Ocean warming alleviates iron limitation of marine nitrogen fixation

The cyanobacterium Trichodesmium fixes as much as half of the nitrogen (N 2 ) that supports tropical open-ocean biomes, but its growth is frequently limited by iron (Fe) availability 1 , 2 . How future ocean warming may interact with this globally widespread Fe limitation of Trichodesmium N 2 fixation is unclear 3 . Here, we show that the optimum growth temperature of Fe-limited Trichodesmium is ~5 °C higher than for Fe-replete cells, which results in large increases in growth and N 2 fixation under the projected warmer Fe-deplete sea surface conditions. Concurrently, the cellular Fe content decreases as temperature rises. Together, these two trends result in thermally driven increases of ~470% in Fe-limited cellular iron use efficiencies (IUEs), defined as the molar quantity of N 2 fixed by Trichodesmium per unit time per mole of cellular Fe (mol N 2 fixed h –1 mol Fe –1 ), which enables Trichodesmium to much more efficiently leverage the scarce available Fe supplies to support N 2 fixation. Modelling these results in the context of the IPCC representative concentration pathway (RCP) 8.5 global warming scenario 4 predicts that IUEs of N 2 fixers could increase by ~76% by 2100, and largely alleviate the prevailing Fe limitation across broad expanses of the tropical Pacific and Indian Oceans. Thermally enhanced cyanobacterial IUEs could increase future global marine N 2 fixation by ~22% over the next century, and thus profoundly alter the biology and biogeochemistry of open-ocean ecosystems. The growth of nitrogen-fixing marine cyanobacteria Trichodesmium is limited by iron availability under current conditions. However warmer temperatures reduce the iron requirement, allowing greater growth rates and increased nitrogen fixation.