Publications

2016
Hellmer, Hartmut H., et al.Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade”. Ocean Dynamics 66.11 (2016): , 66, 11, 1379-1413. Web. Publisher's VersionAbstract
In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program ‘Antarctic Research’ (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals in the ocean with special emphasis on the Weddell Sea.
Kim, Ja-Myung, Oliver Baars, and François M. M. Morel. “The effect of acidification on the bioavailability and electrochemical lability of zinc in seawater”. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374.2081 (2016): n. pag. Print.Abstract
A poorly studied but potentially important consequence of the CO2-induced acidification of the surface ocean is a possible change in the bioavailability of trace metals, which play a critical role in the productivity and population dynamics of marine ecosystems. We report laboratory and field experiments designed to compare quantitatively the effects of acidification on the bioavailability of Zn, a metal essential to the growth of phytoplankton and on the extent of its complexation by model and natural ligands. We observed a good correspondence between the effects of pH on the rate of Zn uptake by a model diatom and the chemical lability of Zn measured by anodic stripping voltammetry (ASV). In model laboratory systems, the chemical lability and the bioavailability of Zn could either increase or decrease at low pH depending on the mix of complexing ligands. In a sample of coastal surface water, we observed similar increases in the ASV-labile and bioavailable Zn concentrations upon acidification, a result contrary to previous observations. These results, which can likely be generalized to other bioactive trace metals, mutatis mutandis, demonstrate the intricacy of the effects of ocean acidification on the chemistry and the ecology of surface seawater.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
Baars, Oliver, and David H Perlman. “Small Molecule LC-MS/MS Fragmentation Data Analysis and Application to Siderophore Identification”. Applications from Engineering with MATLAB Concepts. Ed. Jan Valdman. Rijeka, Croatia: InTech, 2016. 191-214. Web. Publisher's Version
2015
Kim, Ja-Myung, Oliver Baars, and François MM Morel. “Bioavailability and Electroreactivity of Zinc Complexed to Strong and Weak Organic Ligands”. Environmental Science & Technology 49 (2015): , 49, 10894-10902. Print.
Mawji, Edward, et al.The GEOTRACES Intermediate Data Product 2014”. Marine Chemistry 177 (2015): , 177, 1-8. Print.Abstract

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes.

Pižeta, I, et al.Interpretation of complexometric titration data: An intercomparison of methods for estimating models of trace metal complexation by natural organic ligands”. Marine Chemistry 173 (2015): , 173, 3-24. Print.Abstract

