Spatially resolved quantification of metal ion concentration in a biofilm-mediated ion exchanger

Quantitative monitoring of paramagnetic contrast agents and their allocation in plant tissues via DCE-MRI

BACKGROUND

Studying dynamic processes in living organisms with MRI is one of the most promising research areas. The use of paramagnetic compounds as contrast agents (CA), has proven key to such studies, but so far, the lack of appropriate techniques limits the application of CA-technologies in experimental plant biology. The presented proof-of-principle aims to support method and knowledge transfer from medical research to plant science.

RESULTS

In this study, we designed and tested a new approach for plant Dynamic Contrast Enhanced Magnetic Resonance Imaging (pDCE-MRI). The new approach has been applied in situ to a cereal crop (Hordeum vulgare). The pDCE-MRI allows non-invasive investigation of CA allocation within plant tissues. In our experiments, gadolinium-DTPA, the most commonly used contrast agent in medical MRI, was employed. By acquiring dynamic T₁-maps, a new approach visualizes an alteration of a tissue-specific MRI parameter T₁ (longitudinal relaxation time) in response to the CA. Both, the measurement of local CA concentration and the monitoring of translocation in low velocity ranges (cm/h) was possible using this CA-enhanced method.

CONCLUSIONS

A novel pDCE-MRI method is presented for non-invasive investigation of paramagnetic CA allocation in living plants. The temporal resolution of the T₁-mapping has been significantly improved to enable the dynamic in vivo analysis of transport processes at low-velocity ranges, which are common in plants. The newly developed procedure allows to identify vascular regions and to estimate their involvement in CA allocation. Therefore, the presented technique opens a perspective for further development of CA-aided MRI experiments in plant biology.

Solute transport within grape berries inferred from the paramagnetic properties of manganese

Tracer compounds used for studying solute transport should ideally have identical functions and transport properties to the molecules they are designed to mimic. Unfortunately, the xylem-mobile tracer compounds currently used to infer solute transport mechanisms in botanical specimens such as the fruit of the grapevine, Vitis vinifera L., are typically xenobiotic and have difficulty exiting the xylem during berry ripening. Here it is demonstrated that the transport of paramagnetic Mn ions can be indirectly observed within the grape berry, using relaxation magnetic resonance imaging (MRI). Mn ions from a 10mM Mn chloride solution were taken up into the grape berry via the pedicel and moved through the peripheral vasculature before exiting into surrounding pericarp tissue. Mn did not exit evenly across the berry, implying that the berry xylem influences which sites Mn exits the vasculature 'downstream' of the berry pedicel. It was also found that when the cellular membranes of pericarp tissues were disrupted, the distribution of Mn through the pericarp tissue became noticeably more homogenous. This indicates that the cellular membranes of extra-vascular cells affect the spatial distribution of Mn across the berry extra-vascular pericarp tissue upon exiting the grape berry vasculature.

MRI of chemical reactions and processes

As magnetic resonance imaging (MRI) can spatially resolve a wealth of molecular information available from nuclear magnetic resonance (NMR), it is able to non-invasively visualise the composition, properties and reactions of a broad range of spatially-heterogeneous molecular systems. Hence, MRI is increasingly finding applications in the study of chemical reactions and processes in a diverse range of environments and technologies. This article will explain the basic principles of MRI and how it can be used to visualise chemical composition and molecular properties, providing an overview of the variety of information available. Examples are drawn from the disciplines of chemistry, chemical engineering, environmental science, physics, electrochemistry and materials science. The review introduces a range of techniques used to produce image contrast, along with the chemical and molecular insight accessible through them. Methods for mapping the distribution of chemical species, using chemical shift imaging or spatially-resolved spectroscopy, are reviewed, as well as methods for visualising physical state, temperature, current density, flow velocities and molecular diffusion. Strategies for imaging materials with low signal intensity, such as those containing gases or low sensitivity nuclei, using compressed sensing, para-hydrogen or polarisation transfer, are discussed. Systems are presented which encapsulate the diversity of chemical and physical parameters observable by MRI, including one- and two-phase flow in porous media, chemical pattern formation, phase transformations and hydrodynamic (fingering) instabilities. Lastly, the emerging area of electrochemical MRI is discussed, with studies presented on the visualisation of electrochemical deposition and dissolution processes during corrosion and the operation of batteries, supercapacitors and fuel cells.

