Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Measurement of the redox speciation of iron in seawater by catalytic cathodic stripping voltammetry

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe_R) in the absence and presence of 2 μM Dp, Fe_R being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H₂O₂ and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe_R. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe_R, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe_R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.     

The carbonate system in hypersaline solutions: dead sea brines

Various investigators reported a decrease in pH as seawater is concentrated. A similar phenomenon was reported for Dead Sea waters which are about ten times more saline than seawater. The reasons for the low pH values of Dead Sea waters (pH 5.9–6.5), which precipitate CaCO₃, were investigated by determining the apparent dissociation constants of carbonic acid in these brines. A new method, based on alkalinity titration and least-squares fitting, was used to estimate the proton activity coefficient (γ_H+) and the first and second dissociation constants of carbonic acid (K₁′, K₂′) in natural and artificial Dead Sea waters. It was found that as the salt content increases, pK′₁ and pK′₂ values progressively decrease whereas γ_H+ sharply increase. At the highest salinity investigated (TDS = 330 gl^?1) γ_H+ pK′₁ and pK′₂ values are 24.5, 5.09 and 6.23, respectively, as compared to about 0.8, 5.9, 9.1 respectively for normal seawater (19‰ chlorinity) at the same temperature (30°C).The implication of the results of this study regarding solubility of CaCO₃ and the general behavior of the carbonate system in hypersaline solutions is discussed.     

Voltammetric study of adsorption of cu(II) species on solid particles added to seawater

The adsorption on solid particles of natural organic ligand in seawater of Cu(II) ions, and of the inert Cu(II) complexes has been studied. Model solids, γ-Al₂O₃, Na⁺-0.392-γ-Al₂O₃, ‘Aerosil 200’, chrysotile, northupite and CaCO₃ were added to seawater. It was observed that at pH 8 natural organic matter was strongly adsorbed on chrysotile and was not adsorbed on Na⁺ -0.392-γ-Al₂O₃; it was also adsorbed on γ-Al₂O₃ over the range of 3 < pH < 7. In this pH range, the complexing capacity and adsorption of Cu is at a minimum because Cu(II) is not adsorbed on γ-Al₂O₃ and natural organic matter is adsorbed. Inert CuL complexes were adsorbed at pH 8.0 on γ-Al₂O₃, ‘Aerosil 200’, CaCO₃, and chrysotile but they were not adsorbed on northupite. The voltammetric method can be recommended for use in natural waters for distinguishing between metal ionic and metal inert complex species which are adsorbed from solution onto various solid particles.     

Dissociation of hydrogen sulphide in seawater and comparison of pH scales

The protonization constant of HS^? (K₁₂) has been determined potentiometrically (glass electrode) at atmospheric pressure in synthetic seawater in the salinity range 2.5–40‰ at 5 and 25°C and in NaCl solutions in the formal ionic strength of 0.1–0.8 M at 5 and 25°C. The difference between synthetic seawater and an NaCl solution with the same formal ionic strength can be explained in terms of the complexation of H⁺ by sulphate in seawater. These results can be used to compare the pH scales suggested by Hansson (1973c) and Bates (1975). Furthermore, comparison between the present values of K₁₂ and those of Goldhaber and Kaplan (1975) makes it possible to compare the conventional pH scale with Hansson's titration pH scale. The conditional protonization constant of HS^? in seawater of different salinities can be used to modify the Gran plots (Hansson and Jagner, 1973) for alkalinity measurements in anoxic seawater. Ion-pair formation between HS^? and Mg²⁺ or Ca²⁺ seems to be very weak.     

Calibration of a chalcogenide glass membrane ion-selective electrode for the determination of free Fe in seawater: I. Measurements in UV photooxidised seawater

