Dissolved Fe Concentrations Research Articles

PHYSIOLOGICAL CHARACTERIZATION OF A SYNECHOCOCCUS SP. (CYANOPHYCEAE) STRAIN PCC 7942 IRON‐DEPENDENT BIOREPORTER FOR FRESHWATER ENVIRONMENTS1

The complex chemical speciation of Fe in aquatic systems and the uncertainties associated with biological assimilation of Fe species make it difficult to assess the bioavailability of Fe to phytoplankton in relation to total dissolved Fe concentrations in natural waters. We developed a cyanobacterial Fe‐responsive bioreporter constructed in Synechococcus sp. strain PCC 7942 by fusing the Fe‐responsive isiAB promoter to Vibrio harveyi luxAB reporter genes. A comprehensive physiological characterization of the bioreporter has been made in defined Fraquil medium at free ferric ion concentrations ranging from pFe 21.6 to pFe 19.5. Whereas growth and physiological parameters are largely constrained over this range of Fe bioavailability, the bioreporter elicits a luminescent signal that varies in response to Fe deficiency. A dose‐response characterization of bioreporter luminescence made over this range of Fe3+ bioavailability demonstrates a sigmoidal response with a dynamic linear range extending between pFe 21.1 and pFe 20.6. The applicability of using this Fe bioreporter to assess Fe availability in the natural environment has been tested using water samples from Lake Huron (Laurentian Great Lakes). Parallel assessment of dissolved Fe and bioreporter response from these samples reinforces the idea that measures of dissolved Fe should not be considered alone when assessing Fe availability to phytoplankton communities.

Iron in the Sargasso Sea: Implications for the processes controlling dissolved Fe distribution in the ocean

In the surface water of the winter Sargasso Sea, “dissolved” Fe concentrations (defined as iron that passes through a filter with an 0.4 micrometer diameter pore size) decrease with increasing latitude from 0.8 nM at 26°N to 0.2 nM at 31°N. It does not appear that this pattern can be explained solely by spatial variations of eolian Fe deposition. Vertical mixing, phytoplankton growth and particle scavenging all appear to play important roles in controlling dissolved Fe distributions in the surface water. Concentrations of dissolved Fe in the subsurface waters increase with increasing depth from ∼0.2 nM at ∼200 m to ∼0.6 nM at depths below 1000 m, reflecting an imbalance between Fe removal by particle scavenging and Fe regeneration from microbial decomposition of organic matter sinking from the euphotic zone. Simple model calculations suggest that scavenging loss of Fe and ventilation of low preformed Fe high P water from source region may create deep ocean Fe:P concentration ratios (5 × 10−4 for the Atlantic and 1.2 × 10−4 for the Pacific) that are much lower than the ratios of Fe to P regenerated from the organic matter sinking from the euphotic zone (24–74 × 10−4 for the Atlantic and 11–32 × 10−4 for the Pacific). We hypothesize that this low Fe:P ratio may be the driving force for Fe limitation of algal growth observed in high nutrient low chlorophyll oceans and that regional changes of Fe:C ratios in organic matter sinking from the euphotic zone may play an important role in regulating dissolved Fe concentrations in the deep waters of the world ocean.

Iron, nutrient, and phytoplankton distributions in Oregon coastal waters

The relationship between iron and nitrate concentrations was examined off the coast of Oregon during the upwelling season. Surface Fe and N (nitrate + nitrite) concentrations measured underway by flow injection analysis ranged from <0.3 to 20 nmol L−1 and <0.1 to 30 μmol L−1, respectively. Total dissolvable Fe concentrations, measured in unfiltered, acidified samples in surface waters and in vertical profiles, ranged from <0.3 to 300 nmol L−1. Surface water Fe and N concentrations were highly variable and uncoupled. Our observations indicate two dominant sources of Fe to Oregon coastal waters: Slope or shelf sediments and the Columbia River. Sedimentary iron, probably largely in the particulate form, appears to be added to surface waters through wind‐induced vertical mixing during strong winds, through thickening of the bottom mixed layer during relaxation or downwelling favorable wind conditions, and through outcropping of shelf bottom waters during upwelling events. The existence of multiple iron sources and the generally high iron concentrations may explain why the distribution of phytoplankton, measured both remotely (by Sea‐viewing Wide Field‐of‐view Sensor) and underway (by in vivo fluorescence), appeared to be driven primarily by physical dynamics and was not strongly linked to the distribution of iron. Nevertheless, at some offshore stations where underway Fe concentrations were <0.3 nmol L−1, underway measurements of the physiological state of phytoplankton by fast repetition rate fluorometry were consistent with mild iron stress, and cross‐shelf nutrient distributions were consistent with iron regulation of the magnitude of phytoplankton blooms.

