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Barbeau, K, Kujawinski EB, Moffett JW.  2001.  Remineralization and recycling of iron, thorium and organic carbon by heterotrophic marine protists in culture. Aquatic Microbial Ecology. 24:69-81.   10.3354/ame024069   AbstractWebsite

To characterize trace metal cycling in marine systems as mediated by heterotrophic protists, we conducted a series of laboratory experiments in 2-organism model systems consisting of bacteria and protistan grazers. Trace metal isotopes (Fe-59 and Th-234),C-14, and bulk organic carbon measurements were used to follow the chemical transformation of bacterial carbon and associated trace metals by several different grazer species. Results indicate that grazers were able to cause repartitioning of Th and regeneration of Fe from bacterial prey into the dissolved phase (<0.2 m), even in particle-rich laboratory cultures. For both Th and Fe, protist grazing led to the formation of relatively stable dissolved and colloidal metal-organic species. Metal/carbon ratios of the particle pool in some model systems with grazers were significantly altered, indicating a decoupling of trace metal and organic carbon cycling through the grazing process. Different protist species exhibited substantial variation (up to a factor of 10) in their ability to quantitatively remobilize trace metals from bacterial prey. The implications of these findings for trace metal cycling in marine systems are discussed.

Barbeau, K, Wollast R.  1994.  Microautoradiography (with Combined Liquid Scintillation) Applied to the Study of Trace-Metal Uptake by Suspended Particles - Initial Results Using NI-63 as a Tracer. Limnology and Oceanography. 39:1211-1222. AbstractWebsite

We report the development of a microautoradiographic method for the study of trace metal-particle interactions in natural waters. This technique, in combination with conventional liquid scintillation counting methods, was applied to surface water samples from the Belgian coastal zone and Scheldt estuary. Ni-63 was used as the metallic radio-tracer. Ni partitioning in our experimental system was shown to be a primarily abiotic process, driven by passive sorption reactions and limited in extent on a 24-h time scale by the slow reaction kinetics of Ni. Small particles (< 1 mum) were important as sorption sites, while large particles exhibited variable and particle-specific scavenging potential.

Barbeau, K.  2006.  Photochemistry of organic iron(III) complexing ligands in oceanic systems. Photochemistry and Photobiology. 82:1505-1516.   10.1562/2006-06-16-ir-935   AbstractWebsite

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (> 99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O-2(-))The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimental/observational studies of the natural seawater ligand pool.

Barbeau, KA, Moffett JW.  1998.  Dissolution of Iron Oxides by Phagotrophic Protists:  Using a Novel Method To Quantify Reaction Rates. Environmental Science & Technology. 32:2969-2975.: American Chemical Society   10.1021/es9802549   AbstractWebsite

In previous work, we have reported the dissolution of iron oxides within the acidic food vacuoles of marine protozoan grazers as evidence of a novel mechanism for the conversion of refractory iron solids to more labile forms in oxic surface waters. This paper expands upon those initial studies and presents a new technique to study the reaction of iron oxides in seawater, based on the synthesis of colloidal ferrihydrite containing an inert tracer. Measuring the accumulation of the tracer in the dissolved phase enables the determination of the rate and extent of iron oxide reaction, even for kinetically slow processes and regardless of the fate of iron in the system. The validity of the method as a means of following the reaction of iron oxides in seawater is shown here in a series of co-dissolution studies and in several photochemical kinetics experiments. In laboratory studies of the dissolution of colloidal ferrihydrite by protozoan grazers, the inert tracer method enables an improved estimate of the rate of protozoan-mediated iron oxide dissolution, confirming our previous results and providing a useful tool for further studies of phagotrophy as a reaction pathway for refractory iron.

Barbeau, K, Rue EL, Bruland KW, Butler A.  2001.  Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands. Nature. 413:409-413.   10.1038/35096545   AbstractWebsite

Iron is a limiting nutrient for primary production in large areas of the oceans(1-4). Dissolved iron(III) in the upper oceans occurs almost entirely in the form of complexes with strong organic ligands(5-7) presumed to be of biological origin(8,9). Although the importance of organic ligands to aquatic iron cycling is becoming clear, the mechanism by which they are involved in this process remains uncertain. Here we report observations of photochemical reactions involving Fe(III) bound to siderophores-high-affinity iron(III) ligands produced by bacteria to facilitate iron acquisition(10-12). We show that photolysis of Fe(III)-siderophore complexes leads to the formation of lower-affinity Fe(III) ligands and the reduction of Fe(III), increasing the availability of siderophore-bound iron for uptake by planktonic assemblages. These photochemical reactions are mediated by the alpha -hydroxy acid moiety, a group which has generally been found to be present in the marine siderophores that have been characterized(13-15). We suggest that Fe(III)-binding ligands can enhance the photolytic production of reactive iron species in the euphotic zone and so influence iron availability in aquatic systems.

