[Show abstract][Hide abstract] ABSTRACT: Marine ammonia-oxidizing archaea (AOA) are among the most abundant of marine microorganisms, spanning nearly the entire water column of diverse oceanic provinces. Historical patterns of abundance are preserved in sediments in the form of their distinctive glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipids. The correlation between the composition of GDGTs in surface sediment and the overlying annual average sea surface temperature forms the basis for a paleotemperature proxy (TEX 86) that is used to reconstruct surface ocean temperature as far back as the Middle Jurassic. However, mounting evidence suggests that factors other than temperature could also play an important role in determining GDGT distributions. We here use a study set of four marine AOA isolates to demonstrate that these closely related strains generate different TEX 86 –temperature relationships and that oxygen (O 2) concentration is at least as important as temperature in controlling TEX 86 values in culture. All of the four strains characterized showed a unique membrane compositional response to temperature, with TEX 86-inferred temperatures varying as much as 12 °C from the incubation temperatures. In addition, both linear and nonlinear TEX 86 – temperature relationships were characteristic of individual strains. Increasing relative abundance of GDGT-2 and GDGT-3 with increasing O 2 limitation, at the expense of GDGT-1, led to significant elevations in TEX 86-derived temperature. Although the adaptive significance of GDGT compositional changes in response to both temperature and O 2 is unclear, this observation necessitates a reassessment of archaeal lipid-based paleotemperature proxies, particularly in records that span low-oxygen events or underlie oxygen minimum zones.
Proceedings of the National Academy of Sciences 08/2015; DOI:10.1073/pnas.1501568112 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Marine picocyanobacteria, comprised of the genera Synechococcus and Prochlorococcus, are the most abundant and widespread primary producers in the ocean. More than 20 genetically distinct clades of marine Synechococcus have been identified, but their physiology and biogeography are not as thoroughly characterized as those of Prochlorococcus. Using clade-specific qPCR primers, we measured the abundance of 10 Synechococcus clades at 92 locations in surface waters of the Atlantic and Pacific Oceans. We found that Synechococcus partition the ocean into four distinct regimes distinguished by temperature, macronutrients and iron availability. Clades I and IV were prevalent in colder, mesotrophic waters; clades II, III and X dominated in the warm, oligotrophic open ocean; clades CRD1 and CRD2 were restricted to sites with low iron availability; and clades XV and XVI were only found in transitional waters at the edges of the other biomes. Overall, clade II was the most ubiquitous clade investigated and was the dominant clade in the largest biome, the oligotrophic open ocean. Co-occurring clades that occupy the same regime belong to distinct evolutionary lineages within Synechococcus, indicating that multiple ecotypes have evolved independently to occupy similar niches and represent examples of parallel evolution. We speculate that parallel evolution of ecotypes may be a common feature of diverse marine microbial communities that contributes to functional redundancy and the potential for resiliency.The ISME Journal advance online publication, 24 July 2015; doi:10.1038/ismej.2015.115.
The ISME Journal 07/2015; DOI:10.1038/ismej.2015.115 · 9.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean. Of these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle. It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources and is thus of limited importance for ocean biogeochemistry. This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its long-range oceanic transport. Such transport has been subsequently inferred from spatially limited oceanographic observations. Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed. Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates. Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export production.
[Show abstract][Hide abstract] ABSTRACT: The Arabian Sea is a productive basin where seasonal upwelling and convective mixing result in high surface nutrient concentrations and widespread algal blooms. The factors controlling primary productivity in the Arabian Sea are of interest because the region contains an intense oxygen minimum zone (OMZ) that is a major sink for nitrate in the ocean. A survey of iron (Fe) distribution and redox chemistry was carried out in 2007 to assess its role in Arabian Sea biogeochemistry, including an investigation of the biological response to Fe additions. Results show that Fe is strongly enriched in the eastern Arabian Sea, associated with the OMZ. Much of the Fe within the OMZ is present as Fe(II), which enhances the residence time and accumulation of Fe. In contrast, Fe concentrations are lower in the western Arabian Sea, separated from the Fe-rich OMZ by a water mass boundary. Consequently, low surface values are associated with monsoon-driven upwelling off the Omani coast. Incubations revealed that primary production during the southwest monsoon was strongly limited by Fe over much of the study area. Incubation of surface waters with Fe resulted in rapid growth of Phaeocystis and up to sixfold increase in chlorophyll. This is the first demonstration of Fe limitation in the Arabian Sea, and the first high resolution zonal survey of dissolved Fe and Fe(II) across the basin. Our findings suggest that models developed to predict the response of the Arabian Sea biogeochemistry to global warming need to consider effects on Fe inputs.
