Kenshi Kuma

Photo-reduction of Fe(III) by dissolved organic substances and existence of Fe(II) in seawater during spring blooms

Abstract Fe(III) in seawater was readily reduced to Fe(II) in sunlight in the presence of hydroxycarboxylic acids, such as sugar acids. Tartaric, gluconic, glucaric acids and glucaric acid-1,4-lactone gave higher reduced Fe(II) concentrations and a more rapid increase for a short illumination time than other acids. The reduced Fe(II) concentration was found to be greater at 5°C than at 20°C. In sunlight the reduction of Fe(III) in seawater occurred even at low concentration of the hydroxycarboxylic acids (1 μM). From field observations, 0.02–0.04 μM of dissolved Fe(II) appeared in oxic surface seawater during spring blooms in Funka Bay (Japan) during 1987, 1988 and 1989 as a result of vertical water mixing by which Fe(III) from sediments was transported to the surface. These observations may result from photo-reduction of Fe(III) in the presence of hydroxycarboxylic acids which were possibly released by phytoplankton during spring blooms.

Availability of colloidal ferric oxides to coastal marine phytoplankton

Cell growth of a coastal marine diatom, Phaeodactylum tricornutum (stock cultures), and two red tide marine flagellates, Heterosigma akashiwo and Gymnodinium mikimotoi (stock cultures), in the presence of soluble chelated Fe(III)-EDTA (1:2) and of four different phases of ferric oxide colloids were experimentally measured in culture experiments at 20°C under 3000 lux fluorescent light. Soluble Fe(III)-EDTA induced the maximal growth rates and cell yields. The short-term uptake rate of iron by H. akashiwo in Fe(III)-EDTA medium was about eight times faster than that in solid amorphous hydrous ferric oxide (Fe2O3·xH2O) medium. In culture experiments supplied with four different ferric oxide forms, the orders of cell yields are amorphous hydrous ferric oxide>γ-FeOOH (lepidocrocite)>Fe5O7(OH)·4H2O (hydrated ferric oxyhydroxide polymer >α-FeOOH (goethite). The specific growth rates (μ) at logarithmic growth phase in Fe(III)-EDTA, amorphous hydrous ferric oxide and γ-FeOOH media were significantly greater than those in Fe5O7 (OH)·4H2O and α-FeOOH media. The thermodynamically stable forms such as Fe5O7(OH)·4H2O and α-FeOOH supported a little or no phytoplankton growth. The iron solublities and/or proton-promoted iron dissolution rates of these colloidal ferric oxides in seawater at 20°C were determined by simple filtration techniques involving γ-activity measurements of 59Fe. The orders of solubilities and estimated dissolution rate constants of these ferric oxides in seawater were consistent with that of cell yields in the culture experiments. These results suggest that the availability of colloidal iron to provide a source of iron for phytoplankton is related to the thermodynamic stability and kinetic lability of the colloidal ferric oxide phases, which probably control the uptake rate of iron by phytoplankton.

Photoreduction of Fe(III) by hydroxycarboxylic acids in seawater

Abstract The photoreduction of Fe(III) in seawater (pH 8.1) has been investigated over a range of natural sunlight intensities at 5, 10 and 15°C in the presence of hydroxycarboxylic acid (HCA), such as sugar acids. The Fe(III) photoreduction abilities of HCA were in the order: glucaric acid-1,4-lactone = glucaric > tartaric > gluconic >> citric > glyceric = malic > glucuronic acids. The order probably depends on the complexation ability of HCA with Fe(III) in seawater and the photo-activity of Fe(III)-HCA complex. Stable soluble Fe(III)-glucaric complex systems indicated a reversible photochemical reaction The photoreduction rate constant (kred) was estimated from the oxidation rate constant (kox) of Fe(II) in the presence of glucaric form and steady state Fe(II) concentration assuming first-order processes. The photoreduction rate constant increases linearly with increasing light intensity and is independent of temperature. The photoreduction mechanism is probably a photoinduced ligand to metal charge transfer (LMCT) reaction. For colloidal hydrous ferric oxide-HCA systems, the net photoreduction rate probably depends on the surface concentration of adsorbed HCA on colloidal ferric oxide (surface complexation), the back oxidation rate of photoproduced Fe(II), and hydrolytic precipitation rate of reoxidized Fe(III) in seawater remaining HCA. These above interpretations are supported from the observed shift in the Fe(III)HCA absorption spectra measured before and after each photo-experiment. The photoreduction of Fe(III)HCA complex systems is probably promoted by the absorption at longer ultraviolet (u.v.) wavelengths (290–400 nm) of sunlight by the second absorption band centered at 310 nm observed in the spectra of soluble Fe(III)HCA complexes in seawater.