Barbeau, K, Moffett JW, Caron DA, Croot PL, Erdner DL.  1996.  Role of protozoan grazing in relieving iron limitation of phytoplankton. Nature. 380:61-64.   10.1038/380061a0   AbstractWebsite

RECENT evidence indicates that iron is a limiting factor in primary production in some areas of the oceans(1,2). In sea water, iron is largely present in the form of particulate and colloidal phases which are apparently unavailable for uptake by phytoplankton(3-5). Several mechanisms have been proposed whereby non-reactive iron may be converted into more labile forms (for example, thermal dissolution(6), photochemical reactions(7,8) and ligand complexation(9)). Here we report that digestion of colloidal iron in the acidic food vacuoles of protozoan grazers may be a mechanism for the generation of 'bioavailable' iron from refractory iron phases. We have demonstrated several grazer-mediated effects on colloidal ferrihydrite, including a decrease in colloid size, an increase in colloid lability as determined by competitive ligand-exchange techniques, and an increase in the bioavailability of colloids to iron-limited diatoms. These results indicate that protozoan grazers may significantly enhance the supply of iron to marine phytoplankton from terrestrial sources.

Barbeau, K, Zhang GP, Live DH, Butler A.  2002.  Petrobactin, a photoreactive siderophore produced by the oil-degrading marine bacterium Marinobacter hydrocarbonoclasticus. Journal of the American Chemical Society. 124:378-379.   10.1021/ja0119088   AbstractWebsite

Petrobactin is a bis-catecholate, α-hydroxy acid siderophore produced by the oil-degrading marine bacterium Marinobacter hydrocarbonoclasticus. The Fe(III)-complexed form of petrobactin is photoreactive in natural sunlight, mediated by the Fe(III)-citrate moiety. The reaction results in decarboxylation of the petrobactin ligand and reduction of Fe(III) to Fe(II). This report is one of the first to show the photoreactivity of Fe(III)-siderophores mediated by the ferric ion-α-hydroxy acid group. The demonstration of light-mediated decarboxylation of an Fe(III)-siderophore complex raises questions about a possible functional role for photoreactivity in siderophore-mediated iron uptake.

Barbeau, K, Moffett JW.  2000.  Laboratory and field studies of colloidal iron oxide dissolution as mediated by phagotrophy and photolysis. Limnology and Oceanography. 45:827-835. AbstractWebsite

In a previous work, we have employed colloidal ferrihydrite impregnated with an inert radiotracer to probe the mechanistics of iron redox cycling in seawater via phagotrophic and photochemical processes. This paper reports further studies using the inert tracer technique, directed towards obtaining a more quantitative sense of the importance of phagotrophy relative to photolysis as a pathway for the production of bioavailable iron in oxygenated seawater. Our results indicate a maximal (i.e., near-surface at noon) rate of 12% per day for the photochemically-mediated dissolution of colloidal ferrihydrite. Protozoan-mediated dissolution of the same iron oxide phase proceeds at a rate ranging from 1-6% per day, depending on grazing turnover rates. Thus, while photolysis should dominate the redox cycling of refractory iron solids in near-surface waters under bright daytime conditions, phagotrophy is likely to be a more important process overall when the entire euphotic zone is considered on a time-averaged basis.

