Enhancement and inhibition of iron photoreduction by individual ligands in open ocean seawater
Enhancement and inhibition of iron photoreduction by individual ligands in open ocean seawater
In laboratory experiments, we investigated the effect of five individual Fe-binding ligands: phaeophytin, ferrichrome, desferrioxamine B (DFOB), inositol hexaphosphate (phytic acid), and protoporphyrin IX (PPIX) on the Fe(II) photoproduction using seawater of the open Southern Ocean. Addition of 10-100 nM Fe(III) to open Southern Ocean seawater without the model ligands and containing; 1.1 nM dissolved Fe(III), 1.75 +/- 0.28 equivalents of nM Fe of natural ligands with a conditional stability constant (log K) of 21.75 +/- 0.34 and a concentration DOC of 86.8 +/- 1.13 M C leads to the formation of amorphous Fe(III) hydroxides. These amorphous Fe(III) hydroxides are the major source for the photoproduction of Fe(II). The addition of the model ligands changed the Fe(II) photoproduction considerably and in various ways. Phaeophytin showed higher Fe(II) photoproduction than ferrichrome and the control, i.e., amorphous Fe(III) hydroxides. Additions of phytic acid between 65 and 105 nM increased the concentration of photoproduced Fe(II) with 0.16 nM Fe(II) per nM phytic acid, presumably due to the co-aggregation of Fe(III) and phytic acid leading via an increasing colloidal surface to an increasing photoreducible Fe(III) fraction. DFOB and PPIX strongly decreased the photoproduced Fe(II) concentration. The low Fe(II) photoproduction with DFOB confirmed reported observations that Fe(III) complexed to DFOB is photo-stable. The PPIX hardly binds Fe(III) in the open Southern Ocean seawater but decreased the photoproduced Fe(II) concentration by complexing the Fe(II) with a binding rate constant of k(Fe(II)PPIX) = 1.04 x 10(-4) 1.53 x 10(-5) s(-1) nM(-1) PPIX. Subsequently, PPIX is suggested to act as a photo sensitizing producer of superoxide, thus increasing the dark reduction of Fe(III) to Fe(II). Our research shows that the photochemistry of Fe(III) and the resulting photoproduced Fe(II) concentration is strongly depending on the identity of the Fe-binding organic ligands and that a translation to natural conditions is not possible without further characterization of the natural occurring ligands.
diatom thalassiosira-weissflogii, cathodic stripping voltammetry, thiol-containing compounds, equatorial pacific-ocean, natural organic-ligands, open southern-ocean, atlantic-ocean, dissolved iron, inositol hexaphosphate, siderophore production
2790-2805
Rijkenberg, Micha J.A.
549cc6d0-6f1a-432f-863a-2988abcb25d3
Gerringa, Loes J.A.
0675b739-d214-4bf0-a923-398557683fcf
Carolus, Vicky E.
3eef8894-465e-46df-9184-b9295764c6d2
Velzeboer, Ilona
16306100-f135-4894-bcf4-ce08c16f7f21
de Baar, Hein J.W.
0fc12f82-c34e-472a-ac96-ad3681c8179e
1 June 2006
Rijkenberg, Micha J.A.
549cc6d0-6f1a-432f-863a-2988abcb25d3
Gerringa, Loes J.A.
0675b739-d214-4bf0-a923-398557683fcf
Carolus, Vicky E.
3eef8894-465e-46df-9184-b9295764c6d2
Velzeboer, Ilona
16306100-f135-4894-bcf4-ce08c16f7f21
de Baar, Hein J.W.
0fc12f82-c34e-472a-ac96-ad3681c8179e
Rijkenberg, Micha J.A., Gerringa, Loes J.A., Carolus, Vicky E., Velzeboer, Ilona and de Baar, Hein J.W.
(2006)
Enhancement and inhibition of iron photoreduction by individual ligands in open ocean seawater.
Geochimica et Cosmochimica Acta, 70 (11), .
(doi:10.1016/j.gca.2006.03.004).
Abstract
In laboratory experiments, we investigated the effect of five individual Fe-binding ligands: phaeophytin, ferrichrome, desferrioxamine B (DFOB), inositol hexaphosphate (phytic acid), and protoporphyrin IX (PPIX) on the Fe(II) photoproduction using seawater of the open Southern Ocean. Addition of 10-100 nM Fe(III) to open Southern Ocean seawater without the model ligands and containing; 1.1 nM dissolved Fe(III), 1.75 +/- 0.28 equivalents of nM Fe of natural ligands with a conditional stability constant (log K) of 21.75 +/- 0.34 and a concentration DOC of 86.8 +/- 1.13 M C leads to the formation of amorphous Fe(III) hydroxides. These amorphous Fe(III) hydroxides are the major source for the photoproduction of Fe(II). The addition of the model ligands changed the Fe(II) photoproduction considerably and in various ways. Phaeophytin showed higher Fe(II) photoproduction than ferrichrome and the control, i.e., amorphous Fe(III) hydroxides. Additions of phytic acid between 65 and 105 nM increased the concentration of photoproduced Fe(II) with 0.16 nM Fe(II) per nM phytic acid, presumably due to the co-aggregation of Fe(III) and phytic acid leading via an increasing colloidal surface to an increasing photoreducible Fe(III) fraction. DFOB and PPIX strongly decreased the photoproduced Fe(II) concentration. The low Fe(II) photoproduction with DFOB confirmed reported observations that Fe(III) complexed to DFOB is photo-stable. The PPIX hardly binds Fe(III) in the open Southern Ocean seawater but decreased the photoproduced Fe(II) concentration by complexing the Fe(II) with a binding rate constant of k(Fe(II)PPIX) = 1.04 x 10(-4) 1.53 x 10(-5) s(-1) nM(-1) PPIX. Subsequently, PPIX is suggested to act as a photo sensitizing producer of superoxide, thus increasing the dark reduction of Fe(III) to Fe(II). Our research shows that the photochemistry of Fe(III) and the resulting photoproduced Fe(II) concentration is strongly depending on the identity of the Fe-binding organic ligands and that a translation to natural conditions is not possible without further characterization of the natural occurring ligands.
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Published date: 1 June 2006
Keywords:
diatom thalassiosira-weissflogii, cathodic stripping voltammetry, thiol-containing compounds, equatorial pacific-ocean, natural organic-ligands, open southern-ocean, atlantic-ocean, dissolved iron, inositol hexaphosphate, siderophore production
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Local EPrints ID: 41433
URI: http://eprints.soton.ac.uk/id/eprint/41433
ISSN: 0016-7037
PURE UUID: a23eaf6b-31dc-45ff-af0b-530ae58265d6
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Date deposited: 12 Sep 2006
Last modified: 15 Mar 2024 08:29
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Author:
Micha J.A. Rijkenberg
Author:
Loes J.A. Gerringa
Author:
Vicky E. Carolus
Author:
Ilona Velzeboer
Author:
Hein J.W. de Baar
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