Calibration of a chalcogenide glass membrane, Fe(III)ISE [Fe_2.5(Ge₂₈Sb₁₂Se₆₀)_97.5], in buffered saline media has been undertaken in order to assess the suitability of this ISE for seawater analyses. The electrode slopes in saline citrate and salicylate buffers were 26.3 and 28.2 mV/decade, respectively, for Fe³⁺ concentrations ranging from 10⁻¹⁰ M to less than 10⁻²⁵ M Fe³⁺. The calibration lines in the citrate and salicylate buffers were essentially collinear with the response in unbuffered chloride-free standards containing >10⁻⁵ M Fe³⁺, demonstrating that the response of the FeISE is unaffected by chloride ions. A mechanism involving a combination of charge transfer and ion-exchange of Fe(III), at the electrode diffusion layer, can be used to explain the ≈30 mV/decade slope of the FeISE. The response of the FeISE in UV photooxidised seawater containing 8 nM total Fe was measured as the pH was changed from 8.27 to 3.51. The slope of the response was 24.2 mV/decade [Fe³⁺] calculated as a function of pH using Fe(III) hydrolysis constants for seawater. Moreover, the response was essentially collinear with that in citrate buffers and in unbuffered solutions containing >10⁻⁵ M Fe³⁺ and the slope for the combined data was 26.2 mV/decade. This study was restricted to organic-free seawater because the certainty in Fe(III)–ligand stability constants is insufficient to warrant the selection of an ideal calibration buffer system, and there is evidence that powerful chelating ligands (e.g., EDTA along with humic and fulvic acids) may alter the response of the Fe(III)ISE. The Fe dissolution rate of the FeISE in UV photooxidised seawater was found to be 1.6×10⁻² nmol Fe/min, as measured by cathodic stripping voltammetry (CSV). This would contaminate a 100-ml sample by 0.8–1.6 nM Fe over a typical measurement period of 5–10 min obtained using a stability criterion of 0.5 mV/min. Various methods are proposed for reducing the level of contamination in open ocean samples that contain sub-nanomolar concentrations of iron. The FeISE has the potential to detect free Fe³⁺ at concentrations typically found in natural seawater.     

Coprecipitation of zinc with silica in seawater and in distilled water

Dissolved silica can coprecipitate with zinc from seawater or distilled water that has been enriched with both elements. More than 2 ppm Si are necessary for the reaction to begin. The coprecipitation shows pH dependence. The addition of pulverized illite or natural sediment as suspended particulate material does not enhance the reaction in seawater. The organic material present in the nearshore seawater samples decreases the rate and extent of reaction, as indicated by comparisons of results of experiments using natural seawater with results obtained using UV-irradiated seawater. In unbuffered distilled water the reaction must compete with hydrolysis of zinc; however, reaction does occur, which indicates that the seawater matrix is not essential for the reaction. The coprecipitation can limit the concentration of zinc in seawater to less than the solubility concentration assumed for ZnCO₃ or Zn(OH)₂. The results suggest that a zinc silicate can precipitate directly from seawater or interstitial water as an authigenic mineral.     

Oxidation of copper(I) in seawater at nanomolar levels

The oxidation and reduction of nanomolar levels of copper in air-saturated seawater and NaCl solutions has been measured as a function of pH (7.17–8.49), temperature (5–35 °C) and ionic strength (0.1–0.7 M). The oxidation rates were fitted to an equation valid at different pH and ionic strength conditions in sodium chloride and seawater solutions:

The reduction of Cu(II) was studied in both media for different initial concentrations of copper(II). When the initial Cu(II) concentration was 200 nM, the copper(I) productions were 20% and 9% for NaCl and seawater, respectively. The effect of speciation of copper(I) reduced from Cu(II) on the rates was studied. The Cu(I) speciation is dominated by the CuCl₂⁻ species. On the other hand, the neutral chloride CuCl species dominates the Cu(I) oxidation in the range of 0.1 M to 0.7 M chloride concentrations.     

Aluminum hydroxide’s solubility and the forms of dissolved aluminum’s occurrence in seawater

The solubility of aluminum hydroxide in seawater of 35‰ salinity at pH = 7.4−8.2 and 25°C was determined experimentally for three samples synthesized in different ways. The solubilities of two phases subjected to ageing and precipitated (a) from a boiling solution of aluminum sulfate and (b) immediately from seawater at room temperature were a little different and showed the minimum within pH = 8.05−8.10. The solubility of aluminum hydroxide precipitated from a solution of sulfate aluminum at room temperature and not subjected to ageing was about twofold at pH∼7.9. The analysis of the pH dependence of the concentration of dissolved aluminum allows one to suppose that an Al(OH)₂⁺ hydroxo complex is the primary form of the aluminum occurrence in seawater at pH < 8.05, whereas the Al(OH)₄⁻ anion is prevailing at pH > 8.10. Electrically neutral Al(OH)₃⁰ hydroxocomplexes may be prevailing within the narrow range of pH = 8.05−8.10 and, in general, are of secondary importance.     

