Retention of phytosiderophores by the soil solid phase - adsorption and desorption

Solid phase speciation controls copper mobilisation from marine sediments by methanobactin

Although marine environments represent huge reservoirs of the potent greenhouse gas methane, they currently contribute little to global net methane emissions. Most of the methane is oxidized by methanotrophs, minimizing escape to the atmosphere. Aerobic methanotrophs oxidize methane mostly via the copper (Cu)-bearing enzyme particulate methane monooxygenase (pMMO). Therefore, aerobic methane oxidation depends on sufficient Cu acquisition by methanotrophs. Because they require both oxygen and methane, aerobic methanotrophs reside at oxic-anoxic interfaces, often close to sulphidic zones where Cu bioavailability can be limited by poorly soluble Cu sulphide mineral phases. Under Cu-limiting conditions, certain aerobic methanotrophs exude Cu-binding ligands termed chalkophores, such as methanobactin (mb) exuded by Methylosinus trichosporium OB3b. Our main objective was to establish whether chalkophores can mobilise Cu from Cu sulphide-bearing marine sediments to enhance Cu bioavailability. Through a series of kinetic batch experiments, we investigated Cu mobilisation by mb from a set of well-characterized sulphidic marine sediments differing in sediment properties, including Cu content and phase distribution. Characterization of solid-phase Cu speciation included X-ray absorption spectroscopy and a targeted sequential extraction. Furthermore, in batch experiments, we investigated to what extent adsorption of metal-free mb and Cu-mb complexes to marine sediments constrains Cu mobilisation. Our results are the first to show that both solid phase Cu speciation and chalkophore adsorption can constrain methanotrophic Cu acquisition from marine sediments. Only for certain sediments did mb addition enhance dissolved Cu concentrations. Cu mobilisation by mb was not correlated to the total Cu content of the sediment, but was controlled by solid-phase Cu speciation. Cu was only mobilised from sediments containing a mono-Cu-sulphide (CuSx) phase. We also show that mb adsorption to sediments limits Cu acquisition by mb to less compact (surface) sediments. Therefore, in sulphidic sediments, mb-mediated Cu acquisition is presumably constrained to surface-sediment interfaces containing mono-Cu-sulphide phases.

Plants, Microorganisms and Their Metabolites in Supporting Asbestos Detoxification-A Biological Perspective in Asbestos Treatment

Many countries banned asbestos due to its toxicity, but considering its colossal use, especially in the 1960s and 1970s, disposing of waste containing asbestos is the current problem. Today, many asbestos disposal technologies are known, but they usually involve colossal investment and operating expenses, and the end- and by-products of these methods negatively impact the environment. This paper identifies a unique modern direction in detoxifying asbestos minerals, which involves using microorganisms and plants and their metabolites. The work comprehensively focuses on the interactions between asbestos and plants, bacteria and fungi, including lichens and, for the first time, yeast. Biological treatment is a prospect for in situ land reclamation and under industrial conditions, which can be a viable alternative to landfilling and an environmentally friendly substitute or supplement to thermal, mechanical, and chemical methods, often characterized by high cost intensity. Plant and microbial metabolism products are part of the green chemistry trend, a central strategic pillar of global industrial and environmental development.

Effect of calcium and trace metals (Cd, Cu, Mn, Ni, Zn) on root iron uptake in relation to chemical properties of the root-excreted ligands

Interferences of major cations (Ca2+, Mg2+) and trace metals (TM, i.e. Cd2+, Cu2+, Mn2+, Ni2+ and Zn2+) in root Fe uptake were evaluated. Root Fe uptake was modelled including the reactions of the root exuded ligand with the soil major and trace cations. Fe uptake was simulated with different ligands representing various affinities for the cations, the latter varying in concentration. The stability constant of Fe complexes (KFeL) does not influence Fe uptake, contrarily to the ligand parameters for Fe-hydroxide dissolution. Fe uptake decreases when KCaL or Ca2+ in solution increases. Presence of TM has nearly no influence on Fe uptake when the TM complexes have low stability constants (KML), as in the case of oxalate and citrate complexes. When ligands have high KML, like EDTA, DFO-B or mugineic acid (MA), TM reduces Fe uptake by 51-55%, and much more in the case of TM contamination. Exudation of Fe ligands with low KML has no negative effect on TM uptake, which can increase if the dissociation rate is high, as for Cu complexes. Ligands with high KML (EDTA, DFO-B, MA) greatly reduce TM uptake, only if their hydrated cations can be absorbed. Calcium does not significantly reduce Fe uptake when Ca-complexes have KCaL < 104. Consequently, ligands like oxalate or MA should be efficient in most soils. TM should perturbate Fe uptake mediated by ligands with high KML such as MA, but not oxalate. Plants exuding phytosiderophores should also absorb TM complexes to avoid micronutrient deficiencies.

