Detection of Iron Ligands in Seawater and Marine Cyanobacteria Cultures by High-Performance Liquid Chromatography–Inductively Coupled Plasma-Mass Spectrometry (original) (raw)
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Vertical distributions of iron-(III) complexing ligands in the Southern Ocean
Deep Sea Research Part II: Topical Studies in Oceanography, 2011
Electrochemically derived iron speciation data from four vertical profiles to 1000 m depth were obtained during the SAZ-Sense voyage to offshore waters south of Australia in summer (January/February, 2007). The dual aims of this study were firstly to devise a new operational definition to represent the 'complexing capacity', or total concentration of iron-complexing ligands, and subsequently derive vertical profiles of these ligand classes. Secondly, to compare the vertical trends for each ligand class with vertical distributions in oceanic properties thought to control ligand production (i.e. siderophores produced by bacteria and particle remineralisation). Based on simulated ligand titrations, we operationally defined SL as the overall class of ligands, which represents all iron-complexing ligands detectable under the analytical conditions chosen. The stability constant of SL is a weighted average for these ligands. The ligand titration data suggests the presence of an excess of iron-complexing ligands throughout the water column with an average concentration of [SL]¼ 0.7570.20 nM (n¼ 47), and an average stability constant of log K FeSL,Fe 3 þ ¼ 21.5070.24 (n¼ 47). Here, based on the range of observed stability constants we define a distinctly different class of extremely strong ligands (L 1 ) to be the ligand class with a stability constant of log K FeL1,Fe 3 þ Z22 , whereas SL ranged from 21.00 to 21.95 for log K FeL1 ,Fe 3 þ . L 1 had an average concentration and stability constant of 0.4270.10 nM (n¼14) and 22.9770.48 (n¼ 14), respectively. L 1 was only found in three of the four depth profiles, and was restricted to the upper ocean (i.e. o200 m depth), whereas SL was observed at all sampling depths down to 1000 m. Heterotrophic bacterial abundances (a proxy for siderophore production) were always the highest in the surface mixed layer (50-72 m depth for the 4 stations) then decreased sharply, whereas POC downward flux (a proxy for remineralisation) was greatest below the surface mixed layer then decreased exponentially. It has been suggested that siderophores control L 1 production whereas the remainder of SL may be set by particle breakdown . Hence we should expect some vertical partitioning of L 1 (presento70 m depth) and SL (present over the water column). However, profiles at all stations in subtropical, subantarctic, and polar waters exhibited distinguishable concentrations of L 1 to 200 m depth (i.e. straddling both regions of putative L 1 and SL production). There remain issues with the separation of different ligand classes, such that since [L 1 ]r [Fe], deeper in the water column, the concentration of L 1 cannot be resolved, and hence the provenance of both L 1 and SL cannot be clearly assigned.
2023
Iron (Fe) is an essential micronutrient for marine phytoplankton, but its poor solubility in oxic waters is responsible for limited primary production across large parts of the ocean. It has been shown that organic compounds are able to bind with Fe to keep it in the dissolved phase (DFe), thought to be the most bioavailable for phytoplankton. The compounds composing the fraction of the dissolved organic matter (DOM) able to bind with Fe are referred to as Fe-binding ligands. While > 99% of DFe is bound to Fe-binding ligands, the knowledge on their identity and cycling is limited as they represent a very small and diverse fraction of the DOM. There is a one-thousand factor difference between the range of concentrations of Fe-binding ligands and the wider DOM pool (nanomolar and micromolar, respectively), the composition and structure of which is impacted by multiple biological and physical processes. One method to investigate Fe-binding properties of the DOM is to titrate Fe-binding ligands against a calibrated added ligand at different DFe concentrations. This method is called Competitive Ligand Exchange (CLE) using Cathodic Stripping Voltammetry (ACSV). The CLE-ACSV approach allows the estimation of the Fe-binding ligand concentration ([L]), and of their average binding strength (Kcond). This approach, however, suffers from technical and practical limitations. In this work, I address several limitations of the CLE-ACSV approach and present techniques to improve accessibility for new users and minimise the risk of user subjectivity within data selection. I then apply these to samples collected from the subtropical South Pacific to assess the role of Fe-binding ligands in the distribution and cycling of DFe in waters impacted by intense diazotrophic and hydrothermal activities. In Chapter 1, I introduce the general background of my work by reviewing the inorganic and organic aspects of DFe speciation. I present in Chapter 2 a description of the CLE-ACSV approach; the concept, the theory, the apparatus, and the technical limitations are discussed to provide general knowledge on the different aspects hampering the application of the CLE-ACSV. In Chapter 3, I address the limitation of the CLE-ACSV approach related to the pH buffering of the sample, a technical requirement of the current methods which are potentially impacting and hampering our understanding of the DFe speciation. In Chapter 4, limitations related to the interpretation and comparability of the CLE-CSV 2 titrations are addressed. I present a procedure developed to limit the subjectivity of the analyst on the results produced, aiming to ease the comparability of the results between laboratories. Finally, in Chapter 5, I present the application of my CLE-ACSV development on natural samples collected in the Western Tropical South Pacific. Fe-binding ligand data are combined with electrochemical and fluorescence data of the humic fraction, a known contributor to the Fe-binding ligand pool, to interpret the composition and cycling of the fraction of the DOM implicated in DFe distribution in this region. I finally conclude this thesis by sharing some thoughts about how to move forward in this challenging but important research area that marine Fe speciation is.