Some kinetic characteristics of exoproteolytic activity in coastal seawater

The response of the exoproteolytic activity of seawater to proteolytic inhibitors suggests that metalloproteases are the main enzymes involved. The K_m of exoproteolytic enzymes for the hydrolysis of indigenous proteins in coastal north seawater has been evaluated as 80 μg l⁻¹ and the maximum rate of proteolysis lies in the range 0·1–0·35 μgC l⁻¹-enzymatic-unit⁻¹. Enrichment experiments suggest that both species selection and metabolic regulation may play a role in the exoproteolytic activity/biomass ratio. However, in situ exoproteolytic activity/biomass ratios observed in a broad range of natural marine environments lie in a very narrow range, which is intermediate between those observed after amino acid or protein enrichment.     

An ion-association model for estimating acidity constants (at 25°C and 1 ATM total pressure) in electrolyte mixtures related to seawater (ionic strength < 1 mol kg−1 H2O)

The ion-association model of Garrels and Thompson (1962) has been extended in order to incorporate the major acid—base systems present in seawater. The data for the activity coefficients of the various species considered have been presented as a function of ionic strength, thus making it possible to apply the model to solutions with ionic strengths in the range 0–1 mol kg^?1 H₂O. The model therefore allows the prediction of a variety of equilibrium thermodynamic data for natural waters.The model was used to estimate the stoichiometric acidity constants of the major weak acids present in seawater; the conditions simulated correspond to actual measurements made in seawater media at a variety of ionic strengths (salinities). The estimates obtained are generally in good agreement with the experimental results (all within 10% and usually within 5%), thus confirming the usefulness of this model.     

Kinetics of phosphate adsorption by iron oxyhydroxides in aqueous systems

The rate of uptake of phosphate onto synthetic Fe(III)- and Fe(II)-derived oxyhydroxides has been studied using reaction conditions similar to those encountered in natural waters. Kinetic analyses were carried out on the adsorption profiles and both first-order and second-order conditional rate constants were obtained. The temperature dependence of some of the rate constants was investigated and corresponding apparent activation energies calculated. Similar experiments and analyses were undertaken using Fe from natural sources and in general the conditional rate constants obtained in seawater were in agreement with the synthetic ones. The results of this study are of value when comparing the time scales of adsorption processes in natural waters with the time scales of mixing and advection.     

The effect of ionic interaction on the rates of oxidation in natural waters

The effect of ionic interactions on the kinetics of disproportionation of HO₂, and the oxidation of Fe(II) and Cu(I) has been examined. The interactions of O₂ with Mg²⁺ and Ca²⁺ ions in seawater increases the lifetime by 3–5 times compared to water. The effect of OH⁻ on the oxidation of Fe(II) in water and seawater shows a second degree dependence from 5 to 45°C. The effect of salinity on the oxidation of Fe(II) was found to be independent of temperature, while the effect of temperature was found to be independent of salinity. The energy of activation for the overall rate constant was found to be 7 ± 0.5 kcal mol⁻¹.The effect of pH, temperature, salinity and ionic composition on the oxidation of Cu(I) has also been examined. In NaCl solutions from 0.5 to 6 M, the log k for the oxidation was a linear function of pH (6–8) with a slope of 0.2 ± 0.05. The reaction was strongly dependent on the Cl⁻ concentration with variation of from 0.3 to 340 min from 0.5 to 6 M Cl⁻. The rates of oxidation of Cu⁺ and CuCl⁰ responsible for these effects are dependent upon ionic strength. The energy of activation for the reaction was 8.5–9.9 kcal mol⁻¹ from 0.5 to 6 M. Studies of the oxidation in various NaX salts (X = I⁻, Br⁻ and Cl⁻) give rates in the order Cl⁻ > Br⁻ > I⁻ as expected, due to complex formation of Cu⁺ with X⁻.     

An improved gas chromatographic method for the measurement of hydroxylamine in marine and fresh waters

We present here an improved method for the analysis of hydroxylamine at nanomolar levels, which involves oxidation by Fe(III) and the subsequent measurement of nitrous oxide by electron-capture gas chromatography. The relationship between the pH and salinity of natural waters and the conversion of hydroxylamine to nitrous oxide by Fe(III) is defined, the rates of the reaction are evaluated, and the effects of dissolved O₂, Cu(II), and Hg(II) on the reaction are investigated. The method is linear to more than 300 nM and the standard deviation for a single measurement is 1 nM in the 0–40 nM range, thus exceeding the sensitivity of the spectrophotometric methods by almost an order of magnitude. This method eliminates the effects of pH and salinity that have burdened an earlier gas chromatographic approach, making possible the investigation of this labile substance not only in seawater, but in fresh and brackish waters as well.     

