Iron In Seawater Research Articles

Role of superoxide in the photochemical reduction of iron in seawater

We have conducted a series of laboratory studies to investigate the generation of ferrous iron and reactive oxygen species when solutions of seawater containing natural organic matter (NOM) and ferric iron are exposed to simulated sunlight. Total ferrous iron and hydrogen peroxide were measured at nanomolar concentrations with high temporal resolution using chemiluminescence-based methods. In all cases, ferrous iron concentrations rapidly peaked at several nanomoles per litre after a few minutes, and then declined over time, while hydrogen peroxide concentrations increased in a non-linear manner. Although concentrations of both species depended on the concentration of NOM, hydrogen peroxide concentrations were only minimally affected by the presence of iron. Increasing the NOM concentration while the total iron concentration was maintained constant led to an increase in the maximum ferrous iron concentration, suggesting that superoxide-mediated reduction of iron may be a major pathway for ferrous iron formation. This was supported by measurements of superoxide production from irradiation of NOM in the absence of iron and kinetic calculations, as well as an experiment in which superoxide dismutase was added. Further analysis of the data suggested that dissolved oxygen and photo-produced hydrogen peroxide were the primary oxidants of the Fe(II) formed. Thus we propose that superoxide and ferrous iron may be intricately coupled in the system, and that their generation is determined by the supply of NOM available to harvest light and donate electrons.

Direct determination of iron in acidified (pH 1.7) seawater samples by flow injection analysis with catalytic spectrophotometric detection: Application and intercomparison

A sensitive flow injection method for determining iron in seawater developed by Measures et al. (1995) has been substantially modified to allow the direct preconcentration of dissolved iron in acidified seawater samples (pH 1.7) onto a nitrilotriacetic acid (NTA) chelating resin. This removes the need to adjust the pH and buffer samples before the preconcentration step, and the low pH eliminates potential interference from the presence of strong iron-binding organic ligands. As part of an international intercalibration exercise for the Sampling and Analysis of Fe (SAFe), we investigated at sea the precision and accuracy of this flow injection method with its preconcentration step plus catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (FI-NTA-DPD). Acidified seawater samples analyzed using FI-NTA-DPD were shown to be in excellent agreement with other shipand lab-based methods. The acidification of seawater samples to pH 1.7 is an important protocol if total dissolved iron in seawater is to be determined within hours of collection. A ship- and lab-based analytical intercomparison of two flow injection methods (FI-NTA-DPD and FI-NTA-ICP-SFMS) for the determination of total dissolved iron in seawater was carried out on SAFe samples collected from surface waters and at 1000 m depth from the North Pacific Ocean. For the two methods, total dissolved iron concentrations in surface samples were 0.101 ± 0.009 and 0.098 ± 0.009 nM, respectively, and in samples from 1000 m, 0.93 ± 0.04 and 0.92 ± 0.08 nM. No statistical difference between the FI-NTA-DPD and FI-NTA-ICP-SFMS methods was observed (P = 0.05).

Advantages of using a mercury coated, micro-wire, electrode in adsorptive cathodic stripping voltammetry

A mercury coated, gold, micro-wire electrode is used here for the determination of iron in seawater by catalytic cathodic stripping voltammetry (CSV) with a limit of detection of 0.1 nM Fe at a 60 s adsorption time. It was found that the electrode surface is stable for extended periods of analyses (at least five days) and that it is reactivated by briefly (2 s) applying a negative potential prior to each scan. Advantages of this electrode over mercury drop electrodes are that metallic mercury use is eliminated and that it can be readily used for flow analysis. This is demonstrated here by the determination of iron in seawater by continuous flow analysis. It is likely that this method can be extended to other elements. Experiments using bismuth coated, carbon fibre, electrodes showed that the bismuth catalyses the oxidation of the important oxidants bromate and hydrogen peroxide, which makes it impossible to use bismuth based electrodes for catalytic CSV involving these oxidants. For this reason mercury coated electrodes retain a major advantage for catalytic voltammetric analyses.