With the common goal of more accurately and consistently quantifying ambient concentrations of free metal ions and natural organic ligands in aquatic ecosystems, researchers from 15 laboratories that routinely analyze trace metal speciation participated in an intercomparison of statistical methods used to model their most common type of experimental dataset, the complexometric titration. All were asked to apply statistical techniques that they were familiar with to model synthetic titration data that are typical of those obtained by applying state-of-the-art electrochemical methods – anodic stripping voltammetry (ASV) and competitive ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-ACSV) – to the analysis of natural waters. Herein, we compare their estimates for parameters describing the natural ligands, examine the accuracy of inferred ambient free metal ion concentrations ([Mf]), and evaluate the influence of the various methods and assumptions used on these results. The ASV-type titrations were designed to test each participant's ability to correctly describe the natural ligands present in a sample when provided with data free of measurement error, i.e., random noise. For the three virtual samples containing just one natural ligand, all participants were able to correctly identify the number of ligand classes present and accurately estimate their parameters. For the four samples containing two or three ligand classes, a few participants detected too few or too many classes and consequently reported inaccurate ‘measurements’ of ambient [Mf]. Since the problematic results arose from human error rather than any specific method of analyzing the data, we recommend that analysts should make a practice of using one's parameter estimates to generate simulated (back-calculated) titration curves for comparison to the original data. The root–mean–squared relative error between the fitted observations and the simulated curves should be comparable to the expected precision of the analytical method and upon visual inspection the distribution of residuals should not be skewed. Modeling the synthetic, CLE-ACSV-type titration dataset, which comprises 5 titration curves generated at different analytical windows or levels of competing ligand added to the virtual sample, proved to be more challenging due to the random measurement error that was incorporated. Comparison of the submitted results was complicated by the participants' differing interpretations of their task. Most adopted the provided ‘true’ instrumental sensitivity in modeling the CLE-ACSV curves, but several estimated sensitivities using internal calibration, exactly as is required for actual samples. Since most fitted sensitivities were biased low, systematic error in inferred ambient [Mf] and in estimated weak ligand (L2) concentrations resulted. The main distinction between the mathematical approaches taken by participants lies in the functional form of the speciation model equations, with their implicit definition of independent and dependent or manipulated variables. In ‘direct modeling’, the dependent variable is the measured [Mf] (or Ip) and the total metal concentration ([M]T) is considered independent. In other, much more widely used methods of analyzing titration data – classical linearization, best known as van den Berg/Ružić, and isotherm fitting by nonlinear regression, best known as the Langmuir or Gerringa methods – [Mf] is defined as independent and the dependent variable calculated from both [M]T and [Mf]. Close inspection of the biases and variability in the estimates of ligand parameters and in predictions of ambient [Mf] revealed that the best results were obtained by the direct approach. Linear regression of transformed data yielded the largest bias and greatest variability, while non-linear isotherm fitting generated results with mean bias comparable to direct modeling, but also with greater variability. Participants that performed a unified analysis of ACSV titration curves at multiple detection windows for a sample improved their results regardless of the basic mathematical approach taken. Overall, the three most accurate sets of results were obtained using direct modeling of the unified multiwindow dataset, while the single most accurate set of results also included simultaneous calibration. We therefore recommend that where sample volume and time permit, titration experiments for all natural water samples be designed to include two or more detection windows, especially for coastal and estuarine waters. It is vital that more practical experimental designs for multi-window titrations be developed. Finally, while all mathematical approaches proved to be adequate for some datasets, matrix-based equilibrium models proved to be most naturally suited for the most challenging cases encountered in this work, i.e., experiments where the added ligand in ACSV became titrated. The ProMCC program (Omanović et al., this issue) as well as the Excel Add-in based KINETEQL Multiwindow Solver spreadsheet (Hudson, 2014) have this capability and have been made available for public use as a result of this intercomparison exercise.

Baars, O, et al.The Siderophore Metabolome of Azotobacter vinelandii”. Appl Environ Microbiol 82 (2015): , 82, 27-39. Print.Abstract

In this study, we performed a detailed characterization of the siderophore metabolome, or "chelome," of the agriculturally important and widely studied model organism Azotobacter vinelandii. Using a new high-resolution liquid chromatography-mass spectrometry (LC-MS) approach, we found over 35 metal-binding secondary metabolites, indicative of a vast chelome in A. vinelandii. These include vibrioferrin, a siderophore previously observed only in marine bacteria. Quantitative analyses of siderophore production during diazotrophic growth with different sources and availabilities of Fe showed that, under all tested conditions, vibrioferrin was present at the highest concentration of all siderophores and suggested new roles for vibrioferrin in the soil environment. Bioinformatic searches confirmed the capacity for vibrioferrin production in Azotobacter spp. and other bacteria spanning multiple phyla, habitats, and lifestyles. Moreover, our studies revealed a large number of previously unreported derivatives of all known A. vinelandii siderophores and rationalized their origins based on genomic analyses, with implications for siderophore diversity and evolution. Together, these insights provide clues as to why A. vinelandii harbors multiple siderophore biosynthesis gene clusters. Coupled with the growing evidence for alternative functions of siderophores, the vast chelome in A. vinelandii may be explained by multiple, disparate evolutionary pressures that act on siderophore production.