[Show abstract][Hide abstract] ABSTRACT: Copper (Cu) distribution and speciation were characterized along a zonal section in the North Atlantic Ocean from Lisbon, Portugal, to Woods Hole, Massachusetts as part of the U.S. GEOTRACES program. Dissolved Cu profiles displayed many of the same features identified by other researchers, including sub-surface scavenging and a linear increase with depth, but many also exhibited unique properties and geographic trends. Concentrations ranged from 0.43 nM at the surface to 3.07 nM near the seafloor. The highest concentrations were measured in deep waters to the west of Cape Verde and northwest of the Canary Islands while the lowest concentrations were measured in upper waters, mostly between Mauritania and Cape Verde. The westernmost sampling sites overlying or adjacent to the U.S. east coast continental shelf featured surface maxima that decreased in magnitude moving east toward Bermuda, reflecting declining inputs from Cu-enriched coastal waters and North American aerosols. Free Cu (Cu2+) concentrations were tightly controlled by organic complexation and scavenging across the section with values varying between 1.54 fM and 1.07 pM. These results provide the first evidence that Cu2+ concentrations are strongly complexed throughout the water column, even in boundary zones where dissolved Cu concentrations are elevated because of local sources. Strong organic ligands (L) acted as a buffer for Cu2+, restricting concentrations to a narrow range (10–100 fM) throughout most of the water column. Cu2+ and dissolved Cu were strongly scavenged by suspended particulate matter within several benthic nepheloid layers and a hydrothermal plume above the Trans-Atlantic Geotraverse (TAG) vent field on the Mid-Atlantic Ridge (MAR).
Deep Sea Research Part II Topical Studies in Oceanography 11/2014; 116. DOI:10.1016/j.dsr2.2014.11.013 · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The distributions of dissolved iron (Fe), Fe(II) and Fe(III)-binding organic ligands were investigated in the upper 1000 meters of the eastern tropical South Pacific from January to March 2010, during an El Niño event. Dissolved Fe concentrations were exceedingly low in surface waters, showed minima near the chlorophyll maxima, and increased below that depth. High rates of nitrogen fixation have been inferred for this region from models, but our data suggest that surface Fe is frequently too low to support diazotrophs. Dissolved Fe and organic Fe(III) ligand concentrations at mid-depth were elevated in the nearshore stations, where virtually all dissolved Fe(III) was bound to these ligands. Subsurface Fe(II) maxima were observed in the secondary nitrite maxima of the oxygen minimum zone (OMZ), comprising 8 to 68% of dissolved Fe. Dissolved Fe concentrations displayed local maxima coinciding with the maxima in Fe(II) and nitrite. We propose that this zone, the most reducing part of the OMZ, plays an important role in Fe transport in the upper 400 m of the water column.
[Show abstract][Hide abstract] ABSTRACT: The geochemistry of manganese (Mn) in seawater is dominated by its redox chemistry, as Mn(II) is soluble and Mn(IV) forms insoluble oxides, and redox transformations are mediated by a variety of processes in the oceans. Dissolved Mn (DMn) accumulates under reducing conditions and is depleted under oxidizing conditions. Thus the Peruvian upwelling region, characterized by highly reducing conditions over a broad continental shelf and a major oxygen minimum zone extending far offshore, is potentially a large source of Mn to the eastern Tropical South Pacific. In this study, DMn was determined on cruises in October 2005 and February 2010 in the Peruvian Upwelling and Oxygen Minimum Zone, to evaluate the relationship between Mn, oxygen and nitrogen cycle processes. DMn concentrations were determined using simple dilution and matrix-matched external standardization inductively coupled mass spectrometry. Surprisingly, DMn was depleted under the most reducing conditions along the Peruvian shelf. Concentrations of dissolved Mn in surface waters increased offshore, indicating that advection of Mn offshore from the Peruvian shelf is a minor source. Subsurface Mn maxima were observed within the oxycline rather than within the oxygen minimum zone (OMZ), indicating they arise from remineralisation of organic matter rather than reduction of Mn oxides. The distribution of DMn appears to be dominated by non-redox processes and inputs from the atmosphere and from other regions associated with specific water masses. Lower than expected DMn concentrations on the shelf probably reflect limited fluvial inputs from the continent and efficient offshore transport. This behavior is in stark contrast to Fe, reported in a companion study which is very high on the shelf and undergoes dynamic redox cycling.
