James W. Moffett

University of Southern California, Los Angeles, California, United States

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Publications (80)346.39 Total impact

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    ABSTRACT: RATIONALEVitamin B12 is an essential nutrient for more than half of surveyed marine algae species, but methods for directly measuring this important cofactor in seawater are limited. Current mass spectrometry methods do not quantify all forms of B12, potentially missing a significant portion of the B12 pool.METHODS We present a method to measure vitamins B1, B2, B6, B7 and four forms of B12 dissolved in seawater. The method entails solid-phase extraction, separation by ultra-performance liquid chromatography, and detection by triple-quadrupole tandem mass spectrometry using stable-isotope-labeled internal standards. We demonstrated the use of this method in the environment by analyzing B12 concentrations at different depths in the Hood Canal, part of the Puget Sound estuarine system in Washington State.RESULTSRecovery of vitamin B12 forms during the preconcentration steps was >71% and the limits of detection were <0.275 pM in seawater. Standard addition calibration curves in three different seawater matrices were used to determine analytical response and to quantify samples from the environment. Hydroxocobalamin was the main form of B12 in seawater at our field site.CONCLUSIONS We developed a method for quantifying four forms of B12 in seawater by liquid chromatography/mass spectrometry with the option of simultaneous analysis of vitamins B1, B2, B6, and B7. We validated the method and demonstrated its application in the field. Copyright © 2014 John Wiley & Sons, Ltd.
    Rapid Communications in Mass Spectrometry 11/2014; 28(22). DOI:10.1002/rcm.7040 · 2.64 Impact Factor
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  • Jeremy E. Jacquot, James W. Moffett
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    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; DOI:10.1016/j.dsr2.2014.11.013 · 2.76 Impact Factor
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    Yoshiko Kondo, James W. Moffett
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    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.
    Marine Chemistry 11/2014; DOI:10.1016/j.marchem.2014.11.007 · 3.20 Impact Factor
  • Jagruti Vedamati, Catherine Chan, James W. Moffett
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    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.
    Geochimica et Cosmochimica Acta 11/2014; 156. DOI:10.1016/j.gca.2014.10.026 · 4.25 Impact Factor
  • Jagruti Vedamati, Tyler Goepfert, James W. Moffett
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    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 · 3.62 Impact Factor
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    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.
    Environmental Microbiology 10/2014; DOI:10.1111/1462-2920.12677 · 6.24 Impact Factor
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    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 · 9.81 Impact Factor
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    ABSTRACT: The distribution and speciation of copper (Cu) in Hood Canal, a fjord in western Puget Sound, Washington, was 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 - 6.12 nM. 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 × 10-15 M to 1.36 × 10-12 M. This is a range that encompasses the threshold (< ~ 2 × 10-13 M) 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 20 m in the deeper, saline waters, and exhibiting minima in the upper 15 m. 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.
    Marine Chemistry 05/2014; 162:37 - 49. DOI:10.1016/j.marchem.2014.02.002 · 3.20 Impact Factor
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    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 · 3.62 Impact Factor
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    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 · 3.62 Impact Factor
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    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; DOI:10.1038/ismej.2013.75 · 9.27 Impact Factor
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    Peter L Croot, James W Moffett, George W Luther
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    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.
    Marine Chemistry 12/2012; DOI:10.1016/S0304-4203(99)00054-7 · 3.20 Impact Factor
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    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.
