LETTER Copiotrophic marine bacteria are associated with strong iron-binding ligand production during phytoplankton blooms Shane L. Hogle,* 1a Randelle M. Bundy, 1b Jessica M. Blanton, 2 Eric E. Allen, 2 Katherine A. Barbeau 1 1 Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, California; 2 Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, California Abstract Although marine bacteria were identified nearly two decades ago as potential sources for strong iron-binding organic ligands detected in seawater, specific linkages between ligands detected in natural water and the microbial community remain unclear. We compared the production of different classes of iron-binding ligands, dissolved iron and macronutrient concentrations, and phytoplankton and bacterioplankton assemblages in a series of iron amended 6-d incubations. Incubations with high iron additions had near complete macronutrient consumption and higher phytoplankton biomass compared with incubations with low iron additions, but both iron treatments were dominated by diatoms. However, we only detected the strongest ligands in high-iron treatments, and strong iron-binding ligands were generally correlated with an increased abundance of copiotrophic bacteria, particularly Alteromonas strains. Ultimately, these robust correlations suggest a potential linkage between copiotrophic bacteria and strong iron-binding ligand production after iron fertilization events in the marine environment. *Correspondence: shogle@mit.edu Present address: a Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A b Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A Author Contribution Statement: SLH, RMB, EEA, and KAB designed research; SLH, RMB, JMB, and KAB performed research; SLH, JMB, and RMB analyzed data; SLH, RMB, and KAB wrote the article. Data Availability Statement: Data and code for reproducing analyses are available from Figshare, Github, NCBI Bioproject Archive, NCBI Biosam- ple Archive, and the NCBI Sequence Read Archive. https://dx.doi.org/10.6084/m9.figshare.3184534 https://github.com/slhogle/code-LO-letters-2016 http://www.ncbi.nlm.nih.gov/bioproject/PRJNA331054 Additional Supporting Information may be found in the online version of this article. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance Statement Organic ligands shape the marine biogeochemical cycle of iron by controlling its solubility in the ocean. The strongest of the detectable marine organic ligands are presumed to be produced by iron-limited marine microbes, but some field studies have shown that these same strong ligands paradoxically increase in concentration after iron fertilization when microbes are thought to be iron replete. Currently, the specific microbes and mechanisms responsible for increases in strong iron-binding ligands remain unknown. We present evidence that bacterial taxa typically associated with nutrient-rich environments are robustly associated with strong iron-binding ligands detected during the early stages of phytoplankton bloom collapse in experimental incubations. These results may potentially explain observations of spikes in strong ligand concentrations after iron additions in the field. 1 Limnology and Oceanography Letters 00, 2016, 00–00 V C The Authors. Limnology and Oceanography Letters published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography doi: 10.1002/lol2.10026