Award: OCE-1356747

Iron, copper, cobalt, and nickel serve as reaction centers in enzymes that catalyze many key metabolic processes such as photosynthesis, respiration, and nitrogen fixation. These metals are therefore essential for the growth and health of marine microbes. In many regions of the ocean, especially those regions closest to land, concentrations of metals in seawater are high enough for microbes to easily meet their metabolic needs. But across large regions of the ocean, particularly in remote areas, metal concentrations are vanishingly low, and their abundance limits what microbes can grow, and how fast they are able to do so. It has long been believed that one way microbes compete for limited amounts of iron and other essential metals is through the production of organic compounds specifically designed to complex metals and facilitate their uptake. Some microbes have evolved receptors that allow them to "see" specific metal-organic complexes, and target their uptake through dedicated transport systems. This iron acquisition strategy is known to occur in soils and in freshwaters, and is even a factor in the virulence of human pathogens. In all three systems, bacteria produce siderophores, organic compounds with a high affinity for iron, to extract this element from their environment. However, there was little evidence that this process occurs in the ocean. The goal of this project was to develop and deploy new methods designed to detect and identify siderophores and other trace metal organic complexes in seawater. To meet this goal, we designed and built a targeted approach that relies on the separation of organic compounds using chromatography, detection of metals using mass spectrometry, and algorithm assisted identification of metal-organic complexes. We applied our system to detect and identify iron, copper, and nickel complexes across the tropical South Pacific Ocean using samples collected as part of the US GEOTRACES program. We found a suite of iron, copper, and nickel organic complexes across the region. This result confirms that marine microbes deliberately synthesize organic compounds to target metals for uptake. With this knowledge in hand, we were able to interrogate public genomic data catalogues to show that the siderophore synthesis pathways were widely distributed across the Atlantic and Pacific Oceans. The presence of siderophore synthesis in so many regions of the ocean confirms that iron limitation is a persistent and widespread problem for marine microbes. We know very little about the metabolic needs of microbes for copper and nickel, but our results clearly show that marine microbes manipulate their environment to change the chemistry of these metals, perhaps to facilitate uptake, perhaps to render them less toxic. We found copper and nickel complexes were most abundant in oligotrophic areas where exceedingly low nutrient concentrations place severe constraints on microbial productivity. It seems likely that under such severe nutrient stress, microbes are particularly sensitive to copper toxicity, and therefore produce specific compounds designed to bind copper to make it less toxic to living cells. A number of graduate and undergraduate students received training in environmental analytical chemistry, oceanography, and microbial physiology as part of this project. We also developed a tool that is widely applicable for targeting and characterizing organically complexed metals, metalloids, halogens, and nutrients in environmental samples. A number of colleagues and scientists in the US, Europe, and Asia have expressed interest in or are actively building similar systems in their laboratories. Results from the project have been published in peer-reviewed journals, and disseminated through presentations at scientific meetings and at venues open to the general public. All data from the project have been deposited in publically available databases, and codes for targeted search algorithms have been likewise made publically available through github and other websites. Last Modified: 04/04/2018 Submitted by: Daniel J Repeta