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279 results on '"Repeta, Daniel J."'

51. Ultrasonic Nebulization for the Elemental Analysis of Microgram-Level Samples with Offline Aerosol Mass Spectrometry

Author

O'Brien, Rachel E., primary, Ridley, Kelsey J., additional, Canagaratna, Manjula R., additional, Jayne, John T., additional, Croteau, Philip L., additional, Worsnop, Douglas R., additional, Budisulistiorini, Sri Hapsari, additional, Surratt, Jason D., additional, Follett, Christopher L., additional, Repeta, Daniel J., additional, and Kroll, Jesse H., additional

Published
2018
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52. Supplementary material to "Ultrasonic Nebulization for the Elemental Analysis of Microgram-Level Samples with Offline Aerosol Mass Spectrometry"

Author

O'Brien, Rachel E., primary, Ridley, Kelsey J., additional, Canagaratna, Manjula R., additional, Jayne, John T., additional, Croteau, Philip L., additional, Worsnop, Douglas R., additional, Budisulistiorini, Sri Hapsari, additional, Surratt, Jason D., additional, Follett, Christopher L., additional, Repeta, Daniel J., additional, and Kroll, Jesse H., additional

Published
2018
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63. Phosphorus dynamics in biogeochemically distinct regions of the southeast subtropical Pacific Ocean

Author

Duhamel, Solange, Björkman, Karin M., Repeta, Daniel J., and Karl, David M.

Subjects
Ecology, Microorganisms, Phosphorus, Marine ecology
Abstract

The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of Chile and Easter Island. This remote area of the world’s ocean presents strong gradients in physical (e.g., temperature, density and light), chemical (e.g., salinity and nutrient concentrations) and microbiological (e.g., cell abundances, biomass and specific growth rates) properties. The goal of this study was to describe the phosphorus (P) dynamics in three main ecosystems along this transect: the upwelling regime off the northern Chilean coast, the oligotrophic area associated with the southeast subtropical Pacific gyre and the transitional area in between these two biomes. We found that inorganic phosphate (Pi) concentrations were high and turnover times were long (>210 nmol l−1 and >31 d, respectively) in the upper water column, along the entire transect. Pi uptake rates in the gyre were low (euphotic layer integrated rates were 0.26 mmol m−2 d−1 in the gyre and 1.28 mmol m−2 d−1 in the upwelling region), yet not only driven by decreases in particle mass or cell abundance (particulate P- and cell- normalized Pi uptake rates in the euphotic layer were ∼1–4 times and ∼3–15 times lower in the gyre than in the upwelling, respectively). However these Pi uptake rates were at or near the maximum Pi uptake velocity (i.e., uptake rates in Pi amended samples were not significantly different from those at ambient concentration: 1.5 and 23.7 nmol l−1 d−1 at 50% PAR in the gyre and upwelling, respectively). Despite the apparent Pi replete conditions, selected dissolved organic P (DOP) compounds were readily hydrolyzed. Nucleotides were the most bioavailable of the DOP substrates tested. Microbes actively assimilated adenosine-5′-triphosphate (ATP) leading to Pi and adenosine incorporation as well as Pi release to the environment. The southeast subtropical Pacific Ocean is a Pi-sufficient environment, yet DOP hydrolytic processes are maintained and contribute to P-cycling across the wide range of environmental conditions present in this ecosystem.

Published
2016
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68. Ultrasonic Nebulization for the Elemental Analysis of Microgram-Level Samples with Offline Aerosol Mass Spectrometry.

Author

O'Brien, Rachel E., Ridley, Kelsey J., Canagaratna, Manjula R., Jayne, John T., Croteau, Philip L., Worsnop, Douglas R., Budisulistiorini, Sri Hapsari, Surratt, Jason D., Follett, Christopher L., Repeta, Daniel J., and Kroll, Jesse H.

