W. D. C. Schenkeveld

Root exudation of phytosiderophores from soil‐grown wheat

For the first time, phytosiderophore (PS) release of wheat (Triticum aestivum cv Tamaro) grown on a calcareous soil was repeatedly and nondestructively sampled using rhizoboxes combined with a recently developed root exudate collecting tool. As in nutrient solution culture, we observed a distinct diurnal release rhythm; however, the measured PS efflux was c. 50 times lower than PS exudation from the same cultivar grown in zero iron (Fe)-hydroponic culture. Phytosiderophore rhizosphere soil solution concentrations and PS release of the Tamaro cultivar were soil-dependent, suggesting complex interactions of soil characteristics (salinity, trace metal availability) and the physiological status of the plant and the related regulation (amount and timing) of PS release. Our results demonstrate that carbon and energy investment into Fe acquisition under natural growth conditions is significantly smaller than previously derived from zero Fe-hydroponic studies. Based on experimental data, we calculated that during the investigated period (21–47 d after germination), PS release initially exceeded Fe plant uptake 10-fold, but significantly declined after c. 5 wk after germination. Phytosiderophore exudation observed under natural growth conditions is a prerequisite for a more accurate and realistic assessment of Fe mobilization processes in the rhizosphere using both experimental and modeling approaches.

The influence of pH on iron speciation in podzol extracts: Iron complexes with natural organic matter, and iron mineral nanoparticles

The quantities of natural organic matter (NOM) and associated iron (Fe) in soil extracts are known to increase with increasing extractant pH. However, it was unclear how the extraction pH affects Fe speciation for particles below 30 nm. We used flow field-flow fractionation (FlowFFF) and transmission electron microscopy (TEM) to investigate the association of Fe and trace elements with NOM and nanoparticulate iron (oxy)hydroxides in podzol extracts. For extracts prepared at the native soil pH (~4), and within a 1-30 nm size range, Fe was associated with NOM. In extracts with a pH≥7 from the E and B soil horizons, Fe was associated with NOM as well as with iron (oxy)hydroxide nanoparticles with a size of approximately 10 nm. The iron (oxy)hydroxide nanoparticles may have either formed within the soil extracts in response to the increase in pH, or they were mobilized from the soil. Additionally, pH shift experiments showed that iron (oxy)hydroxides formed when the native soil pH (~4) was increased to 9 following the extraction. The iron (oxy)hydroxide nanoparticles aggregated if the pH was decreased from 9 to 4. The speciation of Fe also influenced trace element speciation: lead was partly associated with the iron (oxy)hydroxides (when present), while copper binding to NOM remained unaffected by the presence of iron (oxy)hydroxide nanoparticles. The results of this study are important for interpreting the representativeness of soil extracts prepared at a pH other than the native soil pH, and for understanding the changes in Fe speciation that occur along a pH gradient.

Metallophores and Trace Metal Biogeochemistry

Trace metal limitation not only affects the biological function of organisms, but also the health of ecosystems and the global cycling of elements. The enzymatic machinery of microbes helps to drive critical biogeochemical cycles at the macroscale, and in many cases, the function of metalloenzyme-mediated processes may be limited by the scarcity of essential trace metals. In response to these nutrient limitations, some organisms employ a strategy of exuding metallophores, biogenic ligands that facilitate the uptake of metal ions. For example, bacterial, fungal, and graminaceous plant species are known to use Fe(III)-binding siderophores for nutrient acquisition, providing the best known and most thoroughly studied example of metallophores. However, recent breakthroughs have suggested or established the role of metallophores in the uptake of several other metallic nutrients. Furthermore, these metallophores may influence environmental trace metal fate and transport beyond nutrient acquisition. These discoveries have resulted in a deeper understanding of trace metal geochemistry and its relationship to the cycling of carbon and nitrogen in natural systems. In this review, we provide an overview of the current state of knowledge on the biogeochemistry of metallophores in trace metal acquisition, and explore established and potential metallophore systems.