2014
Baars, Oliver, François MM Morel, and David H Perlman. “ChelomEx: Isotope-Assisted Discovery of Metal Chelates in Complex Media Using High-Resolution LC-MS”. Analytical Chemistry 86 (2014): , 86, 11298-11305. Web. Publisher's VersionAbstract
Chelating agents can control the speciation and reactivity of trace metals in biological, environmental, and laboratory-derived media. A large number of trace metals (including Fe, Cu, Zn, Hg, and others) show characteristic isotopic fingerprints that can be exploited for the discovery of known and unknown organic metal complexes and related chelating ligands in very complex sample matrices using high-resolution liquid chromatography mass spectrometry (LC-MS). However, there is currently no free open-source software available for this purpose. We present a novel software tool, ChelomEx, which identifies isotope pattern-matched chromatographic features associated with metal complexes along with free ligands and other related adducts in high-resolution LC-MS data. High sensitivity and exclusion of false positives are achieved by evaluation of the chromatographic coherence of the isotope pattern within chromatographic features, which we demonstrate through the analysis of bacterial culture media. A built-in graphical user interface and compound library aid in identification and efficient evaluation of results. ChelomEx is implemented in MatLab. The source code, binaries for MS Windows and MAC OS X as well as test LC-MS data are available for download at SourceForge (http://sourceforge.net/projects/chelomex).
Baars, Oliver, and Peter L Croot. “Dissolved cobalt speciation and reactivity in the eastern tropical North Atlantic”. Marine Chemistry (2014): , -. Web. Publisher's VersionAbstract
Abstract Recent studies highlight the role of cobalt (Co) as an important micro-nutrient with a complex scavenged type oceanic distribution. To better understand the biogeochemical cycle of Co we investigate the distribution, speciation and reactivity of dissolved Co in the eastern tropical North Atlantic in the upper 800 m of the water column. For this purpose, we complement classical Co ligand titrations that require a thermodynamic equilibrium with evaluations of ligand-exchange kinetics and reducibility of potential Co(III) species. The experiments include additions of the artificial Co binding ligands dimethylglyoxime or Nioxime and detection by cathodic stripping voltammetry. We find two pools of Co compounds: a labile fraction that exchanges Co within minutes and a strong/inert fraction that does not react within a 24-h period. No intermediate, slowly exchanging fraction is observed. Detection window experiments to determine complex stability constants show that the labile Co fraction is weak and likely consists of Co(II) complexes with no detectable free Co(II) ligands. The fraction of inert Co is always highest at the depth of the chlorophyll-a maximum. Addition of the reductant ascorbate increases the fraction of Co with rapid ligand-exchange kinetics and indicates the presence of dissolved reducible Co(III). The apparent Co(III) reducibility is highest at the chlorophyll-a maximum and decreases in deeper waters. Our results are in agreement with phytoplankton and associated bacteria being a source of Co(III) species, such as vitamin B12. The presented results have important implications for our understanding of the biological availability and the marine cycle of Co.
Baars, Oliver, et al.Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate”. Limnology and Oceanography 59 (2014): , 59, 385–399. Web. Publisher's VersionAbstract
We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is > 100 times higher in near-surface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep-water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end-member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace-nutrient—limited Southern Ocean.
2011
Baars, Oliver, and Peter L Croot. “Comparison of Alternate Reactants for pM Level Cobalt Analysis in Seawater by the Use of Catalytic Voltammetry”. Electroanalysis 23 (2011): , 23, 1663–1670. Web. Publisher's VersionAbstract

The analysis of Cobalt (Co) at low pM concentrations in seawater with Adsorptive Cathodic Stripping Voltammetry involves high concentrations of sodium nitrite (NaNO2) to enhance the signal in an electrocatalytic reaction. In this study we found three substitutes for NaNO2 that critically affected the sensitivity. Optimisation of a method with potassium bromate (KBrO3) resulted in an excellent detection limit (0.9 pM) after a 90 s adsorption period. Reactant concentration and consumption were 10× reduced compared to protocols with NaNO2 and reagent blanks were lower. Accuracy and precision were verified with SAFe intercalibration standards and the method was applied using open ocean seawater samples. The reaction mechanism is discussed and differences to NaNO2 are shown.

Croot, Peter L, Oliver Baars, and Peter Streu. “The distribution of dissolved zinc in the Atlantic sector of the Southern Ocean”. Deep-Sea Research Part II 58.25–26 (2011): , 58, 25–26, 2707-2719. Web. Publisher's VersionAbstract