Variation in iron(III) solubility and iron concentration in the northwestern North Pacific Ocean

Vertical distributions of Fe(III) hydroxide solubilities (≪0.025 µm) and dissolved Fe (≪0.2 µm) concentrations at 0–250 m depth were studied inside (HP) and outside (LP) a high‐production (phytoplankton spring bloom) patch area in the northwestern North Pacific Ocean during May 1999. In the surface mixed layer, the Fe(III) solubility values at HP were much higher (2–4 nM) than those (0.3–0.9 nM) at LP and strongly correlated with chlorophyll a and nutrient concentrations. The high Fe(III) solubility observed in the surface mixed layer was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators. In the surface waters, the dissolved Fe concentrations were generally lower than the Fe(III) solubility values, resulting from the active biological removal of dissolved Fe and excess concentration of Fe‐binding organic ligands. The Fe(III) solubility minima (0.2–0.4 nM) were present in a narrow depth range (40–125 m) below the surface mixed layer at all stations. The subsequent Fe(III) solubility levels appeared to increase up to 0.6–0.8 nM with depth at 100–250 m, in association with the increase in nutrient concentrations. The strong linear correlations between Fe(III) solubility values and nutrient concentrations in middepth waters suggest that the formation of organic Fe(III) chelators may be related to microbial decomposition of sinking biogenic organic matter. In middepth waters, the dissolved Fe concentrations were generally higher than the Fe(III) solubility values, which suggests that the small colloidal iron phases may be present in the dissolved Fe (≪0.2 µm) fraction.

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.

A persistent subsurface chlorophyll maximum in the Interpolar Frontal Zone south of Australia: Seasonal progression and implications for phytoplankton‐light‐nutrient interactions

A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53° and 58°S along 140°E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (>24 μM) year‐round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (<0.2 nM). Mixed layer chlorophyll concentrations are generally <0.3 μg L−1 and surface waters thus fit the high‐nitrate low‐chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a, L−1), and contributes 30–50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L−1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM.

Collection and detection of natural iron-binding ligands from seawater

Abstract Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms, including phytoplankton, which are responsible for the primary productivity in the world's oceans. However, due to the low solubility of Fe in seawater, phytoplankton are often limited by their inability to incorporate enough Fe to allow for optimal growth rates in regions with dissolved Fe concentrations below 1 nM. It has been postulated that certain phytoplankton may produce compounds to facilitate the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (>99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, origin, and chemical characteristics of these organic chelates are largely unknown. Although it has been implied that some components of natural Fe-binding ligands are siderophores, no direct analyses of such compounds from natural seawater have been conducted. Here, we present a simple solid-phase extraction technique employing Biobeads SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from large volumes (>200 l) of seawater. Additionally, we report on the first successful determination of molecular weight size classes and preliminary iron-binding functional group characterization within those size classes for isolates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affinities (with respect to inorganic iron—Fe′) of K FeL,Fe′ cond =10 11.5 –10 11.9 M −1 , similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In addition, 63% of the extracted compounds from surface-collected samples fall within the defined size range of siderophores (300–1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophores extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-binding functional groups consistent with biologically produced siderophores. These results provide further insight into characteristics of the Fe-binding ligands that are thought to be important in controlling the biological availability of Fe in the oceans.

Factors affecting alkalinity generation by successive alkalinity-producing systems: regression analysis.

