Constraining the oxygen requirements for modern microbial eukaryote diversity.

Eukaryotes originated prior to the establishment of modern marine oxygen (O) 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 levels facilitated the early diversification of microbial eukaryotes, the O 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 levels and redox states, we show that microbial eukaryote taxonomic richness and phylogenetic diversity remain the same until O declines to around 2 to 3% of present atmospheric levels, below which these diversity metrics become significantly reduced. Our observations suggest that increasing O would have only directly promoted early crown-eukaryote diversity if atmospheric O 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 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.