Harnessing belowground processes for sustainable intensification of agricultural systems

Increasing food demand coupled with climate change pose a great challenge to agricultural systems. In this review we summarize recent advances in our knowledge of how plants, together with their associated microbiota, shape rhizosphere processes. We address (molecular) mechanisms operating at the plant-microbe-soil interface and aim to link this knowledge with actual and potential avenues for intensifying agricultural systems, while at the same time reducing irrigation water, fertilizer inputs and pesticide use. Combining in-depth knowledge about above and belowground plant traits will not only significantly advance our mechanistic understanding of involved processes but also allow for more informed decisions regarding agricultural practices and plant breeding. Including belowground plant-soil-microbe interactions in our breeding efforts will help to select crops resilient to abiotic and biotic environmental stresses and ultimately enable us to produce sufficient food in a more sustainable agriculture in the upcoming decades.

Soil-pH and cement influence the weathering kinetics of chrysotile asbestos in soils and its hydroxyl radical yield

Chrysotile asbestos is a toxic and carcinogenic mineral that has been used in a variety of industrial and consumer applications. Much of the fiber- and cement-containing asbestos waste has ended up in terrestrial environments. Chrysotile weathering in soils and the potential for natural attenuation have, however, hardly been examined yet. Here we explored how soil properties influence the dissolution rate of chrysotile, the release of the carcinogenic metals chromium and nickel, and the hydroxyl radical (HO^•) generation by chrysotile fibers. Chrysotile dissolution rates in soil suspensions decreased with increasing soil-pH and were lower than reported rates in soil-free systems. Dissolved organic carbon did not markedly accelerate dissolution at circumneutral pH, whereas cement mixed with soil inhibited dissolution because of its alkalinity. The HO^•-yield of incubated fibers in non-amended soils eventually decreased by 60-75%. The decline was fastest in an acidic podzol soil, yet was followed by a small rebound. Cement amendment induced the largest HO^•-yield reduction (∼90%), presumably due to surface coating of the fibers. Overall, this work demonstrates that the potential for natural attenuation of chrysotile asbestos in soils critically depends on soil chemical parameters and the presence of cement in association with the fibers.

Constraints to Synergistic Fe Mobilization from Calcareous Soil by a Phytosiderophore and a Reductant

Synergistic effects between ligand- and reductant-based Fe acquisition strategies can enhance the mobilization of Fe, but also of competing metals from soil. For phytosiderophores, this may alter the time and concentration window of Fe uptake during which plants can benefit from elevated Fe concentrations. We examined how the size of this window is affected by the ligand and reductant concentration and by non-simultaneous addition. To this end, a series of kinetic batch experiments was conducted with a calcareous clay soil to which the phytosiderophore 2′-deoxymugineic acid (DMA) and the reductant ascorbate were added at various concentrations, either simultaneously or with a one- or two-day lag time. Both simultaneous and non-simultaneous addition of the reductant and the phytosiderophore induced synergistic Fe mobilization. Furthermore, initial Fe mobilization rates increased with increasing reductant and phytosiderophore concentrations. However, the duration of the synergistic effect and the window of Fe uptake decreased with increasing reductant concentration due to enhanced competitive mobilization of other metals. Rate laws accurately describing synergistic mobilization of Fe and other metals from soil were parameterized. Synergistic Fe mobilization may be vital for the survival of plants and microorganisms in soils of low Fe availability. However, in order to optimally benefit from these synergistic effects, exudation of ligands and reductants in the rhizosphere need to be carefully matched.

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

BACKGROUND AND AIMS

Graminaceous 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.

METHODS

For 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.

RESULTS

Fe 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.

CONCLUSION

Rhizosphere 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.