    Marine Chemistry 12/2012; DOI:10.1016/j.marchem.2005.07.002 · 3.20 Impact Factor
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    Yoshiko Kondo, James W Moffett
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    ABSTRACT: Keywords: Iron (Fe) Fe(II) Oxygen minimum zone Arabian Sea Indian Ocean GEOTRACES a b s t r a c t The concentration of iron(II) (Fe(II)) in seawater was investigated throughout the water column in the Arabian Sea and western tropical Indian Ocean including the oxygen minimum zone (OMZ) as part of the 2009 Japanese GEOTRACES cruise using a luminol-chemiluminescence detection based flow injection analysis technique. A novel modification was the adjustment of the sample pH to 7.2 with a 3-(N-morpholino) propanesulfonic acid (MOPS) buffer to minimize Fe(II) oxidation during sampling. At stations in the Arabian Sea OMZ, Fe(II) had subsurface maxima in the oxygen-deficient and high nitrite layers; fully 7–29% of total dissolved Fe existed as Fe(II) in these samples. Subsurface Fe(II) maxima were not observed in stations south of the oxygen minimum zone. Within the OMZ, the distribution of Fe(II) resembled previous data obtained during the 2004 southwest monsoon, indicating that the Fe(II) maxima are seasonally and interannually persistent feature. These results confirm the close relationship between Fe(II) and the secondary nitrite maxima and suggest that the rich microbial community within this feature is closely involved with Fe redox cycling. Fe(II) concentrations near the seafloor were elevated in locations underlying the OMZ but nowhere else, possibly reflecting inputs from reducing sediments. To the south, a clear maximum in dissolved Fe from the Rodriguez Triple Junction hydrothermal system showed no evidence of Fe(II). The center location of the Rodriguez Triple Junction is 251 35 0 S, 701 00 0 E (Gamo et al., 2001), more than 800 km southwest of station ER10 (the closest station), so hydrothermally-derived Fe(II) was probably oxidized.
    Deep Sea Research Part I Oceanographic Research Papers 11/2012; 73:73-83. DOI:10.1016/j.dsr.2012.11.014 · 2.83 Impact Factor
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    ABSTRACT: Iron is an important nutrient in the ocean, but the different sources and sinks of iron are not well constrained. Here, we use measurements of Fe concentration and Fe stable isotope ratios to evaluate the importance of reducing continental margins as a source of Fe to the open ocean. Dissolved iron concentration ([Fe]) and iron stable isotope ratios (δ56Fe) were measured in the San Pedro and Santa Barbara basins. Dissolved δ56Fe ranges from −1.82‰ to 0.00‰ in the San Pedro Basin and from −3.45‰ to −0.29‰ in the Santa Barbara Basin, and in both basins the lowest δ56Fe values and highest Fe concentrations are found at the bottom of the basin reflecting the input of isotopically light Fe from reducing sediment porewaters. In the San Pedro Basin, we are also able to fingerprint an advective source of Fe from shallow continental shelves next to the basin and the atmospheric deposition of Fe into surface waters. A one-dimensional model of the Fe isotope cycle has been constructed for the deep silled San Pedro Basin. By fitting model output to data, values of several important iron cycle parameters are predicted including a flux of Fe from sediment porewaters into the water column of 0.32–1.14 μmol m−2 d−1, a first-order dissolved Fe precipitation rate constant of 0.0018–0.0053 d−1, a flux δ56Fe of −2.4‰, and an isotope effect for Fe precipitation of Δδ56Feparticulate-dissolved = −0.8‰. Applying our model-predicted Fe cycle parameters to the global ocean suggests that continental margins contribute 4–12% of world ocean dissolved Fe and make the ocean’s Fe lighter by −0.08‰ to −0.26‰. The dramatically negative δ56Fe signature seen in the water column of the San Pedro and Santa Barbara basins demonstrate the utility of Fe isotopes as a tracer for continental margin Fe input from reducing sediments to the oceans, while the isotopic fractionation observed during loss of Fe from the dissolved phase suggests that this signature will be modified by subsequent reactions. Our modeling provides an initial framework for testing how these signals are transmitted into the open ocean.
    Geochimica et Cosmochimica Acta 09/2012; 93:14–29. DOI:10.1016/j.gca.2012.06.003 · 4.25 Impact Factor
