The interplay between Cs and K in Pseudanabaena catenata; from microbial bloom control strategies to bioremediation options for radioactive waters

Pseudanabaena dominates cyanobacterial blooms in the First-Generation Magnox Storage Pond (FGMSP) at a UK nuclear site. The fission product Cs is a radiologically significant radionuclide in the pond, and understanding the interactions between Cs and Pseudanabaena spp. is therefore important for determining facility management strategies, as well as improving understanding of microbiological responses to this non-essential chemical analogue of K. This study evaluated the fate of Cs following interactions with Pseudanabaena catenata, a laboratory strain most closely related to that dominating FGMSP blooms. Experiments showed that Cs (1 mM) exposure did not affect the growth of P. catenata, while a high concentration of K (5 mM) caused a significant reduction in cell yield. Scanning transmission X-ray microscopy elemental mapping identified Cs accumulation to discrete cytoplasmic locations within P. catenata cells, indicating a potential bioremediation option for Cs. Proteins related to stress responses and nutrient limitation (K, P) were stimulated by Cs treatment. Furthermore, selected K⁺ transport proteins were mis-regulated by Cs dosing, which indicates the importance of the K⁺ transport system for Cs accumulation. These findings enhance understanding of Cs fate and biological responses within Pseudanabaena blooms, and indicate that K exposure might provide a microbial bloom control strategy.

Cs-131 as an experimental tool for the investigation and quantification of the radiotoxicity of intracellular Auger decays in vitro

PURPOSE

In this work, we set out to provide an experimental setup, using Cs-131, with associated dosimetry for studying relative biological effectiveness (RBE) of Auger emitters.

MATERIAL AND METHODS

Cs-131 decays by 100% electron capture producing K- (9%) and L- (80%) Auger electrons with mean energies of 26 keV and 3.5 keV, respectively, plus ≈ 9.4 very low energy electrons (<0.5 keV) per decay. Cs-131 accumulates in the cells through the Na⁺/K⁺-ATPase. By this uptake mechanism and the alkali chemistry of Cs⁺, we argue for its intracellular homogeneous distribution. Cs-131 was added to the cell culture medium of HeLa and V79 Cells. The bio-kinetics of Cs-131 (uptake, release, intracellular distribution) was examined by measuring its intracellular activity concentration over time. Taking advantage of the 100% confluent cellular monolayer, we developed a new and robust dosimetry that is entrusted to a quantity called S_C-value.

RESULTS

The S_C-values evaluated in the cell nucleus are almost independent of the nuclear size and geometry. We obtained dose-rate controlled RBE-values for intracellular Cs-131 decay. Using the γH2AX assay, the RBE was 1 for HeLa cells. Using the clonogenic cell survival, it was 3.9 for HeLa cells and 3.2 for V79 cells.

CONCLUSION

This experimental setup and dosimetry provides reliable RBE-values for Auger emitters in various cell lines.

Processing of Metals and Metalloids by Actinobacteria: Cell Resistance Mechanisms and Synthesis of Metal(loid)-Based Nanostructures

Metal(loid)s have a dual biological role as micronutrients and stress agents. A few geochemical and natural processes can cause their release in the environment, although most metal-contaminated sites derive from anthropogenic activities. Actinobacteria include high GC bacteria that inhabit a wide range of terrestrial and aquatic ecological niches, where they play essential roles in recycling or transforming organic and inorganic substances. The metal(loid) tolerance and/or resistance of several members of this phylum rely on mechanisms such as biosorption and extracellular sequestration by siderophores and extracellular polymeric substances (EPS), bioaccumulation, biotransformation, and metal efflux processes, which overall contribute to maintaining metal homeostasis. Considering the bioprocessing potential of metal(loid)s by Actinobacteria, the development of bioremediation strategies to reclaim metal-contaminated environments has gained scientific and economic interests. Moreover, the ability of Actinobacteria to produce nanoscale materials with intriguing physical-chemical and biological properties emphasizes the technological value of these biotic approaches. Given these premises, this review summarizes the strategies used by Actinobacteria to cope with metal(loid) toxicity and their undoubted role in bioremediation and bionanotechnology fields.