The rate of oxidation of ferrous iron in seawater and the partition of elements between iron oxides and seawater

At the Minamichita Beach Land (Mihama-cho, Aichi, Japan), seawater is pumped up from underground and is supplied to aquaria. The underground seawater containsca. 2 ppm of Fe (II), 0.1 ppm of Mn (II) and a little dissolved oxygen. Iron oxide is formed in the seawater when aerated. The oxidation rate of Fe (II) was measured to be 1.4×10¹⁴ mol^–3 l ³ min^–1, which is comparable to the lowest values in the literature. The slow rate of Fe (II) oxidation obtained here can be attributed to the presence of organically bound iron in the seawater. The distribution coefficient of cations between seawater and iron oxide phase was in the order of Cu>Ni>Co>Cd>Mn, which is consistent with that predicted from their hydrolysis constants. The adsorption affinity sequence of oxyanions was phosphate >vanadate> molybdate. The difference in phosphate from the prediction of the adsorption theory was attributed to the formation of ferriphosphate on the oxide surface. On the basis of these data, the limitation and usefulness in the application of the distribution coefficients to marine environments were discussed.     

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}, R² = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature (C_Θ = 0.009e^0.103Θ, R² = 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).     

The interaction of Np(V)O2+ with common mineral surfaces in dilute aqueous solutions and seawater

The extent and kinetics of Np(V)O₂⁺ adsorption from dilute aqueous solutions and seawater onto a variety of synthetic and natural solids were determined at 25°C and 1 atm total pressure. Extensive and complex adsorption reactions were found, contrary to speculations in the literature that NpO₂⁺ should behave as a simple monovalent ion with a low affinity for surfaces. When normalized to adsorption per unit solid surface area, the ranking for the synthetic solids was aragonite ? calcite > goethite ? MnO₂ ≈ clays. Natural materials generally followed the same behavior patterns as their synthetic counterparts. The dissolved/adsorbed ratio was found to be constant over a wide range (10^?13–10^?7M) of NpO₂⁺ concentrations. At higher concentrations the extent of adsorption decreased until a solubility limit was reached at approximately 10^?5 M.Solution composition had the most significant influence for NpO₂⁺ adsorption on goethite, where much more extensive adsorption occurs in dilute solutions than in seawater. When seawater is added to a dilute solution, extensive desorption of NpO₂⁺ from goethite occurs. Tests conducted on NpO₂⁺ adsorbed on carbonates indicated that it remained in the V oxidation state.There is a growing consensus that Pu dissolved in natural waters also occurs dominantly in the V oxidation state as PuO₂⁺ ion. Consequently, these results for NpO₂⁺ may serve as a guide for Pu behavior when also in the V oxidation state. The fact that most adsorbed Pu is found in the III or IV oxidation states indicates that reduction of Pu may occur subsequent to adsorption in the V oxidation state.     

Simultaneous Measurement of Hydrogen Peroxide and Fe Species (Fe(II) and Fe<Subscript>(tot)</Subscript>) in Okinawa Island Seawater: Impacts of Red Soil Pollution

The northern part of Okinawa Island suffers from red soil pollution—runoff of red soil into coastal seawater—which damages coastal ecosystems and scenery. To elucidate the impacts of red soil pollution on the oxidizing power of seawater, hydrogen peroxide (HOOH) and iron species including Fe(II) and total iron (Fe_(tot), defined as the sum of Fe(II) and Fe(III)) were measured simultaneously in seawater from Taira Bay (red-soil-polluted sea) and Sesoko Island (unpolluted sea), off the northern part of Okinawa Island, Japan. We performed simultaneous measurements of HOOH and Fe(II) because the reaction between HOOH and Fe(II) forms hydroxyl radical (•OH), the most potent environmental oxidant. Gas-phase HOOH concentrations were also measured to better understand the sources of HOOH in seawater. Both HOOH and Fe(II) in seawater showed a clear diurnal variation, i.e. higher in the daytime and lower at night, while Fe_(tot) concentrations were relatively constant throughout the sampling period. Fe(II) and Fe_(tot) concentrations were approximately 58% and 19% higher in red-soil-polluted seawater than in unpolluted seawater. Gas-phase HOOH and seawater HOOH concentrations were comparable at both sampling sites, ranging from 1.4 to 5.4 ppbv in air and 30 to 160 nM in seawater. Since Fe(II) concentrations were higher in red-soil-polluted seawater while concentrations of HOOH were similar, •OH would form faster in red-soil-polluted seawater than in unpolluted seawater. Since the major scavenger of •OH, Br⁻, is expected to have similar concentrations at both sites, red-soil-polluted seawater is expected to have higher steady-state •OH concentrations.     