Barbeau, K, Rue EL, Trick CG, Bruland KT, Butler A.  2003.  Photochemical reactivity of siderophores produced by marine heterotrophic bacteria and cyanobacteria based on characteristic Fe(III) binding groups. Limnology and Oceanography. 48:1069-1078. AbstractWebsite

Siderophores, high-affinity Fe(III) ligands produced by microorganisms to facilitate iron acquisition, might contribute significantly to dissolved Fe(III) complexation in ocean surface waters. In previous work, we demonstrated the photoreactivity of the ferric ion complexes of several alpha-hydroxy carboxylic acid-containing siderophores produced by heterotrophic marine bacteria. Here, we expand on our earlier studies and detail the photoreactivity of additional siderophores produced by both heterotrophic marine bacteria and marine cyanobacteria, making comparisons to synthetic and terrestrial siderophores that lack the alpha-hydroxy carboxylate group. Our results suggest that, in addition to secondary photochemical reaction pathways involving reactive oxygen species, direct photolysis of Fe(III)-siderophore complexes might be a significant source of Fe(II) and reactive Fe(III) in ocean surface waters. Our findings further indicate that the photoreactivity of siderophores is primarily determined by the chemical structure of the Fe(III) binding groups that they possess-hydroxamate, catecholate, or alpha-hydroxy carboxylate moieties. Hydroxamate groups are photochemically resistant regardless of Fe(III) complexation. Catecholates, in contrast, are susceptible to photooxidation in the uncomplexed form but stabilized against photooxidation when ferrated. alpha-Hydroxy carboxylate groups are stable as the uncomplexed acid, but when coordinated to Fe(III), these moieties undergo light-induced ligand oxidation and reduction of Fe(III) to Fe(II). These photochemical properties appear to determine the reactivity and fate of Fe(III)-binding siderophores in ocean surface waters, which in turn might significantly influence the biogeochemical cycling of iron.

Boiteau, RM, Till CP, Ruacho A, Bundy RM, Hawco NJ, McKenna AM, Barbeau KA, Bruland KW, Saito MA, Repeta DJ.  2016.  Structural characterization of natural nickel and copper binding ligands along the US GEOTRACES Eastern Pacific Zonal Transect. Frontiers in Marine Science. 3:243.   10.3389/fmars.2016.00243   Abstract

Organic ligands form strong complexes with many trace elements in seawater. Various metals can compete for the same ligand chelation sites, and the final speciation of bound metals is determined by relative binding affinities, concentrations of binding sites, uncomplexed metal concentrations, and association/dissociation kinetics. Different ligands have a wide range of metal affinities and specificities. However, the chemical composition of these ligands in the marine environment remains poorly constrained, which has hindered progress in modeling marine metal speciation. In this study, we detected and characterized natural ligands that bind copper (Cu) and nickel (Ni) in the eastern South Pacific Ocean with liquid chromatography tandem inductively coupled plasma mass spectrometry (LC-ICPMS), and high resolution electrospray ionization mass spectrometry (ESIMS). Dissolved Cu, Ni, and ligand concentrations were highest near the coast. Chromatographically unresolved polar compounds dominated ligands isolated near the coast by solid phase extraction. Offshore, metal and ligand concentrations decreased, but several new ligands appeared. One major ligand was detected that bound both Cu2+ and Ni2+. Based on accurate mass and fragmentation measurements, this compound has a molecular formula of [C20H21N4O8S2 + M]+ (M = metal isotope) and contains several azole-like metal binding groups. Additional lipophilic Ni complexes were also present only in oligotrophic waters, with masses of 649, 698, and 712 m/z (corresponding to the 58Ni metal complex). Molecular formulae of [C32H54N3O6S2Ni]+ and [C33H56N3O6S2Ni]+ were determined for two of these compounds. Addition of Cu and Ni to the samples also revealed the presence of additional compounds that can bind both Ni and Cu. Although these specific compounds represent a small fraction of the total dissolved Cu and Ni pool, they highlight the compositional diversity and spatial heterogeneity of marine Ni and Cu ligands, as well as variability in the extent to which different metals in the same environment compete for ligand binding.

Brzezinski, MA, Krause JW, Bundy RM, Barbeau KA, Franks P, Goericke R, Landry MR, Stukel MR.  2015.  Enhanced silica ballasting from iron stress sustains carbon export in a frontal zone within the California Current. Journal of Geophysical Research-Oceans. 120:4654-4669.   10.1002/2015jc010829   AbstractWebsite