Use of successive alkalinity-producing systems (SAPS) for treatment of acidic mine drainage (AMD) has grown in recent years. However, inconsistent performance has hampered widespread acceptance of this technology. This research was conducted to determine the influence of system design and influent AMD chemistry on net alkalinity generation by SAPS. Monthly observations were obtained from eight SAPS cells in southern West Virginia and southwestern Virginia. Analysis of these data revealed strong, positive correlations between net alkalinity generation and three variables: the natural log of limestone residence time, influent dissolved Fe concentration, and influent non-Mn acidity. A statistical model was constructed to describe SAPS performance. Subsequent analysis of data obtained from five systems in western Pennsylvania (calibration data set) was used to reevaluate the model form, and the statistical model was adjusted using the combined data sets. Limestone residence time exhibited a strong, positive logarithmic correlation with net alkalinity generation, indicating net alkalinity generation occurs most rapidly within the first few hours of AMD-limestone contact and additional residence time yields diminishing gains in treatment. Influent Fe and non-Mn acidity concentrations both show strong positive linear relationships with net alkalinity generation, reflecting the increased solubility of limestone under acidic conditions. These relationships were present in the original and the calibration data sets, separately, and in the statistical model derived from the combined data set. In the combined data set, these three factors accounted for 68% of the variability in SAPS systems performance.

The incorporation of zinc and iron into the frustule of the marine diatom Thalassiosira pseudonana

Zinc and iron uptake experiments were conducted with the marine diatom Thalassosira pseudonana, to investigate whether Zn and Fe are incorporated into the frustule of this diatom. Our results show that the uptake and deposition of Zn into opal has a sigmoidal relationship with the free Zn2+ concentration of the culture medium. The amount of Zn incorporated into the opal represents only 1–3% of the total amount of Zn taken up by the diatom; however, the exact reasons for Zn incorporation into the opal are not known. Even so, the consistent relationship between Zn levels in the opal and the culture medium suggests that fossil diatoms may be used as a recorder for historical changes in oceanic free Zn2+ concentrations. These results also demonstrate that the Zn:Si(OH)4 relationship observed in the Pacific and Antarctic Oceans is not a result of Zn released from biogenic opal. Fe uptake experiments also revealed that Fe is incorporated into diatom opal; however, the amount incorporated appears to be regulated by the diatom and did not increase with increasing Fe concentration within the diatom culture medium. This therefore eliminates the use of Fe incorporated within diatom opal as a possible proxy for dissolved Fe concentrations.

Effect of natural organic-Fe(III) complex on iron uptake and growth of a brown alga Laminaria religiosa Miyabe

Incoastal waters, enhanced riverine discharge and regener-ative decomposition contribute to significantly higher Feconcentrations, probably due to the higher concentra-tions of organic ligands which were possibly released byriverine input and from coastal marine organisms. Suchnatural organic ligands control the speciation of Fe andthe dissolved Fe concentration and thus the bioavail-ability of Fe in seawater.

Dissolution of Al-substituted goethites by an aerobic Pseudomonas mendocina var. bacteria

Goethite particles in soil environments often contain Al 3+ substituted for Fe 3+ in octahedrally coordinated sites. Al substitution has been shown to alter mineral stability and abiotic dissolution rates. This study focused on the effects of Al substitution (to 8.8 mol%) on synthetic goethite dissolution by an aerobic Pseudomonas mendocina var. bacteria. In contrast to dissimilatory iron reducing bacteria (DIRB), this bacteria is not capable of using Fe as a terminal electron acceptor for oxidative phosphorylation, and hence only requires μM concentrations of Fe for metabolism. Pure and substituted goethites were reacted with microorganisms in Fe-limited growth media wherein the only source of Fe was the solid phase, so that microbial populations could only grow by obtaining Fe through mineral dissolution. Because at least some Fe was taken up by the bacteria, we could not measure Fe release rates directly from dissolved Fe concentrations. Rather, we relied upon microbial growth measurements as indirect indicators of mineral dissolution. Increasing Al substitution resulted in particles with progressively decreasing mean particle length and aspect ratios, as well as fewer domains, as measured by atomic-force microscopy (AFM); but with increasing structural order as determined by XRD line widths. Experiments conducted in the dark at 22°C, exposed to the atmosphere, showed that maximum microbial population did not correlate with particle specific surface area, which is in contrast with previous studies using DIRB. Maximum microbial population increased a small amount with increasing Al content of the goethites, in contrast with several previous investigations of abiotic dissolution. Because dense biofilms formed, we were unable to use AFM to observe mineral dissolution features. AFM imaging suggested that more highly substituted goethites formed denser aggregates, and previous investigations have shown that aggregate structure is important for microbial attachment, which is prerequisite for dissolution. Hence, effects of Al substitution on aggregate structure is a focus of ongoing research.