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    ABSTRACT: The eastern subarctic North Pacific, an area of high nutrients and low chlorophyll, has been studied with respect to the potential for iron to control primary production. The geochemistry of zinc, a critical micronutrient for diatoms, is less well characterized. Total zinc concentrations and zinc speciation were measured in near-surface waters on transects across the subarctic North Pacific and across the Bering Sea. Total dissolved zinc concentrations in the near-surface ranged from 0.10 nmol L-1 to 1.15 nmol L-1 with lowest concentrations in the eastern portions of both the North Pacific and Bering Sea. Dissolved zinc speciation was dominated by complexation to strong organic ligands whose concentration ranged from 1.1 to 3.6 nmol L-1 with conditional stability constants (K'ZnL/Zn') ranging from 109.3 to 1011.0. The importance of zinc to primary producers was evaluated by comparison to phytoplankton pigment concentrations and by performing a shipboard incubation. Zinc concentrations were positively correlated with two pigments that are characteristic of diatoms. At one station in the North Pacific, the addition of 0.75 nmol L-1 zinc resulted in a doubling of chlorophyll after 4 days.
    Global Biogeochemical Cycles 06/2012; 26(2):2015-. DOI:10.1029/2010GB004004 · 4.53 Impact Factor
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    ABSTRACT: Characterization of the speciation of iron and copper is an important objective of the GEOTRACES Science Plan. To incorporate speciation measurements into such a multinational program, standard practices must be adopted that allow data from multiple labs to be synthesized. Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) is the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater. The determination of concentrations and conditional stability constants of metal-binding ligands is particularly challenging, as results can be influenced both by experimental conditions and interpretation of titration data. Here, we report an investigation between four laboratories to study the speciation of iron and copper using CLE-ACSV. Samples were collected on the GEOTRACES II intercomparison cruise in the North Pacific Ocean in May 2009 at 30° N, 140° W. This intercomparison was carried out shipboard and included an assessment of the viability of sample preservation by freezing. Results showed that consensus values could be obtained between different labs, but that some existing practices were problematic and require further attention in future work. A series of recommendations emerged from this study that will be useful in implementing multi-investigator programs like GEOTRACES.
    Limnology and oceanography, methods 04/2012; 10:496-515. DOI:10.4319/lom.2012.10.496 · 1.68 Impact Factor
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    ABSTRACT: The activity of the N(2)-fixing cyanobacterial genus Trichodesmium is critical to the global nitrogen (N) and carbon (C) cycles. Although iron (Fe) has been shown to be an important element limiting the growth and N(2) fixation of Trichodesmium, there have been no specific data demonstrating the in situ affect of Fe on Trichodesmium. We surveyed Trichodesmium populations from the Atlantic and Pacific Oceans for Fe limitation using a novel quantitative reverse transcriptase-PCR (qRT-PCR) method monitoring the expression of an Fe limitation-induced gene, isiB. Here we report the first molecular evidence of in situ Fe limitation of Trichodesmium N(2) fixation, which was evident in samples from the Pacific Ocean, whereas limitation appeared minimal to nonexistent in Atlantic Ocean samples. As our method is Trichodesmium clade specific, we were also able to determine that representatives from the Trichodesmium tenue clade were the most biologically active group of Trichodesmium in the majority of our samples, which speaks to their dominance in open ocean regimes. Furthermore, comparisons of our field expression and chemical data with laboratory studies suggest that the majority of dissolved Fe in the open ocean is available to Trichodesmium colonies regardless of Fe complexation.
    The ISME Journal 03/2012; 6(9):1728-39. DOI:10.1038/ismej.2012.13 · 9.27 Impact Factor

Publication Stats

3k Citations
346.39 Total Impact Points

Institutions

  • 2008–2014
    • University of Southern California
      • Department of Biological Sciences
      Los Angeles, California, United States
    • University of Minnesota Twin Cities
      • Department of Soil, Water, and Climate
      Minneapolis, Minnesota, United States
  • 2008–2013
    • University of California, Los Angeles
      Los Ángeles, California, United States
  • 1993–2012
    • Woods Hole Oceanographic Institution
      • Department of Marine Chemistry and Geochemistry
      Falmouth, MA, United States
  • 2006
    • Pontifical Catholic University of Chile
      CiudadSantiago, Santiago Metropolitan, Chile