Removal of radioactive cesium from an aqueous solution via bioaccumulation by microalgae and magnetic separation

We evaluated the potential sequestration of cesium (Cs⁺) by microalgae under heterotrophic growth conditions in an attempt to ultimately develop a system for treatment of radioactive wastewater. Thus, we examined the effects of initial Cs⁺ concentration (100–500 μM), pH (5–9), K⁺ and Na⁺ concentrations (0–20 mg/L), and different organic carbon sources (acetate, glycerol, glucose) on Cs⁺ removal. Our initial comparison of nine microalgae indicated that Desmodesmus armatus SCK had removed the most Cs⁺ under various environmental conditions. Addition of organic substrates significantly enhanced Cs⁺ uptake by D. armatus, even in the presence of a competitive cation (K⁺). We also applied magnetic nanoparticles coated with a cationic polymer (polyethylenimine) to separate ¹³⁷Cs-containing microalgal biomass under a magnetic field. Our technique of combining bioaccumulation and magnetic separation successfully removed more than 90% of the radioactive ¹³⁷Cs from an aqueous medium. These results clearly demonstrate that the method described here is a promising bioremediation technique for treatment of radioactive liquid waste.

Immobilization of cesium-resistant bacterial cells by radiation polymerization and their bioremoval efficiency

Biological approaches for the removal of heavy metals and radionuclides from contaminated water are reported. The present study was carried out with the objective of identifying bacterial strains for the uptake of cesium that could be used for bioremediation. Polymer carriers prepared by radiation polymerization were used for the immobilization of bacteria and the efficiency of free cells and immobilized cells for the removal of cesium was evaluated. Thirty-five bacterial isolates were screened for resistance to cesium and five bacterial isolates based on resistance to cesium (BR-3, BR-6, BR-21, BR-39, BR-40) were selected for immobilization. Polymer carriers were prepared using 10, 20, 30, 40 and 50% acrylamide at different doses of 1 to 5 kGy gamma radiation. The polymer carriers prepared using 30% and 40% acrylamide at 5 kGy were found to be suitable based on gel fraction and absorption capacity for the immobilization of bacterial cells. Bioremoval of cesium by free and immobilized bacterial cells was evaluated. Significant reductions of 76-81% cesium were observed with bacterial cells immobilized by radiation polymerization.

Impact of effective microorganisms on the transfer of radioactive cesium into lettuce and barley biomass

Soil microorganisms play an important role in determining the physical and chemical properties of soils. Soil microorganisms have both direct and indirect effects on the physical and chemical states of radionuclides and their availability for uptake by plant roots. Controlling the soil microorganisms to immobilize radionuclides is a promising strategy to reduce the content of radionuclides in the food chain. In this study, we evaluated the impact of effective microorganisms (EM) comprising lactic-acid bacteria, photosynthetic bacteria, and yeast on the transfer of 137Cs into the aboveground biomass of barley and lettuce. The application of EM or fermented organic fertilizer (bokashi) alone to sod-podzolic sandy-loam soil significantly reduced the aggregated transfer factor of 137Cs in barley by 37% and 44%, respectively. The combination of EM with bokashi or potassium fertilizer produced the largest reductions in 137Cs transfer into barley biomass (50% and 63%, respectively). EM had a stronger effect on 137Cs transfer into barley compared to lettuce. Laboratory experiments suggested that the effect of microorganisms on 137Cs uptake can be attributed to a reduction in the proportion of bioavailable physicochemical forms of 137Cs in the soils treated with EM and bokashi. This study, to the best of our knowledge, is the first to report the mechanism by which microbial fertilizers reduce the transfer of 137Cs into plants.