In situ electrochemical impedance spectroscopy/synchrotron radiation grazing incidence X-ray diffraction—A powerful new technique for the characterization of electrochemical surfaces and interfaces

A marriage of electrochemical impedance spectroscopy (EIS) and in situ synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) has provided a powerful new technique for the elucidation of the mechanistic chemistry of electrochemical systems. In this study, EIS/SR-GIXRD has been used to investigate the influence of metal ion buffer calibration ligands, along with natural organic ligands in seawater, on the behaviour of the iron chalcogenide glass ion-selective electrode (ISE). The SR-GIXRD data demonstrated that citrate – a previously reported poor iron calibration ligand for the analysis of seawater – induced an instantaneous and total dissolution of crystalline GeSe and Sb 2Se 3 in the modified surface layer (MSL) of the ISE, while natural organic ligands in seawater and a mixture of ligands in a mimetic seawater ligand system protected the MSL's crystalline inclusions of GeSe and Sb 2Se 3 from oxidative attack. Expectedly, the EIS data showed that citrate induced a loss in the medium frequency time constant for the MSL of the ISE, while seawater's natural organic ligands and the mimetic ligand system preserved the medium frequency EIS response characteristics of the ISE's MSL. The new EIS/SR-GIXRD technique has provided insights into the suitability of iron calibration ligands for the analysis of iron in seawater.

Photochemistry of Organic Iron(III) Complexing Ligands in Oceanic Systems

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (>99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O2-). The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimentallobservational studies of the natural seawater ligand pool.

Chemical Speciation of Iron in Seawater by Cathodic Stripping Voltammetry with Dihydroxynaphthalene

The chemical speciation of iron in seawater is determined by cathodic stripping voltammetry using 2,3-dihydroxynaphthalene (DHN) as adsorptive and competing ligand. The optimized conditions include a DHN concentration of 0.5-1 microM, seawater at its original pH of 8, and equilibration overnight. The alpha-coefficient for DHN (=[FeDHN]/[Fe']) was calibrated against EDTA giving values of 166 for 0.5 microM DHN and 366 at 1 microM DHN and a value of 8.51 +/- 0.07 for log K'(Fe'DHN). The dissociation of the natural iron species FeL was found to have a characteristic reaction time of 50 min, indicating that titrations should be equilibrated overnight rather than the shorter periods sometimes used onboard ship. The method was applied to samples from the Pacific giving ligand concentrations of 1.1 and 1.6 nM for deep and surface waters, respectively, with an average value for log K'(FeL) of 11.9 +/- 0.3 compared to a value of 11.5 for the siderophore deferoxamine. The results are similar to those obtained previously for similar samples, but the new method has much greater sensitivity for iron than previous methods, leading to lower limits of detection and shorter analysis time.

A community-wide intercomparison exercise for the determination of dissolved iron in seawater

The first large-scale international intercomparison of analytical methods for the determination of dissolved iron in seawater was carried out between October 2000 and December 2002. The exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24 international laboratories. The comparison was based on a large volume (700 L), filtered surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample), which was acidified, mixed and bottled at sea. Two 1-L sample bottles were sent to each participant. Integrity and blindness were achieved by having the experiment designed and carried out by a small team, and overseen by an independent data manager. Storage, homogeneity and time-series stability experiments conducted over 2.5 years showed that inter-bottle variability of the IRONAGES sample was good (< 7%), although there was a decrease in iron concentration in the bottles over time (0.8–0.5 nM) before a stable value was observed. This raises questions over the suitability of sample acidification and storage. For the complete dataset of 45 results (after excluding 3 outliers not passing the screening criteria), the mean concentration of dissolved iron in the IRONAGES sample was 0.59 ± 0.21 nM, representing a coefficient of variation (%CV) for analytical comparability (“community precision”) of 36% (1s), a significant improvement over earlier exercises. Within-run precision (5–10%), inter-run precision (15%) and inter-bottle homogeneity (< 7%) were much better than overall analytical comparability, implying the presence of: (1) random variability (inherent to all intercomparison exercises); (2) errors in quantification of the analytical blank; and (3) systematic inter-method variability, perhaps related to secondary sample treatment (e.g. measurement of different physicochemical fractions of iron present in seawater) in the community dataset. By grouping all results for the same method, analyses performed using flow injection-luminol chemiluminescence (with FeII detection after sample reduction) [Bowie, A.R., Achterberg, E.P., Mantoura, R.F.C., Worsfold, P.J., 1998. Determination of sub-nanomolar levels of iron in seawater using flow injection with chemiluminescence detection. Anal. Chim. Acta 361, 189–200] and flow injection-catalytic spectrophotometry (using the reagent DPD) [Measures, C.I., Yuan, J., Resing, J.A., 1995. Determination of iron in seawater by flow injection analysis using in-line preconcentration and spectrophotometric detection. Mar. Chem. 50, 3–12] gave significantly ( P = 0.05) higher dissolved iron concentrations than analyses performed using isotope dilution ICPMS [Wu, J.F., Boyle, E.A., 1998. Determination of iron in seawater by high-resolution isotope dilution inductively coupled plasma mass spectrometry after Mg(OH) 2 co-precipitation. Anal. Chim. Acta 367, 183–191]. There was, however, evidence of scatter within each method group (CV up to 59%), implying that better uniformity in procedures may be required. This paper does not identify individual data and should not be viewed as an evaluation of single laboratories. Rather it summarises the status of dissolved iron analysis in seawater by the international community at the start of the 21st century, and can be used to inform future exercises including the SAFE iron intercomparison study in the North Pacific in October 2004.

