Simon J. Ussher

Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability

Oceanic fixed-nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification1, 2, 3, 4. A number of factors, including iron limitation5, 6, 7, can restrict nitrogen fixation, introducing the potential for decoupling of nitrogen inputs and losses2, 5, 8. Such decoupling could significantly affect the oceanic fixed-nitrogen inventory and consequently the biological component of ocean carbon storage and hence air–sea partitioning of carbon dioxide2, 5, 8, 9. However, the extent to which nutrients limit nitrogen fixation in the global ocean is uncertain. Here, we examined rates of nitrogen fixation and nutrient concentrations in the surface waters of the Atlantic Ocean along a north–south 10,000 km transect during October and November 2005. We show that rates of nitrogen fixation were markedly higher in the North Atlantic compared with the South Atlantic Ocean. Across the two basins, nitrogen fixation was positively correlated with dissolved iron and negatively correlated with dissolved phosphorus concentrations. We conclude that inter-basin differences in nitrogen fixation are controlled by iron supply rather than phosphorus availability. Analysis of the nutrient content of deep waters suggests that the fixed nitrogen enters North Atlantic Deep Water. Our study thus supports the suggestion that iron significantly influences nitrogen fixation5, and that subsequent interactions with ocean circulation patterns contribute to the decoupling of nitrogen fixation and loss2, 4, 8.

Marine biogeochemistry of iron

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.

Investigation of iron(III) reduction and trace metal interferences in the determination of dissolved iron in seawater using flow injection with luminol chemiluminescence detection

A detailed investigation into the performance of two flow injection-chemiluminescence (FI-CL) manifolds (with and without a preconcentration column) for the determination of sub-nanomolar dissolved iron (Fe(II)+Fe(III)), following the reduction of Fe(III) by sulphite, in seawater is described. Kinetic experiments were conducted to examine the efficiency of reduction of inorganic Fe(III) with sulphite under different conditions and a rigorous study of the potential interference caused by other transition metals present in seawater was conducted. Using 100microM concentrations of sulphite a reduction time of 4h was sufficient to quantitatively reduce Fe(III) in seawater. Under optimal conditions, cobalt(II) and vanadium(IV)/(III) were the major positive interferences and strategies for their removal are reported. Specifically, cobalt(II) was masked by the addition of dimethylglyoxime to the luminol solution and vanadium(IV) was removed by passing the sample through an 8-hydroxyquinoline column in a low pH carrier stream. Manganese(II) also interfered by suppression of the CL response but this was not significant at typical open ocean concentrations.

Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum

Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (?31°N to 24°N) south of Bermuda in June 2008. A general north-to-south increase in surface concentrations of dFe (0.3–1.6 nM) and dAl (14–42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed for near-surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8–100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44–45%). Hydrographic observations and analysis of time series rain samples from Bermuda indicate that wet deposition accounts for most (>80%) of the total aeolian flux of Co, and hence a significant proportion of the atmospheric input of dCo to our study region. Our aerosol data imply that the atmospheric input of dCo to the Sargasso Sea is modest, although this flux may be more significant in late summer. The water column dCo profiles reveal a vertical distribution that predominantly reflects ‘nutrient-type’ behavior, versus scavenged-type behavior for dAl, and a hybrid of nutrient- and scavenged-type behavior for dFe. Mesoscale eddies also appear to impact on the vertical distribution of dCo. The effects of biological removal of dCo from the upper water column were apparent as pronounced sub-surface minima (21 ± 4 pM dCo), coincident with maxima in Prochlorococcus abundance. These observations imply that Prochlorococcus plays a major role in removing dCo from the euphotic zone, and that the availability of dCo may regulate Prochlorococcus growth in the Sargasso Sea.

Distribution of size fractionated dissolved iron in the Canary Basin

The distribution of size fractionated dissolved iron (DFe, <0.2 mum) species was determined in the upper water column (0-150 m) of the Canary Basin (25-32 degrees N and 18-24 degrees W) on a research cruise in October 2002. A DFe concentration gradient resulting from a decrease in both soluble iron (SFe, <0.02 microm) and colloidal iron (CFe, 0.02-0.2 microm) was shown to extend from the coast of North West Africa into the oligotrophic gyre (varying from approximately 1 nM in the shelf region to 0.15 nM in the most off shore waters). At the time of this study, the dominant dissolved Fe input to the region was deduced to be the advection of shelf and upwelled waters rather than Saharan dust deposition. SFe and CFe fractions had mean concentrations (+/- one standard deviation) of 0.25 +/- 0.11 and 0.21 +/- 0.16 nM, respectively (n = 58). Colloidal iron formed a highly variable fraction of DFe (ca. 0-80%, mean of 42%) in the region but was less variable in the low iron, oligotrophic intermediate waters (0.18 +/- 0.06 nM, 31.7 degrees N, 22.0 degrees W, 0-1300 m depth). The high variability found at the most productive near-shelf stations was driven by biological processing and mixing of different water masses. In contrast, less variability between SFe and CFe at the remote off shore stations suggested that vertical variations in the water column were controlled more by chemical partitioning and vertical particle fluxes with evidence of preferential biological uptake and/or removal of SFe in the most remote surface waters.

