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Front. Mar. Sci. | doi: 10.3389/fmars.2018.00061

Distinct Siderophores Contribute to Iron Cycling in the Mesopelagic at Station ALOHA

  • 1Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, United States
  • 2School of Oceanography, University of Washington, United States
  • 3Pacific Northwest National Laboratory (DOE), United States
  • 4Physical and Biological Sciences, University of California, Santa Cruz, United States

The distribution of dissolved iron (Fe), total organic Fe-binding ligands, and siderophores were measured between the surface and 400 m at Station ALOHA, a long term ecological study site in the North Pacific Subtropical Gyre. Dissolved Fe concentrations were low throughout the water column and strong organic Fe-binding ligands exceeded dissolved Fe at all depths; varying from 0.9 nmol L-1 in the surface to 1.6 nmol L-1 below 150 m. Although Fe does not appear to limit microbial production, we nevertheless found siderophores at nearly all depths, indicating some populations of microbes were responding to Fe stress. Ferrioxamine siderophores were most abundant in the upper water column, with concentrations between 0.1-2 pmol L-1, while a suite of amphibactins were found below 200 m with concentrations between 0.8-11 pmol L-1. The distinct vertical distribution of ferrioxamines and amphibactins may indicate disparate strategies for acquiring Fe from dust in the upper water column and recycled organic matter in the lower water column. Amphibactins were found to have conditional stability constants (log ) ranging from 12.0-12.5, while ferrioxamines had much stronger conditional stability constants ranging from 14.0-14.4, within the range of observed L1 ligands by voltammetry. We used our data to calculate equilibrium Fe speciation at Station ALOHA to compare the relative concentration of inorganic and siderophore complexed Fe. The results indicate that the concentration of Fe bound to siderophores was up to two orders of magnitude higher than inorganic Fe, suggesting that even if less bioavailable, siderophores were nevertheless a viable pathway for Fe acquisition by microbes at our study site. Finally, we observed rapid production of ferrioxamine E by particle-associated bacteria during incubation of freshly collected sinking organic matter. Fe-limitation may therefore be a factor in regulating carbon metabolism and nutrient regeneration in the mesopelagic.

Keywords: Iron, Siderophores, Station ALOHA, organic ligands, Iron limitation

Received: 07 Dec 2017; Accepted: 09 Feb 2018.

Edited by:

Samuel Wilson, University of Hawaii at Manoa, United States

Reviewed by:

Martha Gledhill, University of Southampton, United Kingdom
Oliver Baars, Princeton University, United States  

Copyright: © 2018 Bundy, Boiteau, McLean, Turk-Kubo, McIlvin, Saito, Van Mooy and Repeta. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Daniel J. Repeta, Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, 266 Woods Hole Road, Woods Hole, 02543, MA, United States, drepeta@whoi.edu