Nutrient dynamics, phytoplankton rate processes, and export were examined in a frontal region between an anticyclone and a pair of cyclones 120 km off the coast in the southern California Current System (sCCS). Low silicic acid: nitrate ratios (Si:N) and high nitrate to iron ratios (N: Fe) characteristic of Fe-limiting conditions in the sCCS were associated with the northern cyclone and with the transition zone between the cyclones and the anticyclone. Phytoplankton growth in low-Si:N, high-N:Fe waters responded strongly to added Fe, confirming growth limitation by Fe of the diatom-dominated phytoplankton community. Low Si: N waters had low biogenic silica content, intermediate productivity, but high export compared to intermediate Si: N waters indicating increased export efficiency under Fe stress. Biogenic silica and particulate organic carbon (POC) export were both high beneath low Si: N waters with biogenic silica export being especially enhanced. This suggests that relatively high POC export from low Si: N waters was supported by silica ballasting from Fe-limited diatoms. Higher POC export efficiency in low Si: N waters may have been further enhanced by lower rates of organic carbon remineralization due to reduced grazing of more heavily armored diatoms growing under Fe stress. The results imply that Fe stress can enhance carbon export, despite lowering productivity, by driving higher export efficiency.

Buck, KN, Moffett J, Barbeau KA, Bundy RM, Kondo Y, Wu JF.  2012.  The organic complexation of iron and copper: an intercomparison of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques. Limnology and Oceanography-Methods. 10:496-515.   10.4319/lom.2012.10.496   AbstractWebsite

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 degrees N, 140 degrees 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.

Buck, KN, Selph KE, Barbeau KA.  2010.  Iron-binding ligand production and copper speciation in an incubation experiment of Antarctic Peninsula shelf waters from the Bransfield Strait, Southern Ocean. Marine Chemistry. 122:148-159.   10.1016/j.marchem.2010.06.002   AbstractWebsite

The evolution of dissolved iron (Fe) and copper (Cu) speciation was followed through a simulated spring bloom event in a 15-day incubation experiment of natural seawater collected during austral winter from high macronutrient high Fe waters of Bransfield Strait in the Southern Ocean. The incubation experiment included unamended bottles as well as Fe additions using the stable isotope of Fe, Fe-57. as inorganic ((FeCl3)-Fe-57) and organic (Fe-57-aerobactin, Fe-57-desferrioxamine B) amendments. Exposure to summer light conditions resulted in substantial growth for all treatments, mimicking the initiation of a spring bloom. The addition of Fe resulted in a 30% increase in phytoplankton biomass over unamended controls by day 15, indicating that the unamended waters became Fe limited despite initially elevated dissolved Fe concentrations. Dissolved Cu and Cu speciation remained largely unchanged for all treatments of the incubation, with Cu speciation dominated by exceedingly strong Cu-binding ligands (log K-CuL1.Cu2+(Cond) similar to 16) and low resultant Cu2+ concentrations (10(-16.3 +/- 0.3) mol L-1). In only the unamended light bottles, strong Fe-binding ligands were produced over the course of the experiment. The observed production of strong Fe-binding ligands in the control bottles that became Fe-limited supports the important role of biologically produced siderophore-type natural ligands in the marine Fe cycle. (C) 2010 Elsevier B.V. All rights reserved.

Bundy, RM, Abdulla HAN, Hatcher PG, Biller DV, Buck KN, Barbeau KA.  2015.  Iron-binding ligands and humic substances in the San Francisco Bay estuary and estuarine-influenced shelf regions of coastal California. Marine Chemistry. 173:183-194.   10.1016/j.marchem.2014.11.005   AbstractWebsite

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity <2) to the mouth of the estuary (salinity >26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand,salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5 nmol L-1 and strong ligand (log K-Fel, Fe'(cond) >= 12.0) concentrations equal to 139.5 nmol L-1. The weakest ligands (log K-Fel, Fe'(cond) < 10.0) were always in excess of dFe in low salinity waters, but were rapidly flocculated within the estuary and were not detected at salinities greater than 7. The strongest ligands (log K-Fel, Fe'(cond) > 11.0) were tightly coupled to dFe throughout the estuary, with average excess ligand concentrations ([L]-[dFe]) equal to 0.5 nmol L-1. Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5 mu g L-1 to 67.5 mu g L-1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters. (C) 2014 Elsevier B.V. All rights reserved.