High temporal and spatial resolution environmental monitoring using flow injection with spectroscopic detection

This article describes the use of flow-injection techniques combined with molecular spectroscopic detection (spectrophotometry, fluorescence and chemiluminescence) for the rapid determination of chemical parameters in environmental matrices. The emphasis is on field deployment in order to obtain high temporal and spatial resolution data without the need for discrete sample collection and storage. Specific examples considered are the determination of phosphorus in river water and dissolved iron in open ocean water. The environmental drivers for these determinations are presented and the analytical capabilities of flow-injection instrumentation for remote deployment are discussed.

The impact of changing surface ocean conditions on the dissolution of aerosol iron

The proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH, and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols had the most significant effect on the aerosol Fe solubility, with the most anthropogenically influenced samples having the highest fractional solubility (up to 3.2%). The impact of ocean warming and acidification on aerosol Fe dissolution is therefore unlikely to be as important as changes in land usage and fossil fuel combustion. Our experimental results also reveal important changes in the size distribution of soluble aerosol Fe in solution, depending on the chemical conditions of seawater. Under typical conditions, the majority (77–100%) of Fe released from aerosols into ambient seawater existed in the colloidal (0.02–0.4 µm) size fraction. However, in the presence of a sufficient concentration of strong Fe-binding organic ligands (10 nM) most of the aerosol-derived colloidal Fe was converted to soluble Fe (<0.02 µm). This finding highlights the potential importance of organic ligands in retaining aerosol Fe in a biologically available form in the surface ocean.

Intercomparison between FI-CL and ICP-MS for the determination of dissolved iron in Atlantic seawater

Environmental context. Iron is arguably the most important trace element for the growth of marine organisms and is the limiting micronutrient for primary production in many parts of the world’s oceans. The concentration of dissolved iron in seawater therefore influences the global carbon cycle and consequently Earth’s climate. Hence, it is important to understand the marine biogeochemistry of iron and quantify its spatial and temporal distribution. In order to achieve this objective, it is essential that reported open-ocean concentrations of dissolved iron are accurate. Abstract. Results from a 3-laboratory blind intercomparison exercise with two widely used analytical methods for the determination of iron in seawater are presented. The two methods used are coprecipitation followed by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) and chemical reduction to iron(ii) followed by flow injection with chemiluminescence detection (FI-CL). The five samples used were collected from the South Atlantic Ocean as part of the IRONAGES intercomparison exercise. To avoid any inter-bottle variability, the same sample bottles were sent sequentially to three laboratories in England, Australia and the United States over a 12-month period. The results show that there is no statistical difference (P = 0.05) between the shipboard FI-CL method and the directly traceable, low blank, isotope dilution ICP-MS method for the determination of iron in surface South Atlantic seawater. There was also excellent agreement between the overall mean of the three laboratories (0.54 ± 0.03 nM) and the consensus value from an earlier community-wide separate bottle intercomparison using the same IRONAGES sample water (0.59 ± 0.21 nM).

Design of an Automated Flow Injection-Chemiluminescence Instrument Incorporating a Miniature Photomultiplier Tube for Monitoring Picomolar Concentrations of Iron in Seawater

A flow-injection (FI)-based instrument under LabVIEW control for monitoring iron in marine waters is described. The instrument incorporates a miniature, low-power photomultiplier tube (PMT), and a number of microelectric and solenoid actuated valves and peristaltic pumps. The software allows full control of all flow injection components and processing of the data from the PMT. The optimised system is capable of 20 injections per hour, including preconcentration and wash steps. The detection limit (3 sd of the blank) is 21 pM at sea and the linear range is 21–2000 pM with a 60-second sample load time. Typical precision between replicate FI peaks is 5.9 ± 3.2 % (n =4) over the linear range.

Combined uncertainty estimation for the determination of the dissolved iron amount content in seawater using flow injection with chemiluminescence detection

Abstract This work assesses the components contributing to the combined uncertainty budget associated with the measurement of the Fe amount content by flow injection chemiluminescence (FI‐CL) in <0.2 μm filtered and acidified seawater samples. Amounts of loaded standard solutions and samples were determined gravimetrically by differential weighing. Up to 5% variations in the loaded masses were observed during measurements, in contradiction to the usual assumptions made when operating under constant loading time conditions. Hence signal intensities (V) were normalised to the loaded mass and plots of average normalised intensities (in V kg−1) vs. values of the Fe amount content (in nmol kg−1) added to a “low level” iron seawater matrix were used to produce the calibration graphs. The measurement procedure implemented and the uncertainty estimation process developed were validated from the agreement obtained with consensus values for three SAFe and GEOTRACES reference materials (D2, GS, and GD). Relative expanded uncertainties for peak height and peak area based results were estimated to be around 12% and 10% (coverage factor k = 2), respectively. The most important contributory factors were the uncertainty on the sensitivity coefficient (i.e., calibration slope) and the within‐sequence‐stability (i.e., the signal stability over several hours of operation; here 32 h). For GD, using peak height measurements, these factors contributed respectively 69.7% and 21.6% while the short‐term repeatability accounted for only 7.9%. Therefore, an uncertainty estimation based on the intensity repeatability alone, as is often done in FI‐CL studies, is not a realistic estimation of the overall uncertainty of the procedure.