Impact of environmental factors on in situ determination of iron in seawater by flow injection analysis

A sensitive method for iron determination in seawater has been adapted on a submersible chemical analyser for in situ measurements. The technique is based on flow injection analysis (FIA) coupled with spectrophotometric detection. When direct injection of seawater was used, the detection limit was 1.6 nM, and the precision 7%, for a triplicate injection of a 4 nM standard. At low iron concentrations, on line preconcentration using a column filled with 8-hydroxyquinoline (8HQ) resin was used. The detection limit was 0.15 nM (time of preconcentration = 240 s), and the precision 6%, for a triplicate determination of a 1 nM standard, allowing the determination of Fe in most of the oceanic regimes, except the most depleted surface waters. The effect of temperature, pressure, salinity, copper, manganese, and iron speciation on the response of the analyser was investigated. The slope of the calibration curves followed a linear relation as a function of pressure ( C p = 2.8 × 10 − 5 P + 3.4 × 10 − 2 s nmol − 1 , R 2 = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature ( C Θ = 0.009e 0.103 Θ , R 2 = 0.832, for P = 3 bar). No statistical difference at 95% confidence level was observed for samples of different salinities ( S = 0, 20, 35). Only very high concentration of copper (1000 × [Fe]) produced a detectable interference. The chemical analyser was deployed in the coastal environment of the Bay of Brest to investigate the effect of iron speciation on the response of the analyser. Direct injection was used and seawater samples were acidified on line for 80 s. Dissolved iron (DFe, filtered seawater (0.4 μm), acidified and stored at pH 1.8) corresponded to 29 ± 4% of Fe a (unfiltered seawater, acidified in line at pH 1.8 for 80 s). Most of Fe a (71 ± 4%) was probably a fraction of total dissolvable iron (TDFe, unfiltered seawater, acidified and stored at pH 1.8).

A general kinetic model for iron acquisition by eukaryotic phytoplankton

The conventional model of iron uptake in marine eukaryotic phytoplankton—the Fe′ model—suggests a dependency of uptake rate on the concentration of unchelated iron species (Fe′), and not the concentration of total iron or iron chelated with organic ligands. However, iron in seawater is bound by strong organic ligands that buffer such low Fe′ concentrations that they should not support phytoplankton growth. Studies that show uptake and extracellular reduction of siderophore‐bound iron by diatoms and provide indications that the iron uptake system of phytoplankton may be similar to that of yeast in which extracellular reduction is a prerequisite for uptake, call for revisions of the Fe′ model. In this paper we propose a new model for iron uptake by diatoms in which extracellular reduction of all Fe species is a necessary step for iron acquisition. Experiments verifying the predictions of the model are presented. In particular we show data supporting the fact that Fe(II) is formed as an intermediate during Fe uptake from all experimental media, including those buffered by Fe(III)EDTA. This model reconciles the standing Fe′ model with new data and concepts on reduction of iron chelates and provides a convenient framework for designing and interpreting iron uptake experiments in a variety of natural and artificial media.

Organic Iron Complexation Is Real, The Theory Is Used Incorrectly. Comment on 'Measuring Marine Iron(III) Complexes by CLE-AdSV'

Environmental Context. The theoretical basis on which Town and van Leeuwen (Environ. Chem. 2005, 2, 80) dispute current ideas on the speciation of iron in seawater is valid only for the simplified condition of the binding of ionic Fe3+ with an ionic organic ligand. The possibilities of different pathways for complex formation and dissociation involving mixed hydroxide–organic species, or of different redox states, were not considered. The mismatch with experimental reality shows that the simplification is incorrect.