Spatial variability of Fe(III) hydroxide solubility in the water column of the northern North Pacific Ocean

Solubilities (pH 8.0–8.2, 20°C) of amorphous hydrous ferric oxide (Fe(III) hydroxide) in seawater samples collected on two transects (38°30′–47°30′N along 170°00′E and 175°30′E longitudes) in the northern North Pacific Ocean were experimentally determined by a simple filtration (0.025 μm) involving γ-activity measurement of 59Fe. The vertical profiles of Fe(III) hydroxide solubility in the open-ocean waters have the following features in common: the solubility in the surface mixed layer (0–50 m) is high and variable (0.5–3.6 nM), sometimes corresponding with depth of high chlorophyll a concentrations, and is the highest (2.5–3.6 nM) in the boundary zone (42–44°N) between subtropical and subarctic water masses; the solubility minima (0.14–0.39 nM) occur at depth of 50–200 m, below the surface mixed layer, and there is a northward increase in the minimum value of solubility at the subsurface; the subsequent solubility levels appear to increase with depth in association with the increase in nutrient concentrations at lower latitude (0.3–0.7 nM) or to vary little in middepth waters with high nutrient through a water column at higher latitude (0.5–0.7 nM). The high Fe(III) hydroxide solubility observed in the surface mixed layer in the boundary zone is probably due to higher concentration or stronger affinity of natural organic Fe(III) chelators, which were possibly released by particular phytoplankton or cyanobacteria species through their metabolism. The fact that the solubility minima are present at narrow depth ranges in the subsurface suggests that the produced organic chelators are consumed or degraded in the surface layer. The subsequent increasing solubility in middepth waters would be due to the organic Fe(III) chelators produced through the decomposition and transformation of biogenic organic matter, resulting in a strong correlation between the Fe(III) hydroxide solubility and nutrient concentration in middepth waters (⩾50–100 m) below the depth of the solubility minima. The chemical composition of organic chelators in middepth waters may be different from those in the surface waters. These results support previous conclusions that natural organic Fe(III) chelators exist in significant concentration, controlling the dissolved iron concentration in oceanic waters, and have significant implications for the distribution of phytoplankton and cyanobacteria between oligotrophic and eutrophic systems.

Dissolution rate and solubility of colloidal hydrous ferric oxide in seawater

Dissolution rate constants and solubilities of colloidal hydrous ferric oxide in seawater over the pH range 5.5–8.1 at 20°C were experimentally determined by dialysis techniques involving γ-activity measurements of 59Fe. The Fe(III) dissolution rate was defined as a first-order reaction in proportion to the concentration of particulate Fe(III) in seawater. These rate constants and solubilities within the pH range 6.8–8.1 were independent of pH with values: 0.015 ± 0.002 day−1 and 1.05 ± 0.05 × 10−8 mol l−1, respectively. This result probably indicates the existence of Fe(OH)30 as well as Fe(OH)2+ in the normal pH range of seawater. At lower pHs, the logarithmic dissolution rate constants and solubilities increased linearly with decreasing pH with a slope of −0.55 and −1.0, respectively. The Fe(III) dissolution rate of colloidal hydrous ferric oxide at the normal pH of seawater can be estimated from 0.015 × [Fe (III) p] (conc. day−1) by the concentration of particulate Fe(III) in seawater.