The distribution of dissolved zinc (Zn) was investigated in the Atlantic sector of the Southern Ocean in the austral autumn of 2008 as part of the IPY GEOTRACES expedition ZERO & DRAKE. Research focused on transects across the major frontal systems along the Zero Meridian and across the Drake Passage. There was a strong gradient in surface zinc concentrations observed across the Antarctic Polar Front along both transects and high zinc levels were found in surface waters throughout the Southern Ocean. Vertical profiles for dissolved Zinc showed the presence of local minima and maxima in the upper 200 m consistent with significant uptake by phytoplankton and release by zooplankton grazing, respectively. Highest deep water zinc concentrations were found in the centre of the Weddell Gyre associated with Central Intermediate Water (CIW), a water mass which is depleted in O2, elevated in CO2 and is regionally a CFC minimum. Our data suggests that the remineralization of sinking particles is a key control on the distribution of Zn in the Southern Ocean. Disappearance ratios of zinc to phosphate (Zn:P) in the upper water column increased southwards along both transects and based on laboratory studies they suggest slower growth rates of phytoplankton due to iron or light limitation. Zinc and silicate were strongly correlated throughout the study region but the disappearance ratio (Zn:Si) was relatively uniform overall except for the region close to the ice edge on the Zero Meridian.

Baars, Oliver, and Peter L Croot. “The speciation of dissolved zinc in the Atlantic sector of the Southern Ocean”. Deep-Sea Research Part II 58 (2011): , 58, 2720-2732. Web.Abstract

The speciation of dissolved zinc (Zn) was investigated by voltammetry in the Atlantic sector of the Southern Ocean along two transects across the major frontal systems: along the Zero Meridian and across the Drake Passage. In the Southern Ocean south of the APF we found detectable labile inorganic Zn throughout the surface waters in contrast to studies from lower latitudes. Using a combination of ASV titrations and pseudopolarography revealed the presence of significant concentration of electrochemically inert Zn ligands throughout the Southern Ocean. These ligands however were nearly always saturated due to the presence of excess concentrations of dissolved Zn that were associated with the high nutrient waters south of the Antarctic Polar Front (APF). Only in surface waters did the concentration of Zn complexing ligands exceed the dissolved Zn concentrations suggesting a biological source for these ligands. Our findings have clear implications for the biogeochemical cycling of Zn and for the interpretation of paleo records utilizing Zn in opal as a tracer of Zn speciation in the water column. (C) 2011 Elsevier Ltd. All rights reserved.

2007
Klingele, Julia, et al.Tetranuclear Nickel(II) Complexes Composed of Pairs of Dinuclear LNi2 Fragments Linked by Acetylenedicarboxylate, Terephthalate, and Isophthalate Dianions: Synthesis, Structures and Magnetic Properties”. European Journal of Inorganic Chemistry 2007 (2007): , 2007, 5277–5285. Web. Publisher's VersionAbstract
The tetranuclear nickel(II) complexes [(LNiII2)2(acetylenedicarboxylate)][BPh4]2 (2[BPh4]2), [(LNiII2)2(terephthalate)][BPh4]2 (3[BPh4]2), and [(LNiII2)2(isophthalate)][BPh4]2(4[BPh4]2), where L2– represents a macrocyclic hexaaza-dithiophenolate ligand, have been synthesized and characterised by UV/Vis spectroscopy, IR spectroscopy, X-ray crystallography, and magnetic susceptibility measurements. Each dicarboxylate dianion acts as a quadridentate bridging ligand linking two bioctahedral LNi2 units via μ1,3-bridging carboxylate functions to generate discrete [(LNiII2)2(dicarboxylate)]2+ dications with a central LNi2(O2C–R–CO2)Ni2L core. The structures differ mainly in the distance between the center of the Ni···Ni axes of the isostructural LNi2 units (8.841(1) Å in 2[BPh4]2, 10.712(1) in 3[BPh4]2, and 9.561(1) in 4[BPh4]2) and the tilting angle between the two Ni2O2 planes (86.3° in 2[BPh4]2, 58.2° in 3[BPh4]2, 20.9° in 4[BPh4]2). Magnetic susceptibility measurements on the complexes over the range 2.0–295 K reveal the presence of weak ferromagnetic exchange interactions between the NiII ions within the dinuclear subunits with values for the magnetic exchange constant J1 of 23.1(5), 18.1(5), and 14.2(5) cm–1 for 2[BPh4]2, 3[BPh4]2, and 4[BPh4]2, respectively (H = –2JS1S2). The magnitude of the exchange interaction J2 across the dicarboxylate bridges is in all cases less than 0.1 cm–1, suggesting that no significant interdimer exchange coupling occurs in 2[BPh4]2–4[BPh4]2. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)