmospheric CO₂ levels up to 100-times modern. Such profound physiological stasis implies that an understanding of how modern cyanobacterial CCMs impact net carbon isotope fractionation can inform the Proterozoic C isotope record. We report an isotope flux-balance model and culturing experiments under varying environmental conditions (pCO₂ and light levels) to determine the intracellular controls on carbon isotope fractionation by cyanobacteria during the Proterozoic Eon.