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Summary: The cyanobacterium Raphidiopsis raciborskii is of environmental and social concern in view of its toxicity, bloom‐forming characteristics and increasingly widespread occurrence. However, while availability of macronutrients and micronutrients such as N and Fe are critically important for the growth and metabolism of this organism, the physiological response of toxic and non‐toxic strains of R. raciborskii to varying Fe and N availabilities remains unclear. By determining physiological parameters as a function of Fe and N availability, we demonstrate that R. raciborskii growth and N2‐fixing activity are facilitated at higher Fe availability under N2‐limited conditions with faster growth of the CS‐506 (cylindrospermopsin‐producing) strain compared with that of CS‐509 (the non‐toxic) strain. Radiolabelled Fe uptake assays indicated that R. raciborskii acclimated under Fe‐limited conditions acquires Fe at significantly higher rates than under Fe replete conditions, principally via unchelated Fe(II) generated as a result of photoreduction of complexed Fe(III). While N2‐fixation of both strains occurred during both day and night, the CS‐506 strain overall exhibited higher N2‐fixing and Fe uptake rates than the CS‐509 strain under N‐deficient and Fe‐limited conditions. The findings of this study highlight that Fe availability is of significance for the ecological advantage of CS‐506 over CS‐509 in N‐deficient freshwaters. [ABSTRACT FROM AUTHOR]

The non-enzymatically catalyzed oxidation of dopamine ( DA) and the resultant formation of powerful oxidants such as the hydroxyl radical (• OH) through 'Fenton chemistry' in the presence of iron within dopaminergic neurons are thought to contribute to the damage of cells or even lead to neuronal degenerative diseases such as Parkinson's disease. An understanding of DA oxidation as well as the transformation of the intermediates that are formed in the presence of iron under physiological conditions is critical to understanding the mechanism of DA and iron induced oxidative stress. In this study, the generation of H2O2 through the autoxidation and iron-catalyzed oxidation of DA, the formation of the dominant complex via the direct reaction with Fe( II) and Fe( III) in both oxygen saturated and deoxygenated conditions and the oxidation of Fe( II) in the presence of DA at physiological pH 7.4 were investigated. The oxidation of DA resulted in the generation of significant amounts of H2O2 with this process accelerated significantly in the presence of Fe( II) and Fe( III). At high DA:Fe( II) ratios, the results from this study suggest that DA plays a protective role by complexing Fe( II) and preventing it from reacting with the generated H2O2. However, the accumulation of H2O2 may result in cellular damage as high intracellular H2O2 concentrations will result in the oxidation of remaining Fe( II) mainly through the peroxidation pathway. At low DA:Fe( II) ratios however, it is likely that DA will act as a pro-oxidant by generating H2O2 which, in the presence of Fe( II), will result in the production of strongly oxidizing • OH radicals. [ABSTRACT FROM AUTHOR]

The bloom-forming cyanobacteria species M icrocystis aeruginosa includes toxic and non-toxic (microcystin-producing) strains. Certain stress conditions stimulate synthesis of microcystin ( MCYST) and enhance the binding of the MCYST molecule to proteins. In this quantitative proteomic study, we compared the response of a wild-type toxic strain PCC 7806, an mcyH− knockout non-toxic strain, and a naturally occurring non-toxic strain, PCC 7005, after 8 days in low iron ( Fe) and nitrogen ( N) starvation in order to assess the benefit of MCYST synthesis in non-optimal conditions. Fe limitation increased MCYST synthesis and caused an accumulation of phycobilisome proteins and the ferric iron transporter FutA only in the toxic PCC 7806 but not the non-toxic strains. In N starvation, photosynthetic, C and N metabolism proteins were more abundant in the non-toxic strains, as were chaperones and proteases. Significant interaction between nutrient availability and toxicity existed for thioredoxin peroxidase and several thioredoxin-regulated proteins. We propose a competition of MCYST for binding sites in thioredoxin-regulated proteins during oxidative stress (low Fe) but not in growth-limiting conditions (low N). This then leads to differences in the regulation of C: N metabolism in toxic and non-toxic M . aeruginosa in nutrient-replete and nutrient-limited conditions. [ABSTRACT FROM AUTHOR]

The article presents a study on the applications of time-resolved laser fluorescence spectroscopy (TRLFS) to the environmental biogeochemistry of actinides in the U.S. It cites the advantages of TRLFS over other advanced spectroscopic techniques including its ability to determine in situ metal speciation at environmentally relevant micromolar to picomolar concentrations. It describes the principles of TRLFS and the literature reporting the application of the methodology to the speciation of actinides in systems of biogeochemical interest. It highlights the significant developments in TRLFS methodology and advanced data analysis and outlines how these developments have the potential to further one's understanding of actinide biogeochemistry.