On the relevance of iron adsorption to container materials in small-volume experiments on iron marine chemistry: Fe-aided assessment of capacity, affinity and kinetics

Iron chemistry in seawater has been extensively studied in the laboratory, mostly in small-volume sample bottles. However, little has been reported about iron wall sorption in these bottles. In this paper, radio-iron ⁵⁵Fe was used to assess iron wall adsorption, both in terms of capacity, affinity and kinetics. Various bottle materials were tested. Iron sorption increased from polyethylene/polycarbonate to polymethylmetacrylate (PMMA)/high-density polyethylene/polytetrafluoroethylene to glass/quartz, reaching equilibrium in a 25–70 h period. PMMA was studied in more detail: ferric iron (Fe(III)) adsorbed on the walls of the bottles, whereas ferrous iron (Fe(II)) did not. Considering that in seawater the inorganic iron pool mostly consists of ferric iron, the wall will be a factor that needs to be considered in bottle experiments.The present data indicate that for PMMA with specific surface (S)-to-volume (V) ratio S/V, both iron capacity (42 ± 16 × 10^− 9 mol/m² or 1.7 × 10^− 9 mol/L recalculated for the S/V-specific PMMA bottles used) and affinity (log K_Fe'W = 11.0 ± 0.3 m²/mol or 12.4 ± 0.3 L/mol, recalculated for the S/V-specific PMMA bottles used) are of similar magnitude as the iron capacity and -affinity of the natural ligands in the presently used seawater and thus cannot be ignored.Calculation of rate constants for association and dissociation of both Fe'L (iron bound to natural occurring organic ligands) and Fe'W (iron adsorbed on the wall of vessels) suggests that the two iron complexes are also of rather similar kinetics, with rate constants for dissociation in the order of 10 ⁻⁴–10^− 5 L/s and rate constants for association in the order of 10⁸ L/(mol s). This makes that iron wall sorption should be seriously considered in small-volume experiments, both in assessments of shorter-term dynamics and in end-point observations in equilibrium conditions. Therefore, the present data strongly advocate making use of iron mass balances throughout in experiments in smaller volume set-ups on marine iron (bio) chemistry.     

Production and air–sea flux of halomethanes in the western subarctic Pacific in relation to phytoplankton pigment concentrations during the iron fertilization experiment (SEEDS II)

Iron could play a key role in controlling phytoplankton biomass and productivity in high-nutrient, low-chlorophyll regions. As a part of the iron fertilization experiment carried out in the western subarctic Pacific from July to August 2004 (Subarctic Pacific iron Experiment for Ecosystem Dynamics Study II—SEEDS II), we analysed the concentrations of trace gases in the seawater for 12 d following iron fertilization. The mean concentrations of chlorophyll a in the mixed layer (5–30 m depth) increased from 0.94 to 2.81 μg L^–1 for 8 d in the iron patch. The mean concentrations of methyl bromide (CH₃Br; 5–30 m depth) increased from 6.4 to 13.4 pmol L^–1 for 11 d; the in-patch concentration increased relative to the out-patch concentration. A linear correlation was observed between the concentrations of 19′-hexanoyloxyfucoxanthin, which is a biomarker of several prymnesiophytes, and CH₃Br in the seawater. After fertilization, the air–sea flux of CH₃Br inside the patch changed from influx to efflux from the ocean. There was no clear evidence for the increase in saturation anomaly of methyl chloride (CH₃Cl) due to iron fertilization. Furthermore, CH₃Cl fluxes did not show a tendency to increase after fertilization of the patch. In contrast to CH₃Br, no change was observed in the concentrations of bromoform (in-patch day 11 and out-patch day 11: 1.7 and 1.7 pmol L^–1), dibromomethane (2.1 and 2.2 pmol L^–1), and dibromochloromethane (1.0 and 1.2 pmol L^–1, respectively). The concentration of isoprene, which is known to have a relationship with chlorophyll a, did not change in this study. The responses of trace gases during SEEDS II differed from the previous findings (in situ iron enrichment experiment—EisenEx, Southern Ocean iron experiment—SOFeX, and Subarctic Ecosystem Response to Iron Enrichment Study—SERIES). Thus, in order to estimate the concomitant effect of iron fertilization on the climate, it is important to assess the induction of biological activity and the distributions/air–sea fluxes of trace gases by iron addition.