Research Project

The role of siderophores on the photoreductive dissolution of iron oxides

Investigators: Paul Borer, Stephan Kraemer, Barbara Sulzberger, Stephan Hug,
Ruben Kretzschmar

 

Background

In large parts of the ocean, iron is the limiting growth factor for marine bacteria and algae. In these regions where the essential nutrients (except for iron) are plentiful, more than 99% of dissolved iron is bound by strong organic compounds, which are likely to be excreted by marine bacteria and algea. Due to their high affinity and specificity for iron, these compounds - termed siderophores - are able to complex free iron and therefore keep iron dissolved. Siderophore bound iron can then be incorporated into marine bacteria and algae by different mechanisms. The extent to which iron limits primary productivity in open ocean waters depends on both the abundance of iron and its bioavailability. Particulate iron is less bioavailable (if at all) than dissolved forms, however, the most important source of new iron to oceanic surface waters is by the deposition of atmospheric dust (containing various iron bearing phases). Understanding the fate of the deposited iron bearing phases is a major goal of the ongoing research in marine iron biogeochemistry and associated research fields (e.g. climate research). Any process that converts particulate iron to dissolved iron forms and prevents precipitation of the dissolved iron, may increase the pool of bioavailable iron. Siderophores may play a major role in the solubilization of particulate iron. An important and probably underestimated process contributing to the solubilization is the photoreductive dissolution. So far it has been assumed that photoreductive dissolution is ineffective due to the high pH of marine surface waters. However, the situation may change dramatically in the presence of strong iron complexing ligands.

 

Purpose of this study

So far, photoreductive dissolution studies have not included the role of strong chelating ligands like siderophores that may play a major role in the photochemical cycling of iron. This is where this research project sets its focus.

The aim of this basic research study is to investigate the (photochemical) solubilization of iron oxides in the presence of strong ligands like siderophores. Allthough it is not possible to study the photochemical solubilization of iron oxides under conditions resembling those for remote ocean waters (nM concentrations of soluble and particulate iron and siderophores), this basic research may help to gain insight into the photochemical cycling of iron and the conversion of particulate iron to more bioavailable forms in the presence of strong ligands.

 

Siderophores and model ligands

Over 500 different siderophores from culturable organisms are known today, but it has been impossible to fully elucidate the structures of the extremely strong iron ligands in remote ocean waters. However, these ligands have been shown to be siderophore-like, having similar molecular masses, conditional stability constants and having functional groups typical for siderophores.

Typical metal binding functional groups of siderophores are hydroxamate, chatecholate and less a-hydroxycarboxylate groups. Of these groups the a-hydroxycarboxylic group has a special feature: its Fe(III)-complex is prone to photoreduction. This leads to the question if siderophores containing such a-hydroxycarboxylic groups are substantually involved in the conversion of iron to more bioavailable forms.

The model siderophores in this research project are DFOB (desferrioxamine B) and aerobactin. DFOB is a trihydroxamate siderophore, whereas one of the iron binding groups of aerobactin is a a-hydroxycarboxylic acid functional group.

Besides these model siderophores other smaller model ligands with carboxylic, a-hydroxycarboxylic and hydroxamate groups are used to study the (photo)chemical dissolution of iron oxides.

 

Aerobactin	DFOB (Desferrioxamine B)

 

Objectives

The (photochemical) dissolution of iron oxides depends on many factors, which are all to be studied in this work (points i. ii. and iii.). To reduce work load, only lepidocrocite (g-FeOOH ) is used as the main model iron oxide.

i) photo-formation of Fe(II) in solution and at the oxide surface

- photoreductive ligands or siderophores containing photoreductive metal binding groups may induce the formation of Fe(II) by a LMCT mechanism. Under irradiated conditions an electron from the metal binding group is transferred to the metal center. A premis to this is the formation of an innersphere complex between the binding group of the ligand and the metal center.

