Primary production in the sunlit surface ocean is regulated by the supply of key nutrients, primarily nitrate, phosphate and iron (Fe), required by phytoplankton to fix carbon dioxide into biomass^1,2,3. Below the surface ocean, remineralization of sinking organic matter rapidly regenerates nutrients, and microbial metabolism in the upper mesopelagic ‘twilight zone’ (200–500 m) is thought to be limited by the delivery of labile organic carbon^4,5. However, few studies have examined the role of nutrients in shaping microbial production in the mesopelagic^6,7,8. Here we report the distribution and uptake of siderophores, biomarkers for microbial Fe deficiency⁹ across a meridional section of the eastern Pacific Ocean. Siderophore concentrations are high not only in chronically Fe-limited surface waters but also in the twilight zone underlying the North and South Pacific subtropical gyres, two key ecosystems for the marine carbon cycle. Our findings suggest that bacterial Fe deficiency owing to low Fe availability is probably characteristic of the twilight zone in several large ocean basins, greatly expanding the region of the marine water column in which nutrients limit microbial metabolism, with potential implications for ocean carbon storage.

阳光充足的表层海洋的初级生产受到关键营养物质的供应调节，主要是浮游植物将二氧化碳固定成生物质所需的硝酸盐、磷酸盐和铁 （Fe）^1,2,3。在海洋表层以下，下沉的有机物的再矿化迅速再生营养物质，而中上层“暮光区”（200-500 m）的微生物代谢被认为受到不稳定有机碳输送的限制^4,5。然而，很少有研究检查营养物质在塑造中层微生物产生中的作用^6,7,8。在这里，我们报告了铁载体的分布和摄取，铁载体是微生物 Fe 缺陷⁹ 的生物标志物，在东太平洋的经向剖面上。铁载体浓度很高，不仅在长期 Fe 受限的表层水中，而且在北太平洋和南太平洋副热带环流下面的暮光带中也很高，这是海洋碳循环的两个关键生态系统。我们的研究结果表明，由于 Fe 可用性低而导致的细菌 Fe 缺乏可能是几个大型海洋盆地暮光区的特征，大大扩大了营养物质限制微生物代谢的海水柱区域，对海洋碳储存具有潜在影响。