Phosphorus is believed to be the globally limiting nutrient in the modern ocean, but a number of nutrients have been invoked as limiting the Proterozoic biosphere. Mass balance calculations suggest that Proterozoic net primary productivity must have been 1 to 2 orders of magnitude less than today in order to maintain low oxygen levels despite increased burial efficiency in anoxic environments. The resulting demand for nutrients is so low that nitrogen, molybdenum, and iron could not have limited the rate of primary production following the evolution of extant nitrogenases. Phosphorus demand was approximately equal to the modern riverine flux, making phosphorus the most likely candidate for the limiting nutrient throughout the Proterozoic. Key Points The rate of marine primary productivity must have been at least 10 times smaller during the Proterozoic than it is today This rate is sufficiently small that the biosphere was unlikely to be nitrogen limited, even if molybdenum was efficiently scavenged Phosphorus is the most likely candidate for the limiting nutrient during the Proterozoic eon