[Show abstract][Hide abstract] ABSTRACT: Vertical profiles of total dissolved iron (DFe), Fe(II), and hydrographic parameters were obtained along three transects across the continental shelf off Peru in October 2005. Fe(II) and DFe concentrations were determined using luminol chemiluminescence and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS), respectively. The objective was to study the relationships among redox chemistry, nitrogen-cycle processes, bottom topography, and the distribution and chemistry of iron in the region. The results are the most thorough survey to date of Fe geochemistry in this region. Exceedingly high DFe (∼ 50-75 nmol L−1) was measured over the wide northern continental shelf, with most of the dissolved Fe present as Fe(II) below the oxycline. In southern Peru, the shelf is narrower, and dissolved Fe concentrations were 10-fold lower. Moreover, a smaller fraction of the dissolved Fe was present as Fe(II) in the south, even below the oxycline. At the western ends of the transects, Fe(II) maxima were coincident with deep (i.e., secondary) nitrite maxima. This suggests a relationship between nitrate reduction and Fe(II) accumulation in the water column. However, over the shelf, Fe(II) was also influenced by benthic processes. Large lateral gradients in dissolved Fe across the shelf-slope break reflect Fe removal by oxidative scavenging, and it seems plausible that much of the Fe is “trapped” by redox cycling on the shelf. Nevertheless, the maintenance of dissolved Fe(II) filaments within the secondary nitrite maxima constitutes an important mechanism for Fe transport offshore.
Limnology and Oceanography 11/2014; 59(6):1945-1957. DOI:10.4319/lo.2014.59.6.1945 · 3.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nitrification is a critical process for the balance of reduced and oxidized nitrogen pools in nature, linking mineralization to the nitrogen loss processes of denitrification and anammox. Recent studies indicate a significant contribution of ammonia-oxidizing archaea (AOA) to nitrification. However, quantification of the relative contributions of AOA and ammonia-oxidizing bacteria (AOB) to in situ ammonia oxidation remains challenging. We show here the production of nitric oxide (NO) by Nitrosopumilus maritimus SCM1. Activity of SCM1 was always associated with release of NO with quasi-steady state concentrations between 0.05 and 0.08 μM. Nitric oxide production and metabolic activity were inhibited by the nitrogen free radical scavenger 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). Comparison of marine and terrestrial AOB strains with SCM1 and the recently isolated marine AOA strain HCA1 demonstrated a differential sensitivity of AOB and AOA to PTIO and allylthiourea (ATU). Similar to the investigated AOA strains, bulk water column nitrification at coastal and open ocean sites with sub-micromolar ammonia/ammonium concentrations was inhibited by PTIO and insensitive to ATU. These experiments support predictions from kinetic, molecular and biogeochemical studies indicating that marine nitrification at low ammonia/ammonium concentrations is largely driven by archaea and suggest an important role of NO in the archaeal metabolism.
[Show abstract][Hide abstract] ABSTRACT: Ammonia-oxidizing archaea (AOA) are now implicated in exerting significant control over the form and availability of reactive nitrogen species in marine environments. Detailed studies of specific metabolic traits and physicochemical factors controlling their activities and distribution have not been well constrained in part due to the scarcity of isolated AOA strains. Here, we report the isolation of two new coastal marine AOA, strains PS0 and HCA1. Comparison of the new strains to Nitrosopumilus maritimus strain SCM1, the only marine AOA in pure culture thus far, demonstrated distinct adaptations to pH, salinity, organic carbon, temperature, and light. Strain PS0 sustained nearly 80% of ammonia oxidation activity at a pH as low as 5.9, indicating that coastal strains may be less sensitive to the ongoing reduction in ocean pH. Notably, the two novel isolates are obligate mixotrophs that rely on uptake and assimilation of organic carbon compounds, suggesting a direct coupling between chemolithotrophy and organic matter assimilation in marine food webs. All three isolates showed only minor photoinhibition at 15 µE⋅m(-2)⋅s(-1) and rapid recovery of ammonia oxidation in the dark, consistent with an AOA contribution to the primary nitrite maximum and the plausibility of a diurnal cycle of archaeal ammonia oxidation activity in the euphotic zone. Together, these findings highlight an unexpected adaptive capacity within closely related marine group I Archaea and provide new understanding of the physiological basis of the remarkable ecological success reflected by their generally high abundance in marine environments.
