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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.

Jiang, MS, Barbeau KA, Selph KE, Measures CI, Buck KN, Azam F, Mitchell BG, Zhou M.  2013.  The role of organic ligands in iron cycling and primary productivity in the Antarctic Peninsula: A modeling study. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:112-133.   10.1016/j.dsr2.2013.01.029   AbstractWebsite

Iron (Fe) is the limiting nutrient for primary productivity in the Southern Ocean, with much of the dissolved iron (dFe) bound to organic ligands or colloids. A Fe model for the Southern Ocean (SOFe) is developed to understand the role of bacteria and organic ligands in controlling Fe cycling and productivity. The model resolves the classical food web and microbial loop, including three types of nutrients (N, Si, Fe) and two types of Fe ligands. Simulations of the zero-dimensional (0-D) model are calibrated with detailed results of shipboard grow-out incubation experiments conducted with Antarctic Peninsula phytoplankton communities during winter 2006 to provide the best estimate of key biological parameters. Then a one-dimensional (1-D) model is developed by coupling the biological model with the Regional Oceanic Modeling System (ROMS) for a site on the Antarctic Peninsula shelf, and the model parameters are further calibrated with data collected from two surveys (summer 2004 and winter 2006) in the area. The results of the numerical simulations agree reasonably well with observations. An analysis of the 1-D model results suggests that bacteria and organic ligands may play an important role in Fe cycling, which can be categorized into a relatively fast mode within the euphotic zone dominated by photo-reactions (summer d Fe residence time about 600 days) and complexation and a slow mode below with most of the dFe biologically complexed (summer dFe residence time > 10 years). The dFe removal from the euphotic zone is dominated by colloidal formation and further aggregations with additional contribution from biological uptake, and an increase of organic ligands would reduce Fe export. The decrease of Fe removal rate over depth is due to the continuous dissolution and remineralization of particulate Fe. A number of sensitivity experiments are carried out for both O-D and 1-D models to understand the importance of photo-reactive processes in primary productivity, bacterial activity, Fe speciation, and dFe residence time within the euphotic zone. The bio-availability of ligand-bound Fe (FeL) is critical to modeled high primary productivity, which is consistent with both shipboard measurements and field observations. In addition, model productivity is sensitive to photoreaction rates if FeL is not directly available for phytoplankton uptake. (C) 2013 Elsevier Ltd. All rights reserved.

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, 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.