Subjects
ATMOSPHERIC aerosols, DISSOLVED organic matter, MASS spectrometry
Abstract

The elemental composition of organic material in environmental samples - including atmospheric organic aerosol, dissolved organic matter, and other complex mixtures - provides insights into their sources and environmental processing. However, standard analytical techniques for measuring elemental ratios typically require large sample sizes (milligrams of material or more). Here we characterize a method for measuring elemental ratios in environmental samples, requiring only micrograms of material, using a Small Volume Nebulizer (SVN). The technique uses ultrasonic nebulization of samples to generate aerosol particles (100-300 nm diameter), which are then analyzed using an aerosol mass spectrometer (AMS). We demonstrate that the technique generates aerosol from complex organic mixtures with minimal changes to the elemental composition of the organic and that quantification is possible using internal standards (e.g., NH415NO3). Sample volumes of 2-4 uL with total solution concentrations of at least 0.2-g/L form sufficient particle mass for elemental ratio measurement by the AMS, despite only a small fraction (~ 0.1 %) of the sample forming fine particles while the remainder end up as larger droplets. The method was applied to aerosol filter extracts from the field and laboratory, as well as to dissolved organic matter (DOM) from the North Pacific Ocean. In the case of aerosol particles, the mass spectra and elemental ratios from the SVN-AMS agree with those from online AMS sampling; similarly, for DOM, the elemental ratios determined from the SVN-AMS agree with those determined using combustion analysis. The SVN-AMS provides a platform for the rapid quantitative analysis of the elemental composition of complex organic mixtures and non-refractory inorganic salts from microgram samples with applications that include analysis of aerosol extracts, and terrestrial and atmospheric dissolved organic matter. [ABSTRACT FROM AUTHOR]

Published
2018
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73. Siderophore-based microbial adaptations to iron scarcity across the eastern Pacific Ocean.

Author

McIlvin, Matthew R., Saito, Mak A., Repeta, Daniel J., Boiteau, Rene M., Hawco, Nicholas J., Fitzsimmons, Jessica N., Mende, Daniel R., DeLong, Edward F., and Sedwick, Peter N.

Subjects
SIDEROPHORES, IRON, CHLOROPHYLL, SEAWATER, BIOAVAILABILITY
Abstract

Nearly all iron dissolved in the ocean is complexed by strong organic ligands of unknown composition. The effect of ligand composition on microbial iron acquisition is poorly understood, but amendment experiments using model ligands show they can facilitate or impede iron uptake depending on their identity. Here we show that siderophores, organic compounds synthesized by microbes to facilitate iron uptake, are a dynamic component of the marine ligand pool in the eastern tropical Pacific Ocean. Siderophore concentrations in iron-deficient waters averaged 9 pM, up to fivefold higher than in iron-rich coastal and nutrient-depleted oligotrophic waters, and were dominated by amphibactins, amphiphilic siderophores with cell membrane affinity. Phylogenetic analysis of amphibactin biosynthetic genes suggests that the ability to produce amphibactins has transferred horizontally across multiple Gammaproteobacteria, potentially driven by pressures to compete for iron. In coastal and oligotrophic regions of the eastern Pacific Ocean, amphibactins were replaced with lower concentrations (1–2 pM) of hydrophilic ferrioxamine siderophores. Our results suggest that organic ligand composition changes across the surface ocean in response to environmental pressures. Hydrophilic siderophores are predominantly found across regions of the ocean where iron is not expected to be the limiting nutrient for the microbial community at large. However, in regions with intense competition for iron, some microbes optimize iron acquisition by producing siderophores that minimize diffusive losses to the environment. These siderophores affect iron bioavailability and thus may be an important component of the marine iron cycle. [ABSTRACT FROM AUTHOR]

Published
2016
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79. Seasonal and Annual Fluxes of Nutrients and Organic Matter from Large Rivers to the Arctic Ocean and Surrounding Seas

Author

Holmes, Robert Max, primary, McClelland, James W., additional, Peterson, Bruce J., additional, Tank, Suzanne E., additional, Bulygina, Ekaterina, additional, Eglinton, Timothy I., additional, Gordeev, Viacheslav V., additional, Gurtovaya, Tatiana Y., additional, Raymond, Peter A., additional, Repeta, Daniel J., additional, Staples, Robin, additional, Striegl, Robert G., additional, Zhulidov, Alexander V., additional, and Zimov, Sergey A., additional

Published
2011
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80. Biogeochemical relationships between ultrafiltered dissolved organic matter and picoplankton activity in the Eastern Mediterranean Sea

Author

Meador, Travis B., primary, Gogou, Alexandra, additional, Spyres, Georgina, additional, Herndl, Gerhard J., additional, Krasakopoulou, Evangelia, additional, Psarra, Stella, additional, Yokokawa, Taichi, additional, De Corte, Daniele, additional, Zervakis, Vassilis, additional, and Repeta, Daniel J., additional

Published
2010
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