Marine arsenate evolution during Proterozoic oxygenation events and implications for biological phosphate metabolism

Earth’s atmosphere and oceans experienced two major episodes of ocean-atmosphere oxygenation that transformed the planet and shaped the evolution of the oxygen-rich biosphere between 2.45 and 0.541 billion years ago (Ga). These changes increased the abundance of oxidized chemicals in seawater, but their distribution and effects on the availability and utilization of essential nutrients remain poorly understood. Here we show that these oxygenation events were accompanied by stepwise increases in marine arsenate content—a toxic compound that closely resembles phosphate and interferes with its biological use. Arsenate concentrations rose during the first global oxygenation event and again after the end of the Sturtian global glaciation. Our results suggest that seawater contained much higher arsenate relative to phosphate than today for much of early Earth’s history, likely restricting biological access to phosphate. After the Sturtian glaciation, increasing phosphate availability lowered the arsenate-to-phosphate ratio, reducing arsenate toxicity and enabling more efficient phosphate use in shallow coastal seas, thereby supporting the expansion of oxygen-dependent life.