Effects of Redox Conditions and Flow Processes on the Mobility of Dissolved Organic Carbon and Nitrogen in a Forest Soil

AbstractIn forest ecosystems, organic solutes play prominent roles in pollutant and nutrient transport. This study, conducted in subalpine forested Humaquepts in Switzerland, investigated the influence of redox conditions and flow processes on dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). In the mineral soil, concentrations of DOC were higher under reduced than under oxidized conditions. They averaged 1.2 mmol DOC L−1 in the reduced mineral soil at a 100‐cm depth. A close correlation between DOC and dissolved Fe concentrations (r2 = 0.83; p < 0.001) suggests that reductive dissolution of Fe‐oxides was the major reason for the low retention. However, during aerobic sampling of soil solution from the reduced mineral soil, DOC coprecipitated with Fe. This suggests that the DOC input from reduced mineral soils to aerobic stream water is lower than expected from the high DOC concentrations in the soil. Organic N was the major form of total dissolved N at all soil depths. In the reduced subsoil, dissolved N was completely organically bound. This was probably due to both an immobilization of inorganic N and a low retention of DON. During storms, DOC concentrations increased rapidly with increasing discharge in the subsurface flow. At peak flow, DOC concentrations were doubled, compared with base flow. The molar UV absorptivity of DOC in the subsurface flow corresponded closely to that of the topsoil. These findings suggest that DOC concentrations and properties are sensitive to flow velocities, and that DOC is preferentially transported to the subsoil at high discharge.

Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean

We investigated the relative roles of Fe and silicic acid availabilities in regulating Si uptake rates across the Polar Frontal Zone in the Pacific Sector of the Southern Ocean (59–68°S, 170°W) during the US JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS). Meridional gradients in silicic acid concentration ([Si(OH) 4]) of about 0.25–0.56 μM km −1 were observed in this area during austral spring and summer, 1997–1998, with [Si(OH) 4] ranging from <1 to 15 μM on the north side of the gradient to 40–60 μM on the south side. In two pairs of shipboard bottle-enrichment experiments conducted north and south of the Si gradient in spring and summer, we measured the effects of Fe, Zn and Si additions on 32 Si(OH) 4 and 15 NO 3− uptake rates, biogenic silica concentrations and Si(OH) 4 : NO 3 − uptake ratios. Fe addition had little or no effect on Si uptake rates in enrichments conducted in the low-Si waters north of the Si gradient. However, Fe addition increased Si uptake rates 3–5 times over controls in enrichments conducted in the high-Si waters south of the gradient, in both spring and summer. Fe addition decreased Si(OH) 4 : NO 3 − uptake ratios by 2–5 times, largely due to stimulation of NO 3 − uptake rates. Zn addition had no effect on Si(OH) 4 and NO 3 − uptake rates. Short-term (24 h) Si additions had varying effects on Si uptake rates, depending on season and location. In spring, additions of 40 μM Si to water from bottle enrichments, conducted north of the Si gradient (in situ [Si(OH) 4] ∼15 μM) did not increase Si uptake rates initially, but did increase uptake rates after 8 days. In the summer enrichment north of the Si gradient (in situ [Si(OH) 4] ∼5 μM), 50 μM Si additions doubled in situ Si uptake rates in the initial water collected for the enrichment, and increased Si uptake rates as much as 16-fold during the experiment. South of the Si gradient, where in situ [Si(OH) 4] was >40 μM in both spring and summer, Si addition had no effect on in situ Si uptake rates in the initial enrichment water nor on any Si uptake rates measured during the experiment. Our results indicate that both Fe and Si availabilities regulate Si uptake rates and silica production in the Southern Ocean along 170°W. Fe limitation appears to restrict Si uptake rates south of the Si gradient and plays a role in preventing Si depletion south of the ACC, where ambient [Si(OH) 4] never fell below 40 μM during 1997–1998. Our experiments in the Seasonal Ice Zone at 62° suggest that Si uptake in this area switched from being Fe-limited in the spring, when in situ [Si(OH) 4] was >40 μM, to Si-limited in the summer, when [Si(OH) 4] was <5 μM. Thus, while Fe limitation could be reducing Si uptake rates in this area, it does not prevent eventual Si drawdown. Our experiments also indicate that Si and Fe co-limitation may occur north of the Si gradient, such that Si uptake rates will not reach maximal levels until both Si and Fe limitations are relieved. The interaction between Fe and Si limitation in these waters and the high Si(OH) 4 : NO 3 − uptake ratios observed at in situ dissolved Fe concentrations can have a large impact on Si and N biogeochemistry in the Southern Ocean.