Marine Chemistry, 2014
The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1-0.65 nM dissolved iron (b 0.2 μm) was partitioned into the colloidal iron (cFe) size fraction (10 kDa b cFe b 0.2 μm), as expected for areas of the ocean underlying a dust plume. The 1.3-2.0 nM strong organic iron-binding ligands, however, overwhelmingly (75-77%) fell into the soluble size fraction (b10 kDa). As a result, modeling the dissolved iron size fractionation at equilibrium using the observed ligand partitioning did not accurately predict the iron partitioning into colloidal and soluble pools. This suggests that either a portion of colloidal ligands is missed by current electrochemical methods because they react with iron more slowly than the equilibration time of our CLE-ACSV method, or part of the observed colloidal iron is actually inorganic in composition and thus cannot be predicted by our model of unbound iron-binding ligands. This potentially contradicts the prevailing view that greater than N99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton.
Evidence for organic complexation of iron in seawater
Marine Chemistry, 1995
Iron occurs at very low concentrations in seawater of oceanic origin and its low abundance is thought to limit primary production in offshore waters (Martin and Fitzwater, 1988). A new electrochemical method, cathodic stripping voltammetry (CSV), is used here to determine the speciation of iron in seawater originating from the Western Mediterranean taking advantage of ligand competition of an added electroactive ligand with the natural organic complexing matter to evaluate whether iron is organically complexed. The measurements indicate that iron occurs 99% (or 99.9% depending on which value is selected for aFe,) complexed by organic complexing ligands throughout the water column of the Western Mediterranean and by analogy probably also in other oceanic waters. The composition of the organic complexing ligands is as yet unknown, but the data indicate a major source from microorganisms (bacteria or phytoplankton) in and immediately below the fluorescence maximum in the upper water column. The organic complexes are apparently reversible releasing iron when the competing ligand is added and binding more iron when its concentration is increased. The organic complexing ligands occur at concentrations well above those of iron ensuring full complexation of this biologically essential element, and buffer the free iron concentration at a very low level against fluctuations as a result of removal by primary producers or inputs from atmospheric sources. The new data indicate that a re-evaluation of the concept of the bioavailable fraction of iron is required.
Deep Sea Research Part II: Topical Studies in Oceanography, 2008
During the Kerguelen Ocean and Plateau compared Study (KEOPS; January-February 2005) cruise, the area southeast of the Kerguelen Archipelago in the Indian sector of the Southern Ocean was investigated to identify the mechanisms of natural iron fertilization of the Kerguelen Plateau. In this study, the organic speciation of Fe is described. Samples were determined immediately on board using competing ligand-adsorptive cathodic stripping voltammetry (CL-AdCSV). The dissolved organic ligands were always in excess of the dissolved Fe concentration, increasing the residence time in the water column and the potential availability for phytoplankton. The concentration of the dissolved organic ligands ranged from 0.44 to 1.61 nEq of M Fe ( ¼ complexation site for Fe), with an average concentration of 0.91 nEq of M Fe (S.D. ¼ 0.28, n ¼ 113) and a mean logarithm of conditional stability constant (log K 0 ) of 21.7 (S.D. ¼ 0.28, n ¼ 113). A second weaker dissolved organic ligand group was detected in 32% of the samples, with Fe-binding characteristics at the edge of the detection window of the applied method.