The evolution of the marine ’nitrostat’ across the Great Oxidation Event

The bioavailability of nitrogen and phosphorus is tightly coupled in the modern ocean, a concept known as the ’nitrostat’. Phosphorus is considered the ultimate limiting nutrient on geological timescales, while nitrogen supply is biologically modulated via nitrogen fixation and denitrification. This balance maintains relatively constant N:P ratios in seawater. However, the strong redox dependence of both N and P cycling, as well as the dependence on specific enzymes, suggest that the balance of the marine ‘nitrostat’ may have evolved over Earth’s history in response to oxygenation and biological innovation. Here, we review what is known about N and P biogeochemistry across the Great Oxidation Event (GOE) and discuss cause-effect relationships between the evolving N:P balance and primary production. A key remaining uncertainty is when exactly life transitioned from energy- to nutrient-limitation. However, it is likely that a marine ‘nitrostat’ was established at least locally or temporarily from the Neoarchean onwards. This implies increasing biological control over water chemistry, which may in turn have contributed to stabilizing O 2 -producing ecosystems and thus acted as a positive feedback for the GOE.