Size-fractionated biological iron and carbon uptake along a coastal to offshore transect in the NE Pacific

Iron has been shown to limit phytoplankton growth in high-nutrient low-chlorophyll (HNLC) regions such as the NE subarctic Pacific. We report size-fractionated Fe-uptake rates by the entire plankton community in short (6–8 h) light and dark incubations along an E–W transect from P04 (a coastal ocean station) to OSP (an open-ocean HNLC station) during August–September 1997. Size-fractionated primary productivity and chl a were measured to monitor algal Fe : C uptake ratios and Fe-uptake relative to phytoplankton biomass. The >5.0 μm size-class, which consisted mostly of large diatoms, had the highest Fe-uptake rate at nearshore stations (P04 and P8), but Fe-uptake rates for this size class decreased despite increases in biomass and primary productivity when transecting westwards to HNLC waters. Fe-uptake rates of the small size class (0.2–1.0 μm, including heterotrophic bacteria and autotrophs) were inversely related to the >5.0 μm size-class uptake rates, in that stations with high dissolved Fe (DFe) concentrations had relatively low uptake rates compared to those in the low-Fe offshore region. The 1.0–5.0 μm size-class Fe-uptake rates were low, relatively invariant along the transect, and differed little between light and dark incubations. Dark Fe-uptake rates averaged 10–20% less than those in the light for the >5.0 μm size class. Dark uptake rates however, were higher than light uptake rates for the 0.2–1.0 μm size class at all stations. Fe : C uptake ratios were high for all size classes at P04, but decreased as DFe concentrations decreased offshore. The prokaryote-dominated 0.2–1.0 μm size class had the highest Fe : C uptake ratios at all stations. These data suggest that prokaryotic organisms make an important contribution to biological Fe uptake in this region. Our experiments support the results of previous culture work, suggesting higher Fe : C ratios in coastal phytoplankton compared to open-ocean species, and demonstrate that light can have a large effect on Fe partitioning between size classes in subarctic Pacific HNLC waters.

Biologically enhanced dissolution of a pyrite‐rich black shale concentrate

The acid leaching of a pyrite‐rich black shale concentrate (7% S) was tested in this study. The experiments were performed at 5–30% pulp densities and with inoculations of Fe‐ and S‐oxidizing thiobacilli (Thiobacillus ferrooxidans, and Thiobacillus thiooxidans,),. Cultures supplemented with S0 showed strong acid production, with final pH values of 0.9 in T. ferrooxidans, cultures and 0.4–0.5 in the presence of T. thiooxidans,. Fe dissolution was pronounced in the T. ferrooxidans, culture whereas T. thiooxidans, did not dissolve Fe from the black shale. Total dissolved Fe concentrations were 3 to 50 times higher in the cultures inoculated with T. ferrooxidans, when compared to T. thiooxidans, and sterile controls. The dissolution of Mo was enhanced in the inoculated cultures as compared with the chemical controls. With V, Si, and Al this effect was not as pronounced but was still discernible in solutions acidified by bacterial oxidation of S0. The leaching experiments suggested that the black shale was inhibitory to the inocula. The inhibition was related to the pulp density and was associated with the leach solution. The inhibition could be completely alleviated by replacing the leach solution.