Parameterization of iron and manganese cycling in the Black Sea suboxic and anoxic environment

New and published data on the distribution and speciation of manganese and iron in seawater are analyzed to identify and parameterize major biogeochemical processes of their cycling within the suboxic (∼15.6⩽ σ t⩽∼16.2) and anoxic layers ( σ t⩾∼16.2) of the Black Sea. A steady-state transport-reaction model is applied to reveal layering and parameterize kinetics of redox and dissolution/precipitation processes. Previously published data on speciation of these elements in seawater are used to specify the nature of the transformations. Two particulate species of iron (Fe(III) hydroxide and Fe(II) sulfide) are necessary to adequately parameterize the vertical profile of suspended iron, while three particulate species (hydrous Mn(IV) oxide, Mn(II) sulfide, and Mn(II) carbonate) are necessary to describe the profile of suspended manganese. In addition to such processes as mixing and advection, precipitation, sinking, and dissolution of manganese carbonate are found to be essential in maintaining the observed vertical distribution of dissolved Mn(II). These results are used to interpret the observed difference in the form of vertical distribution for dissolved Mn(II) and Fe(II). Redox transformations of iron and manganese are coupled via oxidation of dissolved iron by sinking suspended manganese at σ t∼16.2±0.2 kg m −3. The particulate manganese, necessary for this reaction, is supplied through oxidation of dissolved Mn(II). The best agreement with observations is achieved when nitrate, rather than oxygen, is set to oxidize dissolved Mn(II) in the lower part of the suboxic layer (∼15.90⩽ σ t⩽∼16.2). The results support the idea that, after sulfides of these metals are formed, they sink with particulate organic matter. The sinking rates of the particles and specific rates of individual redox and dissolved-particulate transformations have been estimated by fitting the vertical profile of the net rate.

The influence of UV irradiation on the photoreduction of iron in the Southern Ocean

An iron enrichment experiment, EisenEx, was performed in the Atlantic sector of the Southern Ocean during the Antarctic spring of 2000. Deck incubations of open ocean water were performed to investigate the influence of ultraviolet B (UVB: 280–315 nm) and ultraviolet A (UVA: 315–400 nm) on the speciation of iron in seawater, using an addition of the radioisotopes 59Fe(III) (1.25 nM) or 55Fe(III) (0.5 nM). Seawater was sampled inside and outside the iron-enriched region. The radioisotopic Fe(II) concentration was monitored during daylight under three different light conditions: the full solar spectrum (total), total minus UVB, and total minus UVB+UVA. A distinct diel cycle was observed with a clear distinction between the three different light regimes. A clear linear relationship was found for the concentration of radioisotopic Fe(II) versus irradiance. UVB produced most of the Fe(II) followed by UVA and visible light (VIS: 400–700 nm), respectively. UVB produced 4.89 and 0.69 pM m 2 W −1 radioisotopic Fe(II) followed by UVA with 0.33 and 0.10 pM m 2 W −1 radioisotopic Fe(II) and VIS with 0.04 and 0.03 pM m 2 W −1 radioisotopic Fe(II).

Inhibition of carbonate synthesis in acidic oceans on early Mars.

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8-4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.

Adsorption and precipitation of iron from seawater on a marine bacteriophage (PWH3A-P1)

The distribution and chemical form of iron in marine environments is of particular interest to oceanographers because of its effect on primary productivity. The high abundance of viruses in the ocean and their small size suggest that they might serve as nucleation centers for iron adsorption and precipitation in the ocean. Hence, we performed laboratory experiments to quantify the adsorption and precipitation of iron by a marine bacteriophage (PWH3a-P1). Acid–base titrations and zeta potential measurements were performed to determine if the virus interacted with protons, and if so, to determine the type, concentration, and reactivity of functional groups present on the viruses. Iron precipitation experiments were conducted to investigate interactions between iron and the viruses. The titrations and zeta potential measurements were well described by a model considering two types of reactive sites on the viruses, with log K values of −4.44 and −8.18, and a total site concentration of 3.64×10 −18 mol per virus. High-magnification observations of the viruses in the presence of iron by whole-mount transmission electron microscopy showed evidence of both adsorption and mineral nucleation. The experimental results clearly demonstrate the potential of marine bacteriophage to serve as nuclei for iron adsorption and precipitation, and the models developed from the data suggest that viruses might represent a significant reservoir of iron in seawater. However, these experiments utilized greatly simplified chemistry and a limited range of iron-to-virus concentration ratios relative to natural seawater, and so the results of this study do not prove that viruses affect the marine iron cycle, but rather serve to justify continued research into iron–virus interactions, in the laboratory and in the field.