Toxin production during cyanobacterial blooms poses a significant public health threat in water bodies globally and requires the development of effective bloom management strategies. Previously, synthesis of the hepatotoxin microcystin has been proposed to be regulated by iron availability, but the contribution of the toxin to the adaptation of cyanobacteria to environmental stresses, such as changing light intensity and nutrient limitation, remains unclear. The aim of this study was to compare the iron stress response in toxic and non-toxic strains of Microcystis aeruginosa subjected to moderate and severe iron limitation. The transcription of a number of genes involved in iron uptake, oxidative stress response, toxin synthesis and transcriptional control of these processes was accessed by quantitative real-time PCR (qRT-PCR). The process of adaptation of M. aeruginosa to iron stress was found to be highly dynamic and strain-specific. Toxin production in PCC 7806 increased in an iron-dependent manner and appeared to be regulated by FurA. The inability to produce microcystin, either due to natural mutations in the mcy gene cluster or due to insertional inactivation of mcyH, affected the remodelling of the photosynthetic machinery in iron-stressed cells, the transport of Fe(II) and transcription of the Fur family of transcriptional regulators. The presence of the toxin appears to give an advantage to microcystin-producing cyanobacteria in the early stages of exposure to severe iron stress and may protect the cell from reactive oxygen species-induced damage. [ABSTRACT FROM AUTHOR]

Significant production of superoxide, a known reductant of both inorganic and organically complexed iron(III), occurs in natural systems by both biotic and abiotic pathways. We have investigated the generation of superoxide by Chattonella marina (Subrahman.) Y. Hara et Chihara, a phytoplankton taxon known to produce high levels of this reactive oxygen species, and examined the role of superoxide in the acquisition of iron by this organism. Additionally, a generalized model for iron acquisition by C. marina has been developed, which includes three pathways of iron acquisition from organically complexed iron(III): nondissociative reductive uptake, dissociative reductive uptake, and nonreductive dissociative uptake. The model is shown to be particularly useful in ascertaining the relative importance of these various iron-uptake pathways as a function of solution parameters including concentration and iron-binding strength of the organic ligand and superoxide concentration. Our results suggest that superoxide can participate in the C. marina iron-uptake process when iron is complexed to weak ligands, such as citrate, but plays only a minor role when iron is bound to a strong ligand. It thus appears that facilitation of iron acquisition is not the sole purpose of superoxide production by these organisms. [ABSTRACT FROM AUTHOR]

We have examined the generation of the reactive oxygen species (ROS) superoxide and hydrogen peroxide (H2O2) by irradiation of dilute aqueous solutions of disodium anthraquinone-2-6-disulfonate (AQDS) with simulated sunlight. Irradiating a solution of AQDS in 2 m M NaHCO3 and 0.01 M NaCl produced superoxide and H2O2 at nanomolar concentrations. Experiments in which initial concentrations of dioxygen, H2O2, the superoxide radical trap nitroblue tetrazolium and the electron donor dimethyl sulfoxide were varied suggested that the interaction of solvent water with photo-excited quinone moieties produces dioxygen-reducing radicals, and that these are the primary source of ROS in the system. A kinetic model for ROS production is proposed based on our experimental data. [ABSTRACT FROM AUTHOR]

A numerical technique for quantifying the key material properties that describe how a flocculated suspension behaves under constant pressure filtration is presented with unrestricted compressive yield stress and permeability models used to describe these material properties. Using an iterative procedure, the optimal parameters for these models are calculated as are pressure and solid fraction distribution profiles. Input parameters to the numerical analysis are flux and final cake height data obtained from batch filtration experiments, which are driven to steady-state. This technique is validated against piston driven filtration (odeometer) and centrifuge experiments for zirconia, soil particles, and yeast assemblages. The compressive yield stress calculated from filtration experiments agrees well with values obtained by odeometer and centrifuge studies for all particle systems studied at both the low and high solid pressure regions. Similarly, the calculated permeability agrees well with the measured permeability. [ABSTRACT FROM AUTHOR]