Iron and manganese in surface waters of the Australian subantarctic region

Iron and manganese concentrations were measured in seawater from the upper water column (≤350 m water depth) in the subantarctic region to the south and west of Tasmania, Australia, during January 1995. Dissolved and total-dissolvable Fe and dissolved Mn were determined in samples from four stations along 140°E ranging from polar to subtropical waters, and from one station over the continental slope off north-west Tasmania. Dissolved Fe concentration were generally low (<1 nM) for all stations, ranging from minimum concentrations between 0.1 and 0.2 nM at the southernmost stations to maximum values near 0.8 nM in subtropical waters and over the continental slope. Total-dissolvable Fe concentrations ranged from 0.17 to 1.3 nM, and were more than twice the dissolved Fe concentrations in some samples, implying a significant proportion of particulate Fe in the water column. Dissolved Mn concentrations were generally low (<0.5 nM) along the 140°E transect, with lowest values of around 0.1 nM measured at the southernmost stations, and much higher concentrations (> 2 nM) observed in shallower waters over the continental slope. Vertical concentration profiles suggest biological removal of dissolved Fe from the photic zone at the deep-ocean stations, except in the subantarctic front. The higher mixed-layer concentrations of Fe and Mn observed over the continental slope and in the deep ocean at 40°S are thought to reflect inputs from the continental shelves or atmospheric deposition. Water-column Fe and Mn profiles from 45°S, 50°S and 53°S show no evidence for significant inputs of these metals via atmospheric deposition or upwelling of metal-rich deep waters, although there was evidence of shelf-derived particulate Fe at 45°S. The low concentrations of dissolved Fe measured at 45°S, 50°S and 53°S indicate that iron deficiency could limit summertime primary production in these high-nitrate lowchlorophyll waters.

The distribution of Fe in the antarctic circumpolar current

The large-scale distributions of dissolved and total Fe in surface and deep waters of the Antarctic Circumpolar Current exhibit strong relationships with hydrography and biological processes. The mean dissolved Fe concentrations are low in surface waters of the Antarctic Circumpolar Current (0.31–0.49 nM, with a minimum of 0.17 nM) and higher (averaging 1.11.9 nM) in the Polar Frontal region. Enhanced dissolved surface water concentrations in the Polar Frontal region are attributed to input from the continental shelf and coincide with phytoplankton spring blooms of large diatoms. The effects of sea-ice melting and iceberg melting on the Fe concentrations were relatively small. Dissolved deep-water concentrations ( > 400 m) in the Antarctic Circumpolar Current ranged from 0.4 to 2.8 nM. Circumpolar Deep Water has relatively high dissolved Fe concentrations in the Polar Frontal region (0.4–2.8 nM) compared with deep waters further to the south (0.6–1.1 nM). Similarly, total dissolvable (unfiltered) Fe concentrations in the Upper Circumpolar Deep Water tend to decrease southward from the Polar Frontal region. In the Lower Circumpolar Deep Water total dissolvable Fe concentrations are higher than in the Upper Circumpolar Deep Water due to the existing nepheloid layer and sources on the Mid-Atlantic Ridge. Dissolved and total dissolvable Fe concentrations in the Antarctic Bottom Water are higher than those of other water masses in the Antarctic Circumpolar Current, consistent with the nepheloid layer as well as diagenetic input from shelf sediments. The High-Nutrient/Low-Chlorophyll areas of the Antarctic Ocean and northeast Pacific Ocean have different major Fe input sources of similar magnitude. In the Antarctic Circumpolar Current upward transport of Fe is the main input source, whereas in the North Pacific Ocean, aerosols are the dominant source.