Proceedings of the National Academy of Sciences 08/2014; DOI:10.1073/pnas.1324115111 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The distribution and speciation of copper (Cu) in Hood Canal, a fjord in western Puget Sound, Washington, were studied over a 2-year period. Cu is required as a cofactor for many enzymatic pathways, including ammonia oxidation. In Hood Canal, ammonia oxidation is largely dominated by ammonia-oxidizing archaea (AOA), which have high Cu requirements for other processes as well. Dissolved Cu was slightly depleted in the upper water column, and concentrations were almost unchanged from measurements made in the late 1970s, ranging from 4.08 to 6.12nM. Although this implies that the biological demand is small relative to the large and relatively constant inventory of dissolved Cu, and that Cu limitation is therefore unlikely to influence rates of biological processes, speciation measurements indicated that dissolved Cu is strongly complexed by organic ligands. As a result, bioavailable Cu2+ concentrations were considerably lower, varying from 6.14×10-15M to 1.36×10-12M. This is a range that encompasses the threshold (<~2×10-13M) for Cu limitation of ammonia oxidation by Nitrosopumilus maritimus SCM1, a representative AOA, in culture (Amin et al., 2013). Furthermore, Cu2+ displayed a clear trend over most sampling periods, with Cu2+ concentrations one to two orders of magnitude higher below 20m in the deeper, saline waters, and exhibiting minima in the upper 15m. The major freshwater input to Hood Canal is not an important source of ligands, which suggests that the ligands are likely produced biologically in the water column and have slow turnover times. In general, ammonia oxidation rates varied considerably but were lowest in the upper water column where Cu2+ concentrations were also lowest. Thus, these findings will facilitate further work to ascertain the relative importance of Cu bioavailability in limiting ammonia oxidation rates versus light inhibition, which has frequently been invoked to account for low rates of nitrification in the upper water column.
[Show abstract][Hide abstract] ABSTRACT: Ammonia oxidizing archaea (AOA) have recently been recognized as the primary nitrifiers in the marine environment; they thus play an important role in the nitrogen cycle. Available genome sequences of AOA indicate that numerous Cu-dependent enzymes are essential for both ammonia oxidation and electron transfer, suggesting a particularly high requirement for copper. However, our knowledge of the copper requirements of AOA and their response to copper limitation in the ocean is nonexistent. Here, we examine the copper requirements of the chemolithoautotrophic AOA Candidatus Nitrosopumilus maritimus SCM1 using a combination of the metal chelators ethylenediaminetetraacetic acid and 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid and show that ammonia oxidation is limited at free cupric ion concentrations < 10−12.7 mol L−1, which are higher than concentrations frequently reported for many coastal and oceanic regimes. Prolonged exposure of cells to copper starvation for up to 6 d had no effect on the recovery of ammonia oxidation by N. maritimus. In addition, we present evidence that N. maritimus does not produce a copper-binding ligand (chalkophore) under copper limitation and therefore probably relies mainly on acquisition of copper ions from surrounding media. Copper limitation may be an important constraint on archaeal ammonia oxidation throughout the marine environment.
Limnology and Oceanography 11/2013; 58(6). DOI:10.4319/lo.2013.58.6.2037 · 3.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20 degrees S and 10 degrees S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as similar to 0.26 nmol L-1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10 degrees S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 x 10(-15) mol L-1 to 1.34 x 10(-13) mol L-1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM-iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation-have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.
Limnology and Oceanography 07/2013; 58(4):1387-1394. DOI:10.4319/lo.2013.58.4.1387 · 3.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Archaeal ammonia oxidizers (AOAs) are increasingly recognized as prominent members of natural microbial assemblages. Evidence that links the presence of AOA with in situ ammonia oxidation activity is limited, and the abiotic factors that regulate the distribution of AOA natural assemblages are not well defined. We used quantitative PCR to enumerate amoA (encodes α-subunit of ammonia monooxygenase) abundances; AOA amoA gene copies greatly outnumbered ammonia-oxidizing bacteria and amoA transcripts were derived primarily from AOA throughout the water column of Hood Canal, Puget Sound, WA, USA. We generated a Michaelis-Menten kinetics curve for ammonia oxidation by the natural community and found that the measured Km of 98±14 nmol l(-1) was close to that for cultivated AOA representative Nitrosopumilus maritimus SCM1. Temperature did not have a significant effect on ammonia oxidation rates for incubation temperatures ranging from 8 to 20 °C, which is within the temperature range for depths of measurable ammonia oxidation at the site. This study provides substantial evidence, through both amoA gene copies and transcript abundances and the kinetics response, that AOA are the dominant active ammonia oxidizers in this marine environment. We propose that future ammonia oxidation experiments use a Km for the natural community to better constrain ammonia oxidation rates determined with the commonly used (15)NH4(+) dilution technique.The ISME Journal advance online publication, 9 May 2013; doi:10.1038/ismej.2013.75.
