Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model

Zhu, Kechen; Birchill, Antony J.; Milne, Angela; Ussher, Simon J.; Humphreys, Matthew; Carr, Nealy; Mahaffey, Claire; Lohan, Maeve C.; Achterberg, Eric P.; Gledhill, Martha

doi:10.1016/j.marchem.2021.104038

Cited by 9 publications

(23 citation statements)

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“…The potential impact of siderophores on the relative abundance of inorganic iron, iron bound to a humic substance like fraction and the formation of solid iron hydroxide phases was calculated by combining an ion-pair model with the non-ideal competitive adsorption (NICA) -Donnan model (Kinniburgh et al, 1996) within the speciation programme ORCHESTRA (Meeussen, 2003). We used the model as applied in Zhu et al (2021a), and consider marine DOM as a heterogeneous pool of binding sites analogous to fulvic acid. To describe iron binding to organic matter, we used generic parameters from Milne et al, (2001;2003) combined with the NICA iron parameters (Supplementary Table 1) optimized for the Celtic Sea (Zhu et al, 2021a).…”

Section: Methodsmentioning

confidence: 99%

“…We used the model as applied in Zhu et al (2021a), and consider marine DOM as a heterogeneous pool of binding sites analogous to fulvic acid. To describe iron binding to organic matter, we used generic parameters from Milne et al, (2001;2003) combined with the NICA iron parameters (Supplementary Table 1) optimized for the Celtic Sea (Zhu et al, 2021a). We use a single dissolved organic carbon concentration of 47 μmol L -1 since scaling binding site concentrations with DOC concentrations had a minor impact on the calculated speciation in our study area (Zhu et al, 2021a).…”

Section: Methodsmentioning

confidence: 99%

“…At least 10% of C atoms are therefore bonded to a heteroatom with an available proton binding group and the total proton binding site concentration in seawater can be estimated to be of the order of a few μmol L -1 . Proton affinities for marine DOM follow a bimodal distribution as observed for humic substances including fulvic and humic acids, although analysis to date suggests that marine DOM is likely less heterogeneous than terrestrial fulvic and humic acids (Lodeiro et al, 2020;Lodeiro et al, 2021;Zhu et al, 2021a;Zhu et al, 2021b). The heterogeneity of marine DOM means that a small portion of the many binding sites present likely have structures favourable for formation of relatively stable iron complexes via e.g., the chelate effect (Fraústo Da Silva, 1983) and has implications for assessing competitive interactions between ions (Tipping, 1993).…”

Section: Introductionmentioning

confidence: 99%

“…The impact of siderophores on iron biogeochemistry is likely influenced by the presence of other iron binding compounds present in seawater such as humic substances (Laglera et al, 2007;Gledhill and Buck, 2012;Slagter et al, 2017;Whitby et al, 2020). Recent work has confirmed that binding site distributions in marine dissolved organic matter are heterogeneous and thus like those of humic substances (Hiemstra and van Riemsdijk, 2006;Lodeiro et al, 2020;Lodeiro et al, 2021;Zhu et al, 2021a;Zhu et al, 2021b). The total concentration of binding sites in the portion of marine DOM that can be extracted from seawater by hydrophobic resins at pH 2 approximates 130 mmol mol -1 C (Lodeiro et al, 2020;Lodeiro et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

“…In this study we combine high performance liquid chromatography (HPLC) with high resolution electrospray ionisation mass spectrometry (ESI-MS) and inductively coupled mass spectrometry (ICP-MS) to identify and quantify siderophores in the Celtic Sea on the Northwest European Shelf. We used the range of determined siderophore concentrations to further assess the competition between siderophores and a humic substance-like component of DOM, which was investigated on the same cruises in parrallel to this study (Zhu et al, 2021a). We therefore add complexation of siderophores to the non-ideal competitive adsorption (NICA) model developed in Zhu et al (2021a) and calculate the resultant species distributions as function of increasing siderophore concentration in order to improve understanding of the impact of siderophore production on iron speciation in our study area (Birchill et al, 2017;Poulton et al, 2017;Davis et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

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Competitive Interactions Between Microbial Siderophores and Humic-Like Binding Sites in European Shelf Sea Waters

et al. 2022

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Siderophores are low molecular weight high affinity iron chelates found at low concentrations in seawater. In this study we determined the total concentrations and identities of siderophores in extracts isolated from a shelf sea environment on the Northwest European shelf by high performance liquid chromatography coupled to high resolution inductively coupled plasma mass spectrometry (ICP-MS) in parallel to high resolution electrospray ionisation mass spectrometry (ESI-MS). We identified a total of 24 different siderophores in our samples via metal isotope profiling of masses detected by ESI-MS. Twenty three of the identified siderophores could be assigned to three siderophore families – ferrioxamines, amphibactins and marinobactins. In contrast, only 12 peaks could be resolved in iron chromatograms obtained via ICP-MS analysis. Comparison of results obtained by the two mass spectrometry detectors showed that neither method was able to detect and identify all siderophores present in the samples on its own. We assessed the impact of our observed total siderophore concentrations on iron speciation by calculating the distribution of iron species as a function of total siderophore concentrations at different iron concentrations representative of our study area. We considered competition for iron between siderophores, a humic like dissolved organic matter (DOM) fraction and hydroxide ions by combining an ion-pair model with a non-ideal competitive interaction (NICA)-Donnan model. We found that the overall impact of siderophores on iron biogeochemistry is low at concentrations of siderophore <100 pmol L-1, and that the dominant iron species present at siderophore concentrations of the order of a few tens of pmol L-1 will be iron bound to the humic like DOM fraction. Furthermore the heterogeneity of binding sites in the humic like DOM fraction means that other binding sites present in organic matter could be effective competitors for siderophores, especially at low iron concentrations. Our findings highlight the importance of binding site heterogeneity when considering the influence of different iron binding groups on iron speciation in the marine environment.

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