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

Abstract

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.

The role of humic-type ligands in the bioavailability and stabilization of dissolved iron in the Western Tropical South Pacific Ocean

“…The remaining 3 nmol eq-Fe L -1 of the L Fe pool may be considered as weaker ligands compared to L FeHS . Our findings are consistent with the results reported by Gledhill et al, 2022, who demonstrated that the heterogeneity of binding sites in humic-like DOM enables humic substances to outcompete siderophores at low iron concentrations.…”

“…In regard to CLE-CSV data, L FeHS are mainly assigned to the intermediate class of ligand (e.g. L 2 ; Laglera and van den Berg, 2009;Gledhill and Buck, 2012) but this complex mixture possibly includes components of other classes (Perdue and Lytle, 1983;Laglera et al, 2019;Gledhill et al, 2022). Field observations (Buck et al, 2018) and numerical simulations (Misumi et al, 2013) suggest that L 2 type ligands regulate the DFe distribution as well as its residence time in the deep ocean (Hunter and Boyd, 2007).…”

Trace metal stoichiometry of dissolved organic matter in the Amazon plume

“…Concentrations of Fe SPE-amb in the marine end-member (0.12 ± 0.03 nM) were an order of magnitude higher than the highest siderophore concentrations reported in the ocean ( 44 ). We further highlight that recoveries of siderophores are typically higher [46%; ( 44 )] than the average recovery observed for DOC in this study, and siderophores may therefore make up a minor component of the total organically bound Fe fraction in coastal waters ( 45 ).…”

New Insights into the Organic Complexation of Bioactive Trace Metals in the Global Ocean from the GEOTRACES Era

“…Such models provide an estimation of metal speciation (Hiemstra and van Riemsdijk, 2006;Stockdale et al, 2011) linking DOM bioavailability with iron solubility (Zhu et al, 2021a). These models may be appropriate in the marine setting given recognition that much of the complexation capacity in the ocean may be attributed to polydisperse degradation products and, in accounting for the complexity of seawater DOM, the NICA-Donnan model approach may better predict iron bioavailability in natural waters (Zhu et al, 2021a;Gledhill et al, 2022aGledhill et al, , 2022b. However, there are still open questions regarding the extent to which speciation is well described by assumptions of thermodynamic equilibrium and the extent to which different metals may compete for the same metal binding sites.…”