Metal mobilization from soils by phytosiderophores – experiment and equilibrium modeling

AimsTo test if multi–surface models can provide a soil-specific prediction of metal mobilization by phytosiderophores (PS) based on the characteristics of individual soils.MethodsMechanistic multi-surface chemical equilibrium modeling was applied for obtaining soil-specific predictions of metal and PS speciation upon interaction of the PS 2’-deoxymugineic acid (DMA) with 6 soils differing in availability of Fe and other metals. Results from multi-surface modeling were compared with empirical data from soil interaction experiments.ResultsFor soils in which equilibrium was reached during the interaction experiment, multi-surface models could well predict PS equilibrium speciation. However, in uncontaminated calcareous soils, equilibrium was not reached within a week, and experimental and modeled DMA speciation differed considerably. In soils with circum-neutral pH, on which Fe deficiency is likely to occur, no substantial Fe mobilization by DMA was predicted. However, in all but the contaminated soils, Fe mobilization by DMA was observed experimentally. Cu and Ni were the quantitatively most important metals competing with Fe for complexation and mobilization by DMA.ConclusionThermodynamics are unable to explain the role of PS as Fe carrier in calcareous soils, and the kinetic aspects of metal mobilization by PS need to be closer examined in order to understand the mechanisms underlying strategy II Fe acquisition.

Analysis of iron-phytosiderophore complexes in soil related samples: LC-ESI-MS/MS versus CE-MS.

Phytosiderophores (PS) form stable complexes with various transition metals. These ligands are exuded by the roots of graminacous plants as a mechanism for mobilizing and acquiring soil iron. To investigate iron mobilization and transport, a novel LC method in combination with ESI‐MS/MS for the determination of three Fe(III)‐complexes with mugineic acid (MA), 2′‐epi‐MA and 2′‐deoxymugineic acid (DMA) has been developed. Liquid chromatographic separation was realized using a silica‐based mixed‐mode reversed phase/weak‐anion exchange type stationary phase and a 50 mM ammonium acetate buffer, pH 6.5. Baseline separation of the two complex diastereomers Fe(III)‐MA and Fe(III)‐epi‐MA could be achieved. ESI‐MS/MS detection allowed for simultaneous quantification of the complexes and the free ligands. Limits of detection were determined to be 0.001 and 0.05 μM for DMA and Fe(III)‐DMA, respectively. The analytical figures of merit of the novel method were evaluated and compared with a CE‐ESI‐MS method that we had published earlier. The LC‐ESI‐MS/MS method has been successfully applied to real samples derived from preliminary extraction experiments.

Geochemical processes constraining iron uptake in Strategy II Fe acquisition.

Phytosiderophores (PS) are natural chelating agents, exuded by graminaceous plants (grasses) for the purpose of Fe acquisition (Strategy II). They can form soluble Fe complexes with soil-Fe that can be readily taken up. PS are exuded in a diurnal pulse release, and with the start of PS release a “window of iron uptake” opens. In the present study we examined how this window is constrained in time and concentration by biogeochemical processes. For this purpose, a series of interaction experiments was done with a calcareous clay soil and the phytosiderophore 2′-deoxymugineic acid (DMA), in which metal and DMA speciation were examined as a function of time and DMA concentration. Various kinetically and thermodynamically controlled processes affected the size of the window of Fe uptake. Adsorption lowered, but did not prevent Fe mobilization by DMA. Microbial activity depleted DMA from solution, but not on time scales jeopardizing Strategy II Fe acquisition. Complexation of competing metals played an important role in constraining the window of Fe uptake, particularly at environmentally relevant PS concentrations. Our study provides a conceptual model that takes into account the chemical kinetics involved with PS-mediated Fe acquisition. The model can help to explain how success or failure of PS-mediated Fe acquisition depends on environmental conditions.

Accurate LC‐ESI‐MS/MS quantification of 2′‐deoxymugineic acid in soil and root related samples employing porous graphitic carbon as stationary phase and a 13C4‐labeled internal standard