Analytical intercomparison between flow injection‐chemiluminescence and flow injection‐spectrophotometry for the determination of picomolar concentrations of iron in seawater

A lab‐ and ship‐based analytical intercomparison of two flow injection methods for the determination of iron in seawater was conducted, using three different sets of seawater samples collected from the Southern Ocean and South Atlantic. In one exercise, iron was determined in three different size‐fractions (&lt;0.03 µm, &lt;0.4 µm, and unfiltered) in an effort to better characterize the operational nature of each analytical technique with respect to filter size. Measured Fe concentrations were in the range 0.19 to 1.19 nM using flow injection with luminol chemiluminescence detection (FI‐CL), and 0.07 to 1.54 nM using flow injection with catalytic spectrophotometric detection with N,N‐dimethyl‐p‐phenylenediamine dihydrochloride (FI‐DPD). The arithmetic mean for the FI‐CL method was higher (by 0.09 nM) than the FI‐DPD method for dissolved (&lt;0.4 µm) Fe, a difference that is comparable to the analytical blanks, which were as high as 0.13 nM (CL) and 0.09 nM (DPD). There was generally good agreement between the FI‐CL determinations for the &lt;0.03 µm size fraction and the FI‐DPD determinations for the &lt;0.4 µm size fraction in freshly collected samples. Differences in total‐dissolvable (unfiltered) Fe concentrations determined by the two FI methods were more variable, reflecting the added complexity associated with the analysis of partially digested particulate material in these samples. Overall, however, the FI‐CL determinations were significantly (P = 0.05) lower than the FI‐DPD determinations for the unfiltered samples. Our results suggest that the observed, systematic inter‐method differences reflect measurement of different physicochemical fractions of Fe present in seawater, such that colloidal and/or organic iron species are better determined by the FI‐CL method than the FI‐DPD method. This idea is supported by our observation that inter‐method differences were largest for freshly collected acidified seawater, which suggests extended storage (&gt;6 months) of acidified samples as a possible protocol for the determination of dissolved iron in seawater.

Dissolution of atmospheric iron in seawater

In vitro dissolution experiments were conducted in seawater in order to quantify the partitioning between dissolved and particulate phases of iron associated to Saharan dust and urban particles. The percentage of iron released was very low (0.05 to 2.2%) depending on the particulate load, the aerosol source and the contact time. This percentage decreased with the amount of particles introduced following a power law, indicating that the process could be modeled. Indeed, these dissolution processes were a function of the lability of iron in relation to the particle source, the particle size and adsorption processes. According to these experiments, the dissolved iron induced in a 10 m mixed layer by Saharan events of various magnitudes ranges from 0.07 to 1 nM. Since these concentrations are in agreement with the natural iron requirements for the phytoplankton, such inputs can profoundly influence the primary production, especially in oligotrophic conditions.

Marine Biogeochemistry of Iron

Environmental Context. Several trace elements are essential to the growth of microorganisms, iron being arguably the most important. Marine microorganisms, which affect the global carbon cycle and consequently indirectly influence the world’s climate, are therefore sensitive to the presence of iron. This link means iron-related oceanic processes are a significant ecological and political issue. Abstract. The importance of the role of iron as a limiting micronutrient for primary production in the World Ocean has become increasingly clear following large-scale in situ iron fertilization experiments in high-nutrient, low-chlorophyll (HNLC) regions.[1] This has led to intensive international research with the aim of understanding the marine biogeochemistry of iron and quantifying the spatial distribution and transport of the element in the oceans. Recent studies have benefited from improved trace metal handling protocols and sensitive analytical techniques, but uncertainties remain concerning fundamental processes such as redox transfer, solubility, adsorption, biological uptake, and remineralization. This review summarizes our present knowledge of iron biogeochemistry. It begins with a discussion of the effects of the physicochemical speciation of iron in seawater from a thermodynamic perspective, including important topics such as inorganic and organic complexation and redox chemistry. This is followed by an overview of the fluxes of iron to the ocean interface and a description of iron cycling within the open ocean water column. Current uncertainties of iron biogeochemistry are highlighted and suggestions of future work provided.

