Oxygen requirements of early eukaryote ecosystems

Eukaryotes originated prior to the establishment of modern marine oxygen (O2) levels. According to the body fossil and lipid biomarker records, modern (crown) microbial eukaryote lineages began diversifying in the ocean approximately 800 million years ago. While it has long been predicted that increasing atmospheric O2 levels facilitated this early diversification of microbial eukaryotes, the O2 levels needed for this remain unconstrained. Using time-resolved geochemical parameter and gene sequence information from a model marine oxygen minimum zone spanning a range of dissolved O2 levels and redox states, we show that microbial eukaryote richness and phylogenetic diversity remain largely unchanged until O2 declines to around 3% of present atmospheric levels, below which minimum diversity values decrease.