Fungal transformations of uranium oxides

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.     

The preparation of tin-molybdenum oxides

The characterisation of ‘tin-molybdenum oxides’ prepared by a variety of methods has been performed by X-ray diffraction. The results show that preparative procedures involving the calcination of precipitates or evaporates containing the two cations at high temperatures are more likely to result in solid solution formation than methods which involve the calcination of slurries or solid state reactions. However, the monophasic tin-molybdenum oxides formed by the moderate temperature calcination of precipitates or evaporates undergo further changes at elevated temperatures which indicate that bulk equilibrium is difficult to achieve in this ternary system.     

Solubility and dissolution of iron oxides

In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (K_sp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. K_sp values are usually extremely low ((Fe³⁺)·(OH)³=10^–37–10^–44). However, for each mineral type, K_sp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in K_sp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily.The concentration of Fe²⁺ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and E_h, the activity of FeII is higher the higher K_sp of the FeIII oxide and thus also varies with the type of Fe oxide present.Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m²g^–1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases.Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe³⁺-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.     

Efficacy of zinc oxides as fertilisers

Background and aims

Human zinc (Zn) deficiency is prevalent in developing countries and Zn biofortification of grains is used to increase the Zn content of food staples. Agronomic interventions to biofortify grain involve fertiliser selection and management. The usefulness of a zinc compound as a fertiliser will depend on its solubility, bioavailability and the effect of its distribution in the soil profile.

Methods

Various sources of Zn oxide and Zn sulfate fertiliser were characterised for nutrient content, morphology, solubility, and fertiliser recovery when applied to the surface, banded near the seed or uniformly mixed.

Results

Compared with Zn sulfates, Zn oxide fertilisers had very low water solubility and slow dissolution rates, because of a higher dissolution pH. This did not translate to a diminished ability to supply Zn to plants when both sources of Zn were mixed through the soil, but there was significantly less fertiliser recovery from Zn oxides than from Zn sulfates when the fertiliser was banded near the seed.

Conclusions

All sources will be equally effective if uniformly mixed through the profile. In no-till systems where fertiliser is banded near seed, Zn sulfate is superior to Zn oxide.     

Review of the genotoxicity of nitrogen oxides

Nitrogen oxides (NO_x) are formed in combustion processes and are major pollutants in urban air. Relatively few studies on the genotoxicity of NO₂ and NO have been performed. These studies indicate that NO₂ is genotoxic in vitro, but the effect of NO seems to be very slight.One in vivo study showed chromosome aberrations and mutations in lung cells after inhalation of NO₂ (and NO), but tests for chromosome aberrations in lymphocytes and spermatocytes or micronuclei in bone marrow were negative after inhalation of NO₂. Based on present studies, there is no clear evidence of a carcinogenic potential of NO₂, although lung adenomas were induced in the susceptible strain A/J mouse.The primary metabolites of NO_x are nitrite and nitrate. Nitrate seems to be devoid of genotoxic properties, but nitrite is genotoxic in vitro, and there are also positive in vivo results. Cancer studies have been mainly negative. However, carcinogenic nitrosamines have been shown to be formed in vivo after inhalation of NO₂.Nitrogen oxides are key components in atomospheric smog formation, which may lead to secondary effects. Strongly mutagenic nitro-PAH compounds are easily formed, and mutagenic reaction products may be formed photochemically from alkenes.     

One step processing of aminofunctionalized gate oxides

A plasma discharge process has been developed that allows the growth of biosensor gate oxides with adapted surface properties for the direct application of biomolecular immobilization cascades. The process involves an accurate selection of processing conditions, mainly, low temperature evaporation of (3-aminopropyl)triethoxysilane (APTS) and dynamic power and flow conditions. Room temperature evaporation of APTS was achieved by designing a vessel with an internal capillary network. The initial high power (100 W) plasma conditions were replaced by milder molecular fragmentation (50 W, 25 W) in a pure Ar discharge. Under these conditions the thin SiO₂ layers presented graded properties with a denser layer at the Si (100) interface and a hybrid organic–inorganic structure at the surface. The chemistry of the films was analysed by Fourier transformed infrared spectroscopy (FTIR) and Rutherford backscattering spectroscopy combined with elastic recoil detection analysis (RBS, ERDA), which confirmed the presence of the SiO₂ and organic phases. Contact angle measurements indicate the higher contribution of the basic polar component to the surface free energy. Furthermore, the higher affinity of the surface towards biomolecular immobilization was confirmed by fluorescence microscopy. Finally, penetration of nitrobenzaldehyde was obtained by application of a molecular permeation method evaluated by UV–vis spectroscopy onto fused silica substrates.     