Kinetics of hydrolysis and precipitation of ferric iron in seawater.

We have investigated the kinetics of iron hydrolysis and precipitation and dissolution of the solid formed via a novel chemical method, namely accessibility of iron to the fungal siderophore desferrioxamine B (DFB), with spectrophotometric detection of the ferrioxamine complex. Our results support a mechanism in which precipitation of dissolved inorganic iron in seawater is first order with respect to total (dissolved and precipitated) iron, with a second-order rate constant of (4.1 +/- 1.1) x 10(7) M(-1) s(-1) at pH 8.1. The rate of dissolution was first order with respect to the total iron concentration and the first-order rate constant decreased from 2.3 x 10(-4) s(-1) after aging for 1 min to 4.8 x 10(-6) s(-1) after aging for 1 week. The proposed reaction mechanism simulated a solubility limit condition in agreement with experimental observations, from which we calculated the solubility of ferric iron to be 1.2 x 10(-13) M when the precipitate had been aged for 1 week. This is approximately 2 orders of magnitude less than reported in previous studies, possibly due to the chemically based method for dissolved iron determination used here compared with traditional physical separation methods. Our results confirm that Fe(III) hydrolysis in seawater is fast and show thatthe precipitated solid is quite labile initially but rapidly becomes much less so, with important implications for sequestration by organic ligands such as siderophores.

The level of iron enrichment required to initiate diatom blooms in HNLC waters

The chemical speciation of iron in seawater is controlled by complexation with organic ligands, the character of which likely regulates which phytoplankton groups can readily access this resource. Evidence from the IronEx II mesoscale iron enrichment experiment provides some insight to this regulation. Dissolved iron (<0.4 μm) in the enriched surface waters during IronEx II was partitioned into colloidal (>1 kDa–0.4 μm) and soluble (<1 kDa) size fractions using cross flow filtration to better delineate iron dynamics with respect to phytoplankton growth. While dissolved concentrations in the patch increased by two orders of magnitude to ∼2 nM Fe, soluble concentrations only roughly doubled to ∼35 pM Fe, and this change occurred only during the first few days of the experiment. The subsequent decrease in soluble iron to ambient levels coincided with a dramatic increase in chlorophyll a, indicating that biological demand was responsible for the disappearance of soluble iron. This decrease also coincided with preferential drawdown of silicic acid over nitrate in the patch, indicating that diatoms were experiencing iron stress even as the bloom developed. Even so, calculations of the diffusional flux and iron uptake kinetics at the cellular level reveal that the concentrations of soluble iron during the later stages of the bloom were above that required to support continued rapid growth of the long, narrow pennate diatoms. This observation implies that the bulk of the organically bound soluble and colloidal iron was unavailable to meet the diatom growth requirements, and further that the measured increase in concentrations of strong Fe(III) complexing ligands impaired iron acquisition by diatoms. These findings indicate that excess macronutrients in HNLC waters might not be fully converted into diatom biomass even with repeated infusions of iron.

Large-scale latitudinal distribution of Trichodesmium spp. in the Atlantic Ocean

The Atlantic Meridional Transect (AMT) programme is a series of bi-annual cruises between the Falkland Islands (50°S) and the UK (50°N). Measurements of the abundance of the N2-fixing, colonial cyanobacterium Trichodesmium along this transect in the years 1995–1999 reveal that it is especially abundant between 0 and 15°N, but by contrast almost completely absent between 5 and 30°S. The cruise path between 0 and 15°N lies close to 20°W, on the African (eastern) side of the Atlantic. The maximum colony abundances we observed (100 000 colonies m–2) are greater than those reported in many other studies. The results of different methods for assessing Trichodesmium abundance are compared. Possible correlations between Trichodesmium abundance and several physical and chemical variables were examined to try and elucidate the factors controlling N2 fixer occurrence. High Trichodesmium abundance was found to be correlated with shallow mixed layer depth and high estimated iron deposition to the surface ocean, but not with temperature, nitrate or the concentration of total dissolvable iron in sea water.