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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.

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, 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, 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.

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.

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Fitzsimmons, JN, Bundy RM, Al-Subiai SN, Barbeau KA, Boyle EA.  2015.  The composition of dissolved iron in the dusty surface ocean: An exploration using size-fractionated iron-binding ligands. Marine Chemistry. 173:125-135.   10.1016/j.marchem.2014.09.002   AbstractWebsite

The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1-0.65 nM dissolved iron (<0.2 mu m) was partitioned into the colloidal iron (cFe) size fraction (10 kDa < cFe <0.2 gm), as expected for areas of the ocean underlying a dust plume. The 1.3-2.0 nM strong organic iron-binding ligands, however, overwhelmingly (75-77%) fell into the soluble size fraction (<10 kDa). As a result, modeling the dissolved iron size fractionation at equilibrium using the observed ligand partitioning did not accurately predict the iron partitioning into colloidal and soluble pools. This suggests that either a portion of colloidal ligands is missed by current electrochemical methods because they react with iron more slowly than the equilibration time of our CLE-ACSV method, or part of the observed colloidal iron is actually inorganic in composition and thus cannot be predicted by our model of unbound iron-binding ligands. This potentially contradicts the prevailing view that greater than >99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton. (C) 2014 Elsevier B.V. All rights reserved.

H
Hopkinson, BM, Barbeau KA.  2007.  Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Marine Chemistry. 106:2-17.   10.1016/j.marchem.2006.02.008   AbstractWebsite

The organic and redox speciation of iron was examined in the strongly layered upper water column of the eastern tropical North Pacific, including oxic and sub oxic waters, in a region 100- 1300 km offshore. Suboxic conditions ([O-2] < 5 mu M) were found to affect the organic speciation of iron, and reduced dissolved iron, Fc(II), was present in the suboxic zone, but conditions were not sufficiently reducing to convert all iron to Fe(II). Dissolved iron concentrations in the suboxic zone were similar to concentrations found in oxic regions. Using a competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) method, natural ligands were found to have distinct characteristics in the oxic and suboxic waters with stronger ligands found in the suboxic zone. It is unusual to find stronger ligands below the euphotic zone, but their strength, logK(Fe'L) = 12.1-12.8, is within the range determined for surface ligands in other regions. These strong ligands may be the result of the unique chemistry of the suboxic zone stabilizing reduced or labile compounds, or they may be actively produced by microbes to enhance iron uptake. No onshore-offshore trends in ligand strength or concentration were detected suggesting the ligands may result from the inherent chemistry of the suboxic zone or production from denitrifiers, rather than the resident suboxic zone population of Prochlorococcus which were more abundant nearshore. A luminol-chemiluminescence based flow injection analysis (FIA) technique capable of detecting picomolar concentrations of Fe(II) was used to assess the redox state of iron in the suboxic zone and overlying oxic waters at a station 1300 km offshore. An elevated signal equivalent to 0.12-0.15 nM Fe(II), 21-24% of dissolved iron, was found only in the suboxic waters. Oxidation kinetics suggest that this Fe(II) is most likely produced by an in-situ process, as opposed to being transported from shelf sediment. The luminol-chemiluminescence Fe(II) method was systematically tested for inferences from reduced species potentially present in the suboxic zone to validate our Fe(II) results. Several species, V(IV) and V(111), produced significant signals, but considerations of the reducing state of the suboxic zone make it unlikely that reduced V is present. With additional information on the identity of the suboxic zone species provided by analysis of signal decay rate, it was determined that Fe(II) was the most reasonable source of the signal, and at minimum the chemiluminescence data allows us to set limits on the Fe(II) concentration in the offshore suboxic water column. (C) 2006 Elsevier B.V. All rights reserved.

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King, AL, Barbeau K.  2007.  Evidence for phytoplankton iron limitation in the southern California Current System. Marine Ecology-Progress Series. 342:91-103.   10.3354/meps342091   AbstractWebsite

Observations of phytoplankton iron limitation in the world's oceans have primarily been confined to high-nutrient, low-chlorophyll (HNLC) regimes, found in the western equatorial and subarctic Pacific, Southern Ocean, and coastal upwelling zones off California and Peru. We investigated the potential for phytoplankton iron limitation in coastal transition zones (50 to 200 km offshore) of the southern California Current System, a weak upwelling regime that is relatively low in nutrients (< 4 mu mol nitrate 1(-1)) and low in chlorophyll (< 1 mu g chl a 1(-1)). In grow-out incubation experiments conducted during summer, July 2003 and 2004, phytoplankton responded to nanomolar iron additions, despite the non-HNLC initial conditions, Observed changes in phytoplankton and nutrient parameters upon iron addition were significant, although markedly lower in amplitude relative to typical grow-out experiments in HNLC regimes. While we cannot disprove alternate explanations for the observed limitation of phytoplankton growth, such as a proximate grazing control, our results indicate that phytoplankton growth in the southern California Current System is, at times, limited by the supply of iron. Based on our findings and the results of previous studies in this region, we suggest that phytoplankton biomass is generally limited by the supply of nitrate, while iron, directly or indirectly, influences macronutrient utilization, community species composition, and phytoplankton spatial and temporal distribution.