Microbial iron limitation in the ocean’s twilight zone

One of the major advances in ocean biogeochemistry achieved over the past three decades is an understanding of how nutrients, primarily nitrate, phosphate and iron (Fe), combine in complex patterns to limit and shape primary production in the surface ocean^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) instead appears to be limited by the delivery of labile organic carbon^4,5. In contrast to the large number of studies describing nutrient limitation in ocean surface waters, nutrient limitation of microbial production in the mesopelagic has been unexplored. Here we report the distribution and uptake of siderophores, biomarkers for microbial Fe limitation⁶, across a meridional section of the eastern Pacific Ocean. Siderophore concentrations were high in chronically Fe limited surface waters, but they were also surprisingly high in the twilight zone underlying the North and South Pacific subtropical gyres, two key ecosystems for the global carbon cycle. Bacterial Fe deficiency due to low Fe availability is likely characteristic of the twilight zone in several large ocean basins, greatly expanding the region of the marine water column where nutrients limit microbial metabolism with potentially significant impacts on ocean carbon storage.