Dewatering and consolidation of saturated swelling soils are governed by pressure-dependent soil hydraulic properties. Existing measurement techniques are difficult and slow. We illustrate a simple, rapid desorption technique, developed for industrial slurries, to measure hydraulic properties of a gel-like sulfidic, estuarine soil (∼70% water content). Measured hydraulic conductivities, K(ψ), were very small, ∼10−10 m/s, giving a representative capillary fringe thickness of ∼7 m and characteristic gravity drainage times around 40 years. Capillarity therefore dominates flow in these soils. Estimated times for dewatering this soil under surface loading with closely spaced, vertical wick drains, are 2 to 70 years, consistent with experience. A Netherlands marine clay soil, saturated with seawater, is unexpectedly wetter than the brackish estuarine soil here at the same matric potential, ψ. However, K(ψ) for both soils overlap, suggesting the engineering approximation, K(ψ) ∝ ∣ψ∣−1, for marine-deposited clays. The functional dependencies of hydraulic properties surprisingly are not consistent with similar-media or double-layer theories. [ABSTRACT FROM AUTHOR]

Superoxide (O2-) and hydrogen peroxide (H2O2) concentrations ranging from 87 to 1120 pmol L-1 and 5 to 107 nmol L-1, respectively, were measured in samples of surface water from the Great Barrier Reef (GBR) lagoon in the absence of photochemistry. Nonphotochemical, particle-associated net production rates of O2- ranging from 1 to 16 pmol L-1 s-1 were also determined and calculated to be similar in magnitude to the likely abiotic photochemical O2- production rates in GBR surface waters. Manipulative experiments using 0.22-µm filtration and addition of biological inhibitors demonstrated that the majority of this particle-associated production was biological and likely driven by photosynthetic organisms. Pseudo-first-order O2- decay rate constants were very low at O2- concentrations < 1 nmol L-1 (values in filtered samples ranged from 0.7 to 4.3 x 10-2 s-1) but increased with increasing O2- concentration toward a value of ~ 0.2 s-1 at O2- concentrations > 10 nmol L-1. This was thought to occur because reduced forms of metals such as iron and copper, or redox-active organic moieties, preferentially react with O2 rather than O2- at low O2- concentrations, thereby inhibiting catalyzed O2- disproportionation. This notion was supported by the observation that addition of superoxide dismutase dramatically increased rates of H2O2 production in samples. We suggest that, under these conditions, O2- can maintain a biologically useful reducing microenvironment around cells without resulting in significant accumulation of potentially harmful H2O2. [ABSTRACT FROM AUTHOR]

Recently there has been recognition of the importance of reductive processes in the acquisition of iron by microorganisms in marine environments with Fe(III) reduction induced by either membrane-bound reductases or by superoxide, a powerful Fe(III) reducing agent generated either by photochemical or biological means. We have measured the relative rates of iron uptake achieved by the cyanobacterium L. majuscula in the presence of a variety of model- and naturally-derived organic ligands exhibiting a broad range of conditional ferric and ferrous stability constants. Additionally, we have investigated the effect upon iron uptake rate of varying the concentration of both iron and the iron-binding ligands. We have reconciled this data with previous work in which we measured rates of reduction by exogenous superoxide of Fe(III) bound to these same complexes. We show that the rate of formation of ferrous iron and the rate of uptake of iron by Lyngbya majuscula are each independent of the concentration of Fe' and demonstrate that this is consistent with our previous finding that this organism acquires iron via nondissociative reduction of ferric complexes. We also show that the rate of reoxidation of organically complexed Fe(II) is a critical determinant of the subsequent bioavailability of iron, a feature not previously addressed in the literature. In view of the central importance of the complexation and redox behavior of the iron- organic complexes to iron uptake rate, we propose a new kinetic model of iron acquisition, termed the FeL model, that is consistent with presented and previously published data and which describes processes both in natural and artificial conditions. [ABSTRACT FROM AUTHOR]