Siderophores adsorbed to an oxide surface may lead to the formation of surface Fe(II) - depending on which types of binding groups (photoreactive or not) are involved in the adsorbed complexes. Therefore knowledge about the surface speciation is of greatest interest. A dissolved Fe(III)-complex may be reduced, but at the same time the surface Fe(III)-complex may not.

- surface Fe(II) may also be formed by the photolysis of hydroxylated surface sites. Here the chemical bond between Fe(III) and the hydroxyl groups (-OH) is broken - leading to the formation of Fe(II) and a hydroxyl radical.

- surface Fe(II) may also be formed inside the iron oxide bulk by a semiconductor mechanism. A formed photoelectron may migrate to the surface and reduce a surface Fe(III).

 

ii) detachment kinetics of the photoproduced surface Fe(II)

- The detachment of Fe(II) depends on many factors including oxic, anoxic conditions, different pH, and of course the labilizing effect of adsorbed ligands.

 

iii) reoxidation of photoproduced surface Fe(II)

- If the detachment is too slow, surface produced Fe(II) may be reoxidized

 

 

Small summary of completed research

from Borer et al. , 2005. Marine Chemistry 93, 179-193:

- We have observed a synergistic effect of DFOB on the photodissolution of lepidocrocite by the photoreductive oxalate ligand. DFOB is very effecient at detaching photoproduced Fe(II) (LMCT from oxalate) from the surface, even at pH 6. Reoxidation of surface Fe(II) is outcompeted by DFOB acting as an efficient shuttle for the transfer of surface Fe(II) into the solution.

- We have shown that the dissolved Fe(III)-aerobactin complex is photoreduced under irradiated conditions, leading to photooxidation of the a-hydroxycarboxylic binding group. Taking DFOB as a reference siderophore without any photoreductive properties and comparing lepidocrocite dissolution rates with aerobactin, we have drawn the conclusion that the a-hydroxycarboxylic group in aerobactin is not involved in the adsorbed surface complex on lepidocrocite.This means that the observed light induced dissolution of lepidocrocite by aerobactin cannot be explained by LMCT of the a-hydroxycarboxylic acid group. In the presence of both aerobactin and DFOB, acceleration of lepidocrocite dissolution under irradiated conditions was observed. This was explained by the formation of surface Fe(II) either by photolysis of surface Fe(III)-hydroxo groups or by the migration of photoelectrons inside the semiconductor bulk to the surface.

 

Current research

The two main apects are

i) to confirm that the a-hydroxycarboxylic acid binding group in aerobactin is not involved in the adsorbed surface complex on lepidocrocite. And to show that other model ligands containing a-hydroxycarboxylic groups are photolysed at the surface, given that the a-hydroxycarboxylic group is involved in the surface complex.

With a new UV -ATR FTIR setup we are able to investigate photochemical processes at the water-oxide interface in situ and in real time at a molecular level.

PP-Beaker mounted on top of a ATR Crystal

 

ii) to elucidate the acceleration of lepidocrocite dissolution in the presence of DFOB under irradiated conditions. Since DFOB has no photoreductive properties, the acceleration is due to the the formation of surface Fe(II) either by photolysis of surface Fe(III)-hydroxo groups or by the migration of photoelectrons inside the semiconductor bulk to the surface.

Other aspects are to quantify the underlying processes (see below) leading to the photodissolution of lepidocrocite in the presence of siderophores and other photoreductive ligands like citrate, oxalate:

i) formation of surface Fe(II)
ii) detachment of surface Fe(II)
iii) reoxidation of surface Fe(II)

 

solar simulator used for dissolution experiments

 

Relevant Publications

Kraemer, S.M, Butler, A., Borer, P., Cervini-Silva, J., 2005. Siderophores and the dissolution of iron-bearing minerals in marine systems. Rev. Min. Geochem. 59, 53-84.

Borer P.M., Sulzberzer B., Reichard P., Kraemer S.M., 2005. Effect of siderophores on the light induced dissolution of colloidal iron(III)(hydr)oxides. Mar. Chem. 93, 179-193. [PDF]