Constraining the oxygen requirements for modern microbial eukaryote diversity

Eukaryotes originated prior to the establishment of modern marine oxygen (O ₂ ) levels. According to the body fossil and lipid biomarker records, modern (crown) microbial eukaryote lineages began diversifying in the ocean no later than ~800 Ma. While it has long been predicted that increasing atmospheric O ₂ levels facilitated the early diversification of microbial eukaryotes, the O ₂ levels needed to permit this diversification remain unconstrained. Using time-resolved geochemical parameter and gene sequence information from a model marine oxygen minimum zone spanning a range of dissolved O ₂ levels and redox states, we show that microbial eukaryote taxonomic richness and phylogenetic diversity remain the same until O ₂ declines to around 2 to 3% of present atmospheric levels, below which these diversity metrics become significantly reduced. Our observations suggest that increasing O ₂ would have only directly promoted early crown-eukaryote diversity if atmospheric O ₂ was below 2 to 3% of modern levels when crown-eukaryotes originated and then later met or surpassed this range as crown-eukaryotes diversified. If atmospheric O ₂ was already consistently at or above 2 to 3% of modern levels by the time that crown-eukaryotes originated, then the subsequent diversification of modern microbial eukaryotes was not directly driven by atmospheric oxygenation.