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 similar to 5 degrees 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 similar to 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 similar to 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 similar to 22% over the next century, and thus profoundly alter the biology and biogeochemistry of open-ocean ecosystems.