Bundy, RM, Barbeau KA, Buck KN.  2013.  Sources of strong copper-binding ligands in Antarctic Peninsula surface waters. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:134-146.   10.1016/j.dsr2.2012.07.023   AbstractWebsite

Copper-binding organic ligands were measured during austral winter in surface waters around the Antarctic Peninsula using competitive ligand exchange-adsorptive cathodic stripping voltammetry with multiple analytical windows. Samples were collected from four distinct water masses including the Antarctic Circumpolar Current, Southern Antarctic Circumpolar Current Front, Bransfield Strait, and the shelf region of the Antarctic Peninsula. Strong copper-binding organic ligands were detected in each water mass. The strongest copper-binding ligands were detected at the highest competition strength in the Antarctic Circumpolar Current, with an average conditional stability constant of logK(CuL,Cu2+)(cond) = 16.00 +/- 0.82. The weakest ligands were found at the lowest competition strength in the shelf region with logK(CuL,Cu2+)(cond) = 12.68 +/- 0.48. No ligands with stability constants less than logK(CuL,Cu2+)(cond) = 13.5 were detected in the Antarctic Circumpolar Current at any competition strength, suggesting a shelf source of weaker copper-binding ligands. Free, hydrated copper ion concentrations, the biologically available form of dissolved copper, were less than 10(-14) M in all samples, approaching levels that may be limiting for some types of inducible iron acquisition. (C) 2012 Elsevier Ltd. All rights reserved.

Bundy, RM, Biller DV, Buck KN, Bruland KW, Barbeau KA.  2014.  Distinct pools of dissolved iron-binding ligands in the surface and benthic boundary layer of the California Current. Limnology and Oceanography. 59:769-787.   10.4319/lo.2014.59.3.0769   AbstractWebsite

Organic dissolved iron (dFe)-binding ligands were measured by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) at multiple analytical windows (side reaction coefficient of salicylaldoxime, alpha(Fe(SA)2) = 30, 60, and 100) in surface and benthic boundary layer (BBL) samples along the central California coast during spring and summer. The weakest ligands were detected in the BBL at the lowest analytical window with average log K-FeL,Fe'(cond) = 10.2 +/- 0.4 in the summer and 10.8 +/- 0.2 in the spring. Between 3% and 18% of the dFe complexation in the BBL was accounted for by HS, which were measured separately in samples by ACSV and may indicate a source of dFe-binding ligands from San Francisco Bay. The strongest ligands were found in nearshore spring surface waters at the highest analytical window with average log K-FeL,Fe'(cond) = 11.9 +/- 0.3, and the concentrations of these ligands declined rapidly offshore. The ligand pools in the surface and BBL waters were distinct from each other based on principal components analysis, with variances in the BBL ligand pool explained by sample location, and variance in surface waters explained by water mass. The use of multiple analytical window analysis elucidated several distinct iron-binding ligand pools, each with unique distributions in the central California Current system.

Bundy, RM, Jiang M, Carter M, Barbeau KA.  2016.  Iron-binding ligands in the southern California Current System: Mechanistic studies. Frontiers in Marine Science. 3:27.   10.3389/fmars.2016.00027   Abstract

The distributions of dissolved iron and organic iron-binding ligands were examined in water column profiles and deckboard incubation experiments in the southern California Current System (sCCS) along a transition from coastal to semi-oligotrophic waters. Analysis of the iron-binding ligand pool by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) using multiple analytical windows (MAWs) revealed three classes of iron-binding ligands present throughout the water column (L1-L3), whose distributions closely matched those of dissolved iron and nitrate. Despite significant biogeochemical gradients, ligand profiles were similar between stations, with surface minima in strong ligands (L1 and L2), and relatively constant concentrations of weaker ligands (L3) down to 500 m. A phytoplankton grow-out incubation, initiated from an iron-limited water mass, showed dynamic temporal cycling of iron-binding ligands. A biological iron model was able to capture the patterns of the strong ligands in the grow-out incubation relatively well with only the microbial community as a biological source. An experiment focused on remineralization of particulate organic matter showed production of both strong and weak iron-binding ligands by the heterotrophic community, supporting a mechanism for in-situ production of both strong and weak iron-binding ligands in the subsurface water column. Photochemical experiments showed a variable influence of sunlight on the degradation of natural iron-binding ligands, providing some evidence to explain differences in surface ligand concentrations between stations. Patterns in ligand distributions between profiles and in the incubation experiments were primarily related to macronutrient concentrations, suggesting microbial remineralization processes might dominate on longer time-scales over short-term changes associated with photochemistry or phytoplankton growth.