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Chappell, PD, Armbrust EV, Barbeau KA, Bundy RM, Moffett JW, Vedamati J, Jenkins BD.  2019.  Patterns of diatom diversity correlate with dissolved trace metal concentrations and longitudinal position in the northeast Pacific coastal-offshore transition zone. Marine Ecology Progress Series. 609:69-86.   10.3354/meps12810   AbstractWebsite

Diatoms are important primary producers in the northeast Pacific Ocean, with their productivity closely linked to pulses of trace elements in the western high nitrate, low chlorophyll (HNLC) region of the oceanographic time series transect 'Line P.' Recently, the coastal-HNLC transition zone of the Line P transect was identified as a hotspot of phytoplankton productivity, potentially controlled by a combination of trace element and macronutrient concentrations. Here we describe diatom community composition in the eastern Line P transect, including the coastal- HNLC transition zone, with a method using high-throughput sequencing of diatom 18S gene amplicons. We identified significant correlations between shifting diatom community composition and longitude combined with concentrations of dissolved copper and 2 other dissolved trace metals (dissolved Fe [dFe] and/or dissolved zinc) and/or a physical factor (salinity or density). None of these variables on its own was significantly correlated with shifts in community composition, and 3 of the factors (dFe, salinity, and density) correlated with one another. Longitude could incorporate multiple factors that may influence diatom communities, including distance from shore, proximity of sampling stations, and an integration of previous pulses of macro- and micro-nutrients. We also evaluated in situ Fe limitation of the diatom Thalassiosira oceanica using a quantitative reverse-transcription polymerase chain reaction method, and found biological evidence of Fe stress in samples from the coastal-HNLC transition zone. Combined, our results support a prior hypothesis that dissolved trace metals as well as longitudinal distance may be important to diatom diversity in the coastal-HNLC transition zone of the Line P transect.

Landry, MR, Ohman MD, Goericke R, Stukel MR, Barbeau KA, Bundy R, Kahru M.  2012.  Pelagic community responses to a deep-water front in the California Current Ecosystem: overview of the A-Front Study. Journal of Plankton Research. 34:739-748.   10.1093/plankt/fbs025   AbstractWebsite

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front.

Kwasnik, M, Fuhrer K, Gonin M, Barbeau K, Fernandez FM.  2007.  Performance, resolving power, and radial ion distributions of a prototype nanoelectrospray ionization resistive glass atmospheric pressure ion mobility spectrometer. Analytical Chemistry. 79:7782-7791.   10.1021/ac071226o   AbstractWebsite

In this article, we describe and characterize a novel ion mobility spectrometer constructed with monolithic resistive glass desolvation and drift regions. This instrument is equipped with switchable corona discharge and nanoelectrospray ionization sources and a Faraday plate detector. Following description of the instrument, pulsing electronics, and data acquisition system, we examine the effects of drift gas flow rate and temperature, and of the aperture grid to anode distance on the observed resolving power and sensitivity. Once optimum experimental parameters are identified, different ion gate pulse lengths, and their effect on the temporal spread of the ion packet were investigated. Resolving power ranged from an average value of 50 ms/ms for a 400-mu s ion gate pulse, up to an average value of 68 ms/ms for a 100-mu s ion gate pulse, and a 26-cm drift tube operated at 383 V cm(-1). Following these experiments, the radial distribution of ions in the drift region of the spectrometer was studied by using anodes of varying sizes, showing that the highest ionic density was located at the center of the drift tube. Finally, we demonstrate the applicability of this instrument to the study of small molecules of environmental relevance by analyzing a commercially available siderophore, deferoxamine mesylate, in both the free ligand and Fe-bound forms. Ion mobility experiments showed a dramatic shift to shorter drift times caused by conformational changes upon metal binding, in agreement with previous reversed-phase liquid chromatography observations.

Hogle, SL, Dupont CL, Hopkinson BM, King AL, Buck KN, Roe KL, Stuart RK, Allen AE, Mann EL, Johnson ZI, Barbeau KA.  2018.  Pervasive iron limitation at subsurface chlorophyll maxima of the California Current. Proceedings of the National Academy of Sciences of the United States of America. 115:13300-13305.   10.1073/pnas.1813192115   AbstractWebsite

Subsurface chlorophyll maximum layers (SCMLs) are nearly ubiquitous in stratified water columns and exist at horizontal scales ranging from the submesoscale to the extent of oligotrophic gyres. These layers of heightened chlorophyll and/or phytoplankton concentrations are generally thought to be a consequence of a balance between light energy from above and a limiting nutrient flux from below, typically nitrate (NO3). Here we present multiple lines of evidence demonstrating that iron (Fe) limits or with light colimits phytoplankton communities in SCMLs along a primary productivity gradient from coastal to oligotrophic offshore waters in the southern California Current ecosystem. SCML phytoplankton responded markedly to added Fe or Fe/light in experimental incubations and transcripts of diatom and picoeukaryote Fe stress genes were strikingly abundant in SCML metatranscriptomes. Using a biogeochemical proxy with data from a 40-y time series, we find that diatoms growing in California Current SCMLs are persistently Fe deficient during the spring and summer growing season. We also find that the spatial extent of Fe deficiency within California Current SCMLs has significantly increased over the last 25 y in line with a regional climate index. Finally, we show that diatom Fe deficiency may be common in the subsurface of major upwelling zones worldwide. Our results have important implications for our understanding of the biogeochemical consequences of marine SCML formation and maintenance.

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, 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, 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.  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.

Hopkinson, BM, Seegers B, Hatta M, Measures CI, Mitchell BG, Barbeau KA.  2013.  Planktonic C:Fe ratios and carrying capacity in the southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:102-111.   10.1016/j.dsr2.2012.09.001   AbstractWebsite

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July-August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4 x 10(5) (mol:mol) for the radiotracer method and 1.7 x 10(5) for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. (c) 2012 Elsevier Ltd. All rights reserved.