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.

束毛藻这种蓝细菌固定了多达一半支撑热带开阔海洋生物群落的氮气（N₂），但其生长经常受到铁（Fe）可利用性的限制（1,2）。未来海洋变暖如何与这种全球广泛存在的束毛藻N₂固定的铁限制相互作用尚不清楚（3）。在此，我们表明，缺铁的束毛藻的最适生长温度比铁充足的细胞高约5℃，这导致在预计的更温暖且缺铁的海面条件下，其生长和N₂固定大幅增加。同时，细胞铁含量随着温度升高而降低。这两种趋势共同导致缺铁细胞的铁利用效率（IUE）受热驱动增加约470%，铁利用效率定义为束毛藻每单位时间每摩尔细胞铁固定的N₂摩尔量（mol N₂固定·h⁻¹·mol Fe⁻¹），这使得束毛藻能够更有效地利用稀缺的可用铁供应来支持N₂固定。在政府间气候变化专门委员会（IPCC）代表性浓度路径（RCP）8.5全球变暖情景下对这些结果进行建模（4）预测，到2100年，N₂固定生物的铁利用效率可能会增加约76%，并在很大程度上缓解热带太平洋和印度洋大片区域普遍存在的铁限制。受热增强的蓝细菌铁利用效率可能会使未来一个世纪全球海洋N₂固定增加约22%，从而深刻改变开阔海洋生态系统的生物学和生物地球化学特性。