Kevin J. Wilkinson

CHARACTERISTIC FEATURES OF THE MAJOR COMPONENTS OF FRESHWATER COLLOIDAL ORGANIC MATTER REVEALED BY TRANSMISSION ELECTRON AND ATOMIC FORCE MICROSCOPY

Organic biopolymers such as humic substances and polysaccharides account for the majority of freshwater NOM. Their role in natural systems is largely dependent upon their supramolecular microscopic structure which cannot be determined by bulk chemical measurements alone. Microscopic techniques were developed so as to permit the systematic observation of several colloidal-sized organic macromolecules with variable structures. This paper describes the characteristic structures of some reference compounds representative of the major organic components of natural waters. Polyacrylic acid, alginic acid and schizophyllan in addition to humic substances and polysaccharides isolated from natural freshwaters were examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The techniques were optimized for the observation of the aquatic biopolymers. Colloidal organic matter isolated from natural freshwaters was also observed by TEM and AFM and compared with the standard images of colloidal organic matter.

Some fundamental (and often overlooked) considerations underlying the free ion activity and biotic ligand models

Trace metal bioavailability is often evaluated on the basis of steady-state models such as the free ion activity model (FIAM) and the biotic ligand model (BLM). Some of the assumptions underlying these models were verified by examining Pb and Zn uptake by the green microalga Chlorella kesslerii. Transporter bound metal ([M-Rcell]) and free ion concentrations ([M(Z+)]) were related to experimentally determined uptake fluxes (Jint). Although the BLM and FIAM correctly predicted Pb uptake in the absence of competing ions, they failed to predict competitive interactions with Ca2+, likely because of modifications of the algal surface charge and the active nature of Ca2+ transport. Zinc transport is also active; in this case, both the internalization rate constant (kint) and the equilibrium constant for the binding of Zn to the transport sites (K(Zn-Rcell)) varied as a function of [Zn2+] in the bulk solution. For this reason, Zn uptake could not be modeled by the steady-state models either in the presence or absence of competitors (Cd and Ca). Furthermore, the role of Cu on Pb and Zn adsorption and uptake could not be predicted by either model because of secondary effects on the algal physiology and membrane permeability. Finally, a 17 degree C reduction in temperature resulted in a two- to fivefold decrease in membrane permeability of the metals, an observation that also is unaccounted for by either the FIAM or BLM. This paper emphasizes the limitations of the models in well-controlled laboratory systems with the goal of extrapolating the results to complex environmental systems.

Sample preparation techniques for the observation of environmental biopolymers by atomic force microscopy

Although the structural characterization of environmental biopolymers is of prime importance in order to understand their roles in the natural environment, only limited analytical techniques are available that allow the determination of their morphologies and size distributions, due mainly to the complexity, heterogeneity and polydispersity of these organic colloids. Atomic force microscopy (AFM) has become an important technique for direct imaging of biomolecules with a sub nanometer resolution. This paper describes a systematic optimization of the AFM sample preparation technique prior to direct observation under ambient conditions. Three preparation methods (drop deposition, adsorption, ultracentrifugation) were tested on two reference compounds: a humic substance and a polysaccharide. The methods were evaluated using, where possible, quantitative analysis of the AFM images. The results indicated that each of the commonly used preparation techniques had potential artefacts, but when used in concert, structural parameters corresponding to those measured in solution could be determined. AFM height measurements performed under conditions of variable humidity demonstrated no observable structural effects on the biopolymers.