Heat-induced formation of nitrogen oxides in water

It was found by the fluorimetric method using 2,3-diaminonaphthalene that moderate heating of water (60–80°C, for up to 4 h) leads to the fixation of atmospheric nitrogen with the formation of nitrite. The kinetic parameters of this process were determined. The energy of activation of ${\rm NO}_{2}^{-}$ formation was estimated to be 139 kJ/mol. It was found that the amount of nitrite formed depends on the concentration of dissolved oxygen and nitrogen. It was shown by two independent methods (Griess reagent/VCl₃ and 2,3-diaminonaphthalene/nitrate reductase) that heating of water (80°C, 1 h) results in the formation of nitrate; with the use of the fluorescent probe dihydrorhodamine 123, the generation of nitrogen dioxide (peroxynitrite) was revealed. Nitrite, nitrate, and nitrogen dioxide are formed in water upon heating in approximately equal amounts. A scheme of reactions proceeding with bidistilled water by the action of heat with the formation of nitrogen oxides is proposed.     

The availability of various manganese oxides to plants

Summary A number of oxides of manganese were prepared and their crystalline structures were indentified by X-rays. They were added to four soils on which plants suffered from manganese deficiency. Oxides of the structures manganite, manganous manganite, and pyrolusite cured manganese deficiency of oats and peas. All the successful oxides consisted of very small particles, as shown by electron micrographs.The amount of the oxides dissolved by aqueous quinol during an hour at room temperature was correlated with their ability to supply plants with manganese.     

Potential-pH behavior of plutonium solutions containing hydrous oxides

The oxidation-reduction potential decreases linearly with increasing pH in solutions that contain both tetravalent and hexavalent plutonium hydrous oxides. An equation predicting the potential is compared to an equation describing some potentials found experimentally by Rai. The corresponding slopes and intercepts of the two equations differ by only a few per cent.     

Evaluation of enantioselectivity in lipase-catalyzed acylation of hydroxyalkylphosphine oxides

The lipase-catalyzed optical resolution of 2-, 3-, and 5-hydroxyalkyl phosphorus compounds 1 provided the corresponding optically pure diastereomers in good yields. (S_P, R)- and (R_P, S)-1 were acylated faster than (S_P, S)- and (R_P, R)-1. The stereoselectivity at the phosphorus atom changed with the flexibility of the active sites in the lipases. The stereoselectivity at the phosphorus atom was higher in the reaction of 1a than in the reaction of 1b,c. The reaction rate of ɛ-hydroxyalkylphosphine oxide 1c was faster than that of 1a, although less enantioselectivity was observed at the phosphorus atom.     

Oscillations and gaseous oxides in invertebrate olfaction

Olfactory systems combine an extraordinary molecular sensitivity with robust synaptic plasticity. Central neuronal circuits that perform pattern recognition in olfaction typically discriminate between hundreds of molecular species and form associations between odor onsets and behavioral contingencies that can last a lifetime. Two design features in the olfactory system of the terrestrial mollusk Limax maximus may be common elements of olfactory systems that display the twin features of broad molecular sensitivity and rapid odor learning: spatially coherent oscillations in the second-order circuitry that receives sensory input; and involvement of the interneuronal messengers nitric oxide (NO) and carbon monoxide (CO) in sensory responses and circuit dynamics of the oscillating olfactory network. The principal odor processing center in Limax, the procerebrum (PC) of the cerebral ganglion, contains on the order of 10⁵ local interneurons and receives both direct and processed input from olfactory receptors. Field potential recordings in the PC show an oscillation at approximately 0.7 Hz that is altered by odor input. Optical recordings of voltage changes in local regions of the PC show waves of depolarization that originate at the distal pole and propagate to the base of the PC. Weak odor stimulation transiently switches PC activity from a propagating mode to a spatially uniform mode. The field potential oscillation in the PC lobe depends on intercellular communication via NO, based on opposing effects of reagents that decrease or increase NO levels in the PC. Inhibition of NO synthase slows the field potential oscillation, while application of exogenous NO increases the oscillation frequency. A role for CO in PC dynamics is suggested by experiments in which CO liberation increases the PC oscillation frequency. These design features of the Limax PC lobe odor processing circuitry may relate to synaptic plasticity that subserves both connection of new receptors throughout the life of the slug and its highly developed odor learning ability. © 1996 John Wiley & Sons, Inc.