Microbial iron limitation in the ocean's twilight zone

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^1-3. 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^6-8. 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.