Dissolved organic Fe(III) and Fe(II) complexes in salt marsh porewaters

Fe(III) and Fe(II) organic complexes were determined by spectroscopic methods after sephadex gel fractionation of salt marsh porewaters during June and July 1993. Fe(III), which is a significant anaerobic oxidant for sulfide mineral oxidation, was typically found in the 100–5000 molecular weight (MW) fraction indicative of humic and other organic complexing agents. Fe(II) was found in both the <100 and the 100–5000 MW fractions with most of the Fe(II) found in the smaller MW class. Both forms of Fe precipitated with humic material when the pH of the porewaters became less than 3. There was a twofold decrease in the <5000 MW fractions, a ninefold increase in the >5000 MW class, and a tenfold decrease in dissolved Fe concentration in sephadex gel fractions as the pH decreased from 5.0 to <3 for porewater samples. Low pH values are attributed to sulfide mineral oxidation from severe drought conditions that caused significant dessication of the vegetated marsh. Microelectrode measurements demonstrated that O 2 was not detected below 2 mm and that Fe(III) organic complexes should be significant oxidants in anoxic waters.

Response of equatorial Pacific phytoplankton to subnanomolar Fe enrichment

In the equatorial Pacific Ocean along 160° W, surface-water samples with natural plankton communities were placed in incubation bottles to which subnanomolar levels of Fe were added under ultraclean conditions. Addition of 0.1-0.8 nM Fe to seawater samples containing high NO 3 increased stocks of Chl a and POC, NO 3 consumption and net growth rate of phytoplankton in incubation bottles relative to the controls. A large increase in the Chl a concentration of large- (> 10 μm) and medium-size (3-10 μm) fractions was observed in the Fe-enriched samples. POC concentrations doubled even with 0.1 nM Fe at the equator. The net growth rates of large- and medium-size phytoplankton increased systematically with added Fe concentration. The dissolved Fe concentration in the incubation bottles, which was determined on board by flow injection analysis, decreased significantly during the first 3 days of incubation. However, 50-90% of the added Fe remained in the dissolved fraction (< 0.2 μm) at the end of the experiments. These results indicate that changes in subnanomolar Fe levels affect the equatorial phytoplankton communities by promoting the growth of large phytoplankton.

Iron, primary production and carbon-nitrogen flux studies during the JGOFS North Atlantic bloom experiment

Primary production was measured every other day towards the end (18–31 May) of the 1989 North Atlantic spring bloom. Rates varied with light and averaged 90.4 mmol C m−2 day−1 at the 47°N, 20°W station. Productivities measured south of Iceland (59°30′N, 20°45′W) were somewhat lower, averaging 83.6 mmol C m−2 day−1. Carbon and nitrogen fluxes were estimated using free-floating, VERTEX type particle trap arrays. To obtain mean rates representative of the North Atlantic spring bloom, flux data from three trap deployments were combined and fitted to normalized power functions: mmol C m−2 day−1 = 14.35 (z/100)−0.946, mmol N m−2 day−1 = 2.34(z/100)−1.02, with depth z in meters. Regeneration rates were: mmol C m−2 day−1 = 0.136(z/100)−1.946, mmol N m−2 day−1 = 0.0239(z/100)−2.02. The carbon export rate from the upper 35 m for the entire NABE study period (24 April to 1 June) was 39 mmol m−2 day−1. This value divided by the averaged productivity for the entire study (86 mmol N m−2 day−1) gave an F-ratio of 0.45. Concentrations of Cu, Fe, Ni, Pb and Zn were determined in water samples provided by JGOFS NABE scientists involved with primary productivity measurements. Although little contamination was observed for Cu, Ni and Pb, relatively large amounts of Zn (10 nmol kg−1) were found in some cases. In subsequent studies it was learned that this quantity of Zn can depress productivity rates by 25%. North Atlantic dissolved Fe concentrations were similar to those occurring in the Pacific (surface = 0.07; deep = 0.5–0.6 nmol kg−1). Although no evidence of Fe deficiency was found in enrichment experiments, the addition of nmol amounts of Fe did increase CO2 uptake and POC formation by factors of 1.3–1.7. In this region, most of the phytoplankton's Fe requirement is probably met via the lateral transport of Fe from distant continental margins.