Magnetic resonance imaging of chemistry

Magnetic resonance imaging (MRI) has long been recognized as one of the most important tools in medical diagnosis and research. However, MRI is also well placed to image chemical reactions and processes, determine the concentration of chemical species, and look at how chemistry couples with environmental factors, such as flow and heterogeneous media. This tutorial review will explain how magnetic resonance imaging works, reviewing its application in chemistry and its ability to directly visualise chemical processes. It will give information on what resolution and contrast are possible, and what chemical and physical parameters can be measured. It will provide examples of the use of MRI to study chemical systems, its application in chemical engineering and the identification of contrast agents for non-clinical applications. A number of studies are presented including investigation of chemical conversion and selectivity in fixed-bed reactors, temperature probes for catalyst pellets, ion mobility during tablet dissolution, solvent dynamics and ion transport in Nafion polymers and the formation of chemical waves and patterns.

Application of paramagnetically tagged molecules for magnetic resonance imaging of biofilm mass transport processes

Molecules become readily visible by magnetic resonance imaging (MRI) when labeled with a paramagnetic tag. Consequently, MRI can be used to image their transport through porous media. In this study, we demonstrated that this method could be applied to image mass transport processes in biofilms. The transport of a complex of gadolinium and diethylenetriamine pentaacetic acid (Gd-DTPA), a commercially available paramagnetic molecule, was imaged both in agar (as a homogeneous test system) and in a phototrophic biofilm. The images collected were T(1) weighted, where T(1) is an MRI property of the biofilm and is dependent on Gd-DTPA concentration. A calibration protocol was applied to convert T(1) parameter maps into concentration maps, thus revealing the spatially resolved concentrations of this tracer at different time intervals. Comparing the data obtained from the agar experiment with data from a one-dimensional diffusion model revealed that transport of Gd-DTPA in agar was purely via diffusion, with a diffusion coefficient of 7.2 x 10(-10) m(2) s(-1). In contrast, comparison of data from the phototrophic biofilm experiment with data from a two-dimensional diffusion model revealed that transport of Gd-DTPA inside the biofilm was by both diffusion and advection, equivalent to a diffusion coefficient of 1.04 x 10(-9) m(2) s(-1). This technology can be used to further explore mass transport processes in biofilms, either by using the wide range of commercially available paramagnetically tagged molecules and nanoparticles or by using bespoke tagged molecules.

Advanced imaging techniques for assessment of structure, composition and function in biofilm systems

Scientific imaging represents an important and accepted research tool for the analysis and understanding of complex natural systems. Apart from traditional microscopic techniques such as light and electron microscopy, new advanced techniques have been established including laser scanning microscopy (LSM), magnetic resonance imaging (MRI) and scanning transmission X-ray microscopy (STXM). These new techniques allow in situ analysis of the structure, composition, processes and dynamics of microbial communities. The three techniques open up quantitative analytical imaging possibilities that were, until a few years ago, impossible. The microscopic techniques represent powerful tools for examination of mixed environmental microbial communities usually encountered in the form of aggregates and films. As a consequence, LSM, MRI and STXM are being used in order to study complex microbial biofilm systems. This mini review provides a short outline of the more recent applications with the intention to stimulate new research and imaging approaches in microbiology.

Magnetic resonance microscopy of iron transport in methanogenic granules

Interactions between anaerobic biofilms and heavy metals such as iron, cobalt or nickel are largely unknown. Magnetic resonance imaging (MRI) is a non-invasive method that allows in situ studies of metal transport within biofilm matrixes. The present study investigates quantitatively the penetration of iron (1.7 5mM) bound to ethylenediaminetetraacetate (EDTA) into the methanogenic granules (spherical biofilm). A spatial resolution of 109x109x218 microm(3) and a temporal resolution of 11 min are achieved with 3D Turbo Spin Echo (TSE) measurements. The longitudinal relaxivity, i.e. the slope the dependence of the relaxation rate (1/T(1)) on the concentration of paramagnetic metal ions, was used to measure temporal changes in iron concentration in the methanogenic granules. It took up to 300 min for the iron-EDTA complex ([FeEDTA](2-)) to penetrate into the methanogenic granules (3-4mm in diameter). The diffusion was equally fast in all directions with irregularities such as diffusion-facilitating channels and diffusion-resistant zones. Despite these irregularities, the overall process could be modeled using Fick's equations for diffusion in a sphere, because immobilization of [FeEDTA](2-) in the granular matrix (or the presence of a reactive barrier) was not observed. The effective diffusion coefficient (D(ejf)) of [FeEDTA](2-) was found to be 2.8x10(-11)m(2)s(-1), i.e. approximately 4% of D(ejf) of [FeEDTA](2-) in water. The Fickian model did not correspond to the processes taking place in the core of the granule (3-5% of the total volume of the granule), where up to 25% over-saturation by iron (compare to the concentration in the bulk solution) occurred.