Iron(III) hydroxide solubility and humic-type fluorescent organic matter in the deep water column of the Okhotsk Sea and the northwestern North Pacific Ocean

Abstract Vertical distributions of Fe(III) hydroxide solubility were studied in the Okhotsk Sea and the northwestern North Pacific Ocean during May and June 2000. Fe(III) solubility minima (0.35– 0.45 nM ) were present in a narrow depth range (80– 100 m ) below the surface mixed layer at all stations. In general, the Fe(III) solubility levels in intermediate and deep waters are characterized by mid-depth maxima (0.76– 0.86 nM ) at 800– 1250 m depth and, below that, a slight decrease to 0.4– 0.6 nM with depth in association with increase in nutrient, apparent oxygen utilization (AOU) and humic-type fluorescence intensity. The most significant correlation between the Fe(III) solubility and humic-type fluorescence in intermediate and deep waters suggests that the distribution of humic-type fluorescent organic matter may control the distribution of Fe(III) solubility in deep ocean waters. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters may be controlled primarily by Fe(III) complexation with natural organic ligands, such as marine dissolved humic substances released through the oxidative decomposition and transformation of biogenic organic matter in intermediate and deep waters. In addition, high Fe(III) hydroxide solubility values (1.0– 1.6 nM ) were observed in the surface mixed layer at a station in the northwestern North Pacific Ocean where a phytoplankton bloom was observed. The higher Fe(III) solubility in the surface waters was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators, which may be released by dominant phytoplankton and/or bacteria during the spring bloom and probably have a different chemical composition from those found in intermediate and deep waters.

Riverine input of bioavailable iron supporting phytoplankton growth in Kesennuma Bay (Japan)

Abstract The effects of riverain iron and nutrient inputs on phytoplankton growth in Kesennuma Bay were studied. The effects of iron and fulvic acid-iron complex additions on phytoplankton growth were studied in iron-enriched and -limited culture experiments of coastal marine diatom Chaetoceros sp. (the dominant species inside and outside of the bay) using media prepared from bay and outer waters. Bay water is not iron-limited. The addition of Fe(III) to bay water or autoclaved bay water gave no increase in cell yield. However, when bay water was autoclaved after UV-irradiation, there was little growth. This suggests that the UV irradiation destroyed organic compounds that affected iron bioavailability. Outer water is iron-limited. The addition of Fe(III) to outer water increased cell yield and iron-enriched outer water prepared by autoclaving after adding fulvic acid-Fe increased also cell yield. When outer water after adding Fe(III) was autoclaved, there was little growth. This suggests that fulvic acid made the iron bioavailable. The riverain inputs of organically bound iron, such as fulvic acid-Fe, and nutrients probably play an important role for supporting phytoplankton growth in the bay.

Variation of size-fractionated Fe concentrations and Fe(III) hydroxide solubilities during a spring phytoplankton bloom in Funka Bay (Japan)

Abstract Vertical distributions of size-fractionated Fe concentrations ( 0.22-μm fractions) and Fe(III) hydroxide solubilities were studied during a spring phytoplankton bloom (February to April, 1995) in Funka Bay, Japan. “Soluble Fe” (

Accumulation of humic-like fluorescent dissolved organic matter in the Japan Sea

Major fraction of marine dissolved organic matter (DOM) is biologically recalcitrant, however, the accumulation mechanism of recalcitrant DOM has not been fully understood. Here, we examine the distributions of humic-like fluorescent DOM, factions of recalcitrant DOM, and the level of apparent oxygen utilization in the Japan Sea. We find linear relationships between these parameters for the deep water (>200 m) of the Japan Sea, suggesting that fluorescent DOM is produced in situ in the Japan Sea. Furthermore, we find that the amount of fluorescent DOM at a given apparent oxygen utilization is greater in the deep water of the Japan Sea than it is in the North Pacific, where the highest level of fluorescent DOM in the open ocean was previously observed. We conclude that the repeated renewal of the deep water contributes to the accumulation of fluorescent DOM in the interior of the Japan Sea.

Vertical distributions of iron(III) hydroxide solubility and dissolved iron in the northwestern North Pacific Ocean

Detailed vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable (0.3-2.4 nM), corresponding with the depth of high chlorophyll a concentrations, and the solubility minima (0.2-0.4 nM) occurred at 75-125 m depth. The vertical profiles of Fe(III) hydroxide solubility in mid-depth and deep waters are characterized by mid-depth maxima (0.6-0.7 nM) and, subsequently, a slight decrease to 0.4-0.5 nM with depth, which is markedly similar to nutrient and dissolved Fe depth profiles. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands, which were released through the oxidative decomposition and transformation of biogenic organic matter in mid-depth and deep waters.