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Iron binding organic ligands

Periodic Report Summary 2 - FEBOL (Iron binding organic ligands)

Concentrations of iron in many regions of the ocean are in the picomolar to low nanomolar ranges, largely due to its low solubility and rapid uptake by phytoplankton. Severe iron limitation of ocean productivity has been identified in the Equatorial and Subarctic Pacific, the Southern Ocean and the coastal upwelling systems adjacent to California and Peru. Over 99 % of iron in seawater is complexed with organic ligands. We hypothesised that these ligands are of biological origin and might function as marine siderophores. We propose to answer key questions such as:
- are siderophores present in the mixture of iron binding ligands in seawater? and
- do marine photoautotrophs use siderophores to acquire iron? or
- are natural iron binding ligands a by-product of generic organic matter cycling with little effect on iron uptake?
To do that it is necessary to know the chemical composition and structure of the iron-ligands extracted from ocean samples.

Our work at the Woods Hole Oceanographic Institution (WHOI) focused on the isolation of naturally occurring organic iron biding ligands from open ocean waters using both hydrophobic resin XAD16 and tangential-flow filtration. These isolates are being analysed in order to determine its chemical composition, total iron concentration and conditional stability constants.

The trace metal technique developed at the WHOI to concentrate the high molecular weight fraction of dissolved organic matter by tangential-flow filtration has been implemented by the researcher at the Instituto de Investigaciones Marinas (IIM-CSIC). Application of this isolation method during the return phase of the IOF has allowed the researcher to initiate a new research line in the home laboratory, securing of a three-year research position, and getting competitive funds to develop her own research.

Our efforts have been focused on:
(i) collecting and processing samples from contrasting marine environments including the Western Mexican Shelf, the Equatorial Pacific, the Chilean upwelling, the South Pacific subtropical gyre, and the Iberian coastal upwelling system; and
(ii) developing an adequate methodology to separate and characterise iron-ligand complexes from those samples, using high pressure liquid chromatography and mass spectroscopy.
In this sense, there are two major contributions from our work so far. First, we have shown that about one third of the iron-ligands are extracted from seawater with these extraction techniques. Second, we have been able to interface liquid chromatography with mass spectroscopy to detect natural iron-ligand complexes. The technique reveals very fast and with picomolar sensitivity, which is at about the expected concentration range for iron-ligand in seawater. Most importantly, we have been able to observe that there are a number of distinct iron ligands in our samples that roughly cluster into three polarity ranges. However, this is a work still very challenging with many issues that have to be sorted out in collaboration with research groups at the University of South California (USC) and Massachusetts Institute of Technology (MIT).

These technical difficulties with the interfacing of the liquid chromatograph and mass spectrometer have not allowed us to chemically characterise the iron-ligands in due time. In compensation, we have conducted an experiment at station Aloha (Pacific Ocean, Hawaii) to determine if the ligands from the iron-ligand complexes affect the iron uptake by plankton. The experiment was designed, assembled, and performed by the researcher and students in the Repeta, DeLong, and Saito laboratories (WHOI and MIT). The meta-genomics analysis of this experiment revealed strong differences between ligand-amended samples and iron-amended treatments, suggesting that ligand amendments may induce a carbon-specific response, observed at the microbial community level, while effects of the iron amendments may strictly be observed at the gene expression level.