Biofouling of spiral-wound nanofiltration and reverse osmosis membranes: a feed spacer problem

Biofouling was studied in full-scale and pilot-scale installations, test-rigs and membrane fouling monitors by conventional methods as well as Magnetic Resonance Imaging (MRI). Independent of permeate production, the feed spacer channel pressure drop and biomass concentration increased similarly in a nanofiltration pilot installation. In the presence of a feed spacer the absolute feed channel pressure drop increase caused by biomass accumulation was much higher than when a feed spacer was absent: in both spiral-wound nanofiltration and reverse osmosis systems biofouling is dominantly a feed spacer problem. This conclusion is based on (i) in-situ visual observations of the fouling accumulation, (ii) in-situ non-destructive observations of the fouling accumulation and velocity distribution profiles using MRI, and (iii) differences in pressure drop and biomass development in monitors with and without feed spacer. MRI studies showed that even a restricted biofilm accumulation on the feed channel spacer influenced the velocity distribution profile strongly. Biofouling control should be focused on the development of low fouling feed spacers and hydrodynamic conditions to restrict the impact of biomass accumulation on the feed channel pressure drop increase.

Comparison of uranium(VI) removal by Shewanella oneidensis MR-1 in flow and batch reactors

To better understand the interactions among metal contaminants, nutrients, and microorganisms in subsurface fracture-flow systems, biofilms of pure culture of Shewanella oneidensis MR-1 were grown in six fracture-flow reactors (FFRs) of different geometries. The spatial and temporal distribution of uranium and bacteria were examined using a tracer dye (brilliant blue FCF) and microscopy. The results showed that plugging by bacterial cells was dependent on the geometry of the reactor and that biofilms grown in FFRs had a limited U(VI)-reduction capacity. To quantify the U(VI)-reduction capacity of biofilms, batch experiments for U(VI) reduction were performed with repetitive U(VI) additions. U(VI)-reduction rates of stationary phase cultures decreased after each U(VI) addition. After the fourth U(VI) addition, stationary phase cultures treated with U(VI) with and without spent medium yielded gray and black precipitates, respectively. These gray and black U precipitates were analyzed using high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Data for randomly selected areas of black precipitates showed that reduced U particles (3-6 nm) were crystalline, whereas gray precipitates were a mixture of crystalline and amorphous solids. Results obtained in this study, including a dramatic limitation of S. oneidensis MR-1 and its biofilms to reduce U(VI) and plugging of FFRs, suggest that alternative organisms should be targeted for stimulation for metal immobilization in subsurface fracture-flow systems.

Biomineralization: linking the fossil record to the production of high value functional materials

The microbial cell offers a highly efficient template for the formation of nanoparticles with interesting properties including high catalytic, magnetic and light-emitting activities. Thus biomineralization products are not only important in global biogeochemical cycles, but they also have considerable commercial potential, offering new methods for material synthesis that eliminate toxic organic solvents and minimize expensive high-temperature and pressure processing steps. In this review we describe a range of bacterial processes that can be harnessed to make precious metal catalysts from waste streams, ferrite spinels for biomedicine and catalysis, metal phosphates for environmental remediation and biomedical applications, and biogenic selenides for a range of optical devices. Recent molecular-scale studies have shown that the structure and properties of bionanominerals can be fine-tuned by subtle manipulations to the starting materials and to the genetic makeup of the cell. This review is dedicated to the late Terry Beveridge who contributed much to the field of biomineralization, and provided early models to rationalize the mechanisms of biomineral synthesis, including those of geological and commercial potential.