Characterization of Norwegian natural organic matter: Size, diffusion coefficients, and electrophoretic mobilities

Eighteen Norwegian NOM samples were analyzed by AFM, TEM, FCS, and CE. The TEM and AFM gave complementary, but not identical, information regarding the conformation of the NOM. A large majority of the material in all samples appeared as points with a size of approximately 2 nm or less. FCS gave values for diffusion coefficients which were in the range 2.1–3.0 × 10−10 m2 s−1. This corresponds to molecular diameters of between 1.6 and 2.0 nm, in reasonable agreement with both TEM and AFM. Electropherograms, using absorbance at 200 nm, were all dissimilar except for the Gjerstad and Hellerudmyra sample pairs, for which no major differences were observed. CE was also carried out using both fluorescence (excitation at 325, 457, and 488 nm) and UV-absorbance (200, 210, 254, and 288 nm) detection on three of the NOM samples. The different modes of detection and wavelengths gave qualitatively similar electropherograms. Calculated EPMs of the major sample components were in the range -2.0 to -5.0 10−8 m2 s−1 V−1.

Cadmium Adsorption by Chlamydomonas reinhardtii and its Interaction with the Cell Wall Proteins

Environmental Context. In natural waters, trace metals levels are largely controlled by microbiology; organisms take up, metabolize, store, and detoxify the metals. However, aquatic organisms may regulate their own uptake via dynamic processes that result in a system that is far from equilibrium. By examining the model title alga with a battery of techniques, a more realistic assessment of metal uptake and metal regulatory processes could be gained. Abstract. Cadmium adsorption by a wild type strain of Chlamydomonas reinhardtii and a cell wall-less mutant was quantified as a function of Cd speciation in a well-defined aqueous medium. For both strains, Cd adsorption to the cell surface was not predicted by a single-site (Langmuirian) model. Indeed, no saturation of the cell wall was observed, even for Cd concentrations in excess of 5 × 10−3 M. A continual production of Cd binding sites appeared to be responsible for the observed increase of Cd adsorption with time. SDS-page separations and measurements of the protein content of algal supernatants demonstrated that organic matter was released by the algae, both in the presence and absence of Cd. Both the nature (e.g. polysaccharides, proteins) and the quantity of exudate production was influenced by the physicochemistry of the external medium. Measurements using the permeation liquid membrane (PLM) and anodic stripping voltammetry (ASV) demonstrated that dissolved cadmium was rapidly complexed by the organic exudates produced by the algae.

β‐Fibrillogenesis from Rigid‐Rod β‐Barrels: Hierarchical Preorganization Beyond Microns

Ordnung und Asthetik kommen in neuartigen supramolekularen Architekturen zum Ausdruck, die aus praorganisierten starren stabformigen Molekulen aufgebaut sind. Nach Selbstorganisation zu einer Fass-artigen Tertiarstruktur (links) bildet sich eine Quartarstruktur hoherer Ordnung (rechts), die aus starren stabformigen s-Fibrillen besteht.

Particles in Natural Surface Waters: Chemical Composition and Size Distribution

Abstract The size distributions and chemical composition of particles in natural surface waters were studied using filtration and laser light scattering. An on-line tangential sequential-stage filtration system was tested using compounds of known particle sizes. With this filtration system six fractions of different particle sizes were obtained which could then be further analysed, e.g. by ICP-AES, resulting in the chemical composition of each particle size fraction. This technique was then applied to natural surface waters. Data were compared with parallel filtration results using three filters. In addition, the particle size distribution of several European river waters was measured by laser light scattering.

Optimization of the Hydrolysis of Freshwater Polysaccharides

Abstract Three techniques of acid hydrolysis are verified for their ability to efficiently hydrolyse freshwater polysaccharides. Polysaccharides isolated from two eutrophic lakes were hydrolysed rapidly as compared to model or marine polysaccharides. No single technique is 100% satisfactory for all polysaccharides (i.e. complete hydrolysis with no degradation). Hydrolysis by 0.1 M HCl appeared to give the best results while acceptable results were also obtained for 1.2 M H2SO4. The results suggest that it is prudent to hydrolyse freshwater polysaccharides under the least drastic conditions and for as short of a time as possible in order to minimize decomposition of the sample, (esp. uronic acids). Interferences due to humic acid; did not appear to increase as a function of hydrolysis time.