For the first time the phytosiderophore 2′‐deoxymugineic acid (DMA) could be accurately quantified by LC‐MS/MS in plant and soil related samples. For this purpose a novel chromatographic method employing porous graphitic carbon as stationary phase combined with ESI‐MS/MS detection in selected reaction monitoring was developed. Isotope dilution was implemented by using in‐house synthesized DMA as external calibrant and 13C4‐labeled DMA as internal standard (concentration levels of standards 0.1–80 μM, determination coefficient of linear regression R2 > 0.9995). Sample preparation involved acidification of the samples in order to obtain complete dissociation of metal‐DMA complexes. Excellent matrix related LOD and LOQ depending on different experimental setups were obtained in the range of 3–34 nM and 11–113 nM, respectively. Standard addition experiments and the implementation of the internal 13C4‐DMA standard proved the accuracy of the quantification strategy even in complex matrices such as soil solution. The repeatability of the method, including sample preparation, expressed as short‐ and long term precision was below 4 and 5% RSD, respectively. Finally, application in the context of plant and soil research to samples from rhizosphere sampling via micro suction cups, from soil solutions and soil adsorption/extraction studies revealed a DMA concentration range from 0.1 to 235 μM.

Equilibrium and kinetic modelling of the dynamic rhizosphere

In a recent article, published in issue 383 of Plant and Soil, Schenkeveld et al. (2014) report that the mobilization of metals from uncontaminated calcareous soils by phytosiderophores (PS) could not be predicted by multisurface equilibrium modeling, because in batch experiments chemical equilibrium was not reached within a relevant time-frame. From this they concluded that follow-up studies should focus on kinetic aspects of metal mobilization by PS. In a commentary related to the aforementioned article, also published in this issue, Terzano and co-workers request an appreciation for the complex and dynamic nature of rhizosphere processes (Terzano et al. 2014). In doing so, they support the conclusion by Schenkeveld et al. (2014) regarding a kinetic approach and put it in the context of exudates in the rhizosphere. Furthermore they question the validity of a thermodynamic approach to describe the chemistry in a complex and dynamic environment like the rhizosphere. It is concluded that, because of the dynamics in supply and depletion of reactants, and the large number of parameters affecting even single reactions, equilibrium is never reached. Hence, according to Terzano et al., thermodynamics cannot provide a realistic description of the chemistry in the rhizosphere system. In this short response to their commentary, we question if the complexity and dynamics of the rhizosphere system render a thermodynamic modelling approach unapt per se, or if equilibrium modeling can in fact help to better understand rhizosphere processes. As Terzano et al. aptly illustrate, the rhizosphere is a complex and dynamic environment, and reaction networks are affected by environmental conditions and availability of reactive compounds. We will first address thermodynamic modeling of complex, closed systems and later address the use of suchmodels in the context of dynamic rhizosphere systems. Complexity is something thermodynamic models can in principal manage well. They can describe simultaneous equilibria for various types of reactions (e.g. complexation, precipitation and adsorption) and for a large number of compounds. Parameterization of complex models may be tedious and labor-intensive, but it is feasible. Once parameterized, thermodynamic models are very suitable for conducting sensitivity analyses on the parameters included, e.g. to examine how sensitive Plant Soil (2015) 386:395–397 DOI 10.1007/s11104-014-2318-z

The effect of pH, electrolytes and temperature on the rhizosphere geochemistry of phytosiderophores

Background and aimsGraminaceous plants are grown worldwide as staple crops under a variety of climatic and soil conditions. They release phytosiderophores for Fe acquisition (Strategy II). Aim of the present study was to uncover how the rhizosphere pH, background electrolyte and temperature affect the mobilization of Fe and other metals from soil by phytosiderophores.MethodsFor this purpose a series of kinetic batch interaction experiments with the phytosiderophore 2′-deoxymugineic acid (DMA), a calcareous clay soil and a mildly acidic sandy soil were performed. The temperature, electrolyte concentration and applied electrolyte cation were varied. The effect of pH was examined by applying two levels of lime and Cu to the acidic soil.ResultsFe mobilization by DMA increased by lime application, and was negatively affected by Cu amendment. Mobilization of Fe and other metals decreased with increasing ionic strength, and was lower for divalent than for monovalent electrolyte cations at equal ionic strength, due to higher adsorption of metal-DMA complexes to the soil. Metal mobilization rates increased with increasing temperature leading to a faster onset of competition; Fe was mobilized faster, but also became depleted faster at higher temperature. Temperature also affected biodegradation rates of metal-DMA complexes.ConclusionRhizosphere pH, electrolyte type and concentration and temperature can have a pronounced effect on Strategy II Fe acquisition by affecting the time and concentration ‘window of Fe uptake’ in which plants can benefit from phytosiderophore-mediated Fe uptake.