OCEAN OXYGENATION, PHOSPHORUS CYCLING, AND THE EXPANSION OF EUKARYOTIC LIFE

The rise of eukaryotes to ecological dominance represents one of the most dramatic shifts in the history of Earth’s biosphere. It has become increasingly clear that eukaryotes were a minor component of marine ecosystems prior to the late Proterozoic, despite the appearance of crown group eukaryotes in the fossil record occurs nearly 1 billion years earlier. In parallel, there has been a secular increase in the availability of the key macronutrient phosphorus (P) in Earth’s oceans. Here, we use an Earth system model equipped with a size-structured marine ecosystem and a model of Earth’s coupled C-P-S-O₂ cycles to explore whether eukaryote expansion could be tied to increasing abundance and availability of marine nutrients. We find a strong dependence of planktonic ecosystem structure on nutrient levels in the ocean interior, with higher nutrient levels leading to greater overall biomass, broader size spectra, and increasing abundance of large zooplankton grazers. We suggest that increases in the ecological impact of eukaryotic algae and trophic complexity in eukaryotic ecosystems were directly linked to restructuring of the global P cycle associated with the protracted rise of surface oxygen levels. This model finds strong support in observations from Earth’s rock record, including microfossil, organic biomarker, and inorganic geochemical data indicating contemporaneous time-dependent changes to the late Proterozoic phosphorus cycle and the composition of shallow marine ecosystems. Our model provides a simple explanation for the puzzling lag between the emergence of crown-group eukaryotic organisms and their much later ecological expansion, and a novel framework linking the evolution of biological complexity to shifts in the Earth’s coupled oxygen, carbon, and nutrient cycles.