The ISME Journal 05/2013; 7(10). DOI:10.1038/ismej.2013.75 · 9.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A methodology has been developed to determine the half-wave potentials of organic Cu chelators in seawater and algal culture media from pseudopolarograms obtained by anodic stripping voltammetry. A linear relationship between half-wave potentials determined from the pseudopolarogram, and thermodynamic stability constants was observed for a collection of model ligands that spanned a wide range of stability constants (log K=12–26.5) and included many functional groups likely to be important in copper complexation in seawater. Results suggested that it is possible to determine thermodynamic stability constants from half-wave potentials, as reported previously for Zn and Fe. Pseudopolarograms from culture media of three Cu stressed phytoplankton species showed very different thermodynamic stability constants, indicating the presence of chemically distinct ligands. The cyanobacterium Synechococcus produced at least two strong Cu chelators. One had a thermodynamic stability constant of log K=36.7±3.0 (obtained by extrapolation of the scale) and complexed from 30%–50% of the total Cu. The second was electrochemically inert, even at −1.6 V. The eukaryotic species produced several chelators with stability constants ranging from log K=22.6–39.1. Seawater samples collected from coastal waters showed similar characteristics. These thermodynamic constants are very high relative to model ligands in the literature, which could reflect coordination by specialized biomolecules. Alternatively, the large negative half-wave potentials could be due to Cu(I) complexes (with log K=10–20) or reflect reductive ligand decomposition of kinetically inert Cu(II) complexes.
[Show abstract][Hide abstract] ABSTRACT: The first large-scale international intercomparison of analytical methods for the determination of dissolved iron in seawater was carried out between October 2000 and December 2002. The exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24 international laboratories. The comparison was based on a large volume (700 L), filtered surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample), which was acidified, mixed and bottled at sea. Two 1-L sample bottles were sent to each participant. Integrity and blindness were achieved by having the experiment designed and carried out by a small team, and overseen by an independent data manager. Storage, homogeneity and time-series stability experiments conducted over 2.5 years showed that inter-bottle variability of the IRONAGES sample was good (< 7%), although there was a decrease in iron concentration in the bottles over time (0.8–0.5 nM) before a stable value was observed. This raises questions over the suitability of sample acidification and storage.For the complete dataset of 45 results (after excluding 3 outliers not passing the screening criteria), the mean concentration of dissolved iron in the IRONAGES sample was 0.59 ± 0.21 nM, representing a coefficient of variation (%CV) for analytical comparability (“community precision”) of 36% (1s), a significant improvement over earlier exercises. Within-run precision (5–10%), inter-run precision (15%) and inter-bottle homogeneity (< 7%) were much better than overall analytical comparability, implying the presence of: (1) random variability (inherent to all intercomparison exercises); (2) errors in quantification of the analytical blank; and (3) systematic inter-method variability, perhaps related to secondary sample treatment (e.g. measurement of different physicochemical fractions of iron present in seawater) in the community dataset. By grouping all results for the same method, analyses performed using flow injection-luminol chemiluminescence (with FeII detection after sample reduction) [Bowie, A.R., Achterberg, E.P., Mantoura, R.F.C., Worsfold, P.J., 1998. Determination of sub-nanomolar levels of iron in seawater using flow injection with chemiluminescence detection. Anal. Chim. Acta 361, 189–200] and flow injection-catalytic spectrophotometry (using the reagent DPD) [Measures, C.I., Yuan, J., Resing, J.A., 1995. Determination of iron in seawater by flow injection analysis using in-line preconcentration and spectrophotometric detection. Mar. Chem. 50, 3–12] gave significantly (P = 0.05) higher dissolved iron concentrations than analyses performed using isotope dilution ICPMS [Wu, J.F., Boyle, E.A., 1998. Determination of iron in seawater by high-resolution isotope dilution inductively coupled plasma mass spectrometry after Mg(OH)2 co-precipitation. Anal. Chim. Acta 367, 183–191]. There was, however, evidence of scatter within each method group (CV up to 59%), implying that better uniformity in procedures may be required. This paper does not identify individual data and should not be viewed as an evaluation of single laboratories. Rather it summarises the status of dissolved iron analysis in seawater by the international community at the start of the 21st century, and can be used to inform future exercises including the SAFE iron intercomparison study in the North Pacific in October 2004.