Bioaccumulation and trophic transfer of 137Cs in marine and freshwater plankton

An experimental study was conducted to investigate the trophic transfer of ¹³⁷Cs in marine and freshwater food chains, focusing on phytoplankton, zooplankton, and planktivorous fish. Algal concentration factors were 278 in freshwater and 69 in seawater. The weight-normalized uptake rate constants of ¹³⁷Cs were similar for both freshwater daphnids and marine copepods. Most of the ¹³⁷Cs in marine copepods was in the exoskeleton followed by polar and non-polar components. In freshwater daphnids, ¹³⁷Cs was highest in the polar fractions followed by exoskeleton and low amounts in the non-polar components. Fish that fed on contaminated marine copepods assimilated ¹³⁷Cs with an efficiency of 88%, while 91% was assimilated from freshwater daphnids. A bioaccumulation model demonstrated that diet accounted for ≤3% of the total body burden of ¹³⁷Cs in marine zooplankton and ≤12% in freshwater zooplankton, but ≥99% of the total body burden in fish. Rate constants of ¹³⁷Cs loss from fish following aqueous exposure were 0.2 d^-1 and 0.4 d^-1 in marine and freshwater conditions, respectively, but only 0.06 d^-1 and 0.3 d^-1, respectively, following dietary exposure. This model also indicates that trophic transfer factors from zooplankton to fish are up to 2.2 for marine conditions and up to 0.6 for freshwater. ¹³⁷Cs is unusual among metals in that it enters marine food chains primarily from the aqueous phase into zooplankton, from which it is highly assimilated by fish, resulting in detectable ¹³⁷Cs in fish tissues.

Stable and radioactive cesium: A review about distribution in the environment, uptake and translocation in plants, plant reactions and plants' potential for bioremediation

Radiocesium in water, soil, and air represents a severe threat to human health and the environment. It either acts directly on living organisms from external sources, or it becomes incorporated through the food chain, or both. Plants are at the base of the food chain; it is therefore essential to understand the mechanisms of plants for cesium retention and uptake. In this review we summarize investigations about sources of stable and radioactive cesium in the environment and harmful effects caused by internal and external exposure of plants to radiocesium. Uptake of cesium into cells occurs through molecular mechanisms such as potassium and calcium transporters in the plasma membrane. In soil, bioavailability of cesium depends on the chemical composition of the soil and physical factors such as pH, temperature and tilling as well as on environmental factors such as soil microorganisms. Uptake of cesium occurs also from air through interception and absorption on leaves and from water through the whole submerged surface. We reviewed information about reducing cesium in the vegetation by loss processes, and we extracted transfer factors from the available literature and give an overview over the uptake capacities of 72 plants for cesium from the substratum to the biomass. Plants with high uptake potential could be used to remediate soil and water from radiocesium by accumulation and rhizofiltration. Inside plants, cesium distributes fast between the different plant organs and cells, but cesium in soil is extremely stable and remains for decades in the rhizosphere. Monitoring of contaminated soil therefore has to continue for many decades, and edible plants grown on such soil must continuously be monitored.

Adsorption of cesium ion by marine actinobacterium Nocardiopsis sp. 13H and their extracellular polymeric substances (EPS) role in bioremediation

This paper evaluates the cesium adsorption of marine actinobacterium Nocardiposis sp. 13H strain isolated from nuclear power plant sites in India. It could remove 88.6 ± 0.72% of Cs⁺ from test solution containing 10 mM CsCl₂. The biosorption of Cs⁺ with different environmental factors such as pH, temperature, and time interval is also determined. Scanning electron microscopy coupled with energy dispersive spectroscopy (EDS) confirmed the Cs⁺ adsorption by Nocardiopsis sp. 13H. Most of the bound cesium was found to be associated extracellular polymeric substances (EPS) suggesting its interaction with the surface active groups. The main component of the EPS was carbohydrate followed by protein and nucleic acid. Further, Fourier transform infrared (FTIR) spectroscopy suggested the carboxyl, hydroxyl, and amide groups on the strain cell surface were likely to be involved in Cs⁺ adsorption. Results from this study show Nocardiopsis sp. 13H microorganism could be useful in exploring the biosorption of radioisotope pollution and developing efficient and eco-friendly biosorbent for environmental cleanup.

Kup-mediated Cs+ uptake and Kdp-driven K+ uptake coordinate to promote cell growth during excess Cs+ conditions in Escherichia coli

The physiological effects of caesium (Cs) on living cells are poorly understood. Here, we examined the physiological role of Cs⁺ on the activity of the potassium transporters in E. coli. In the absence of potassium (K⁺), Kup-mediated Cs⁺ uptake partially supported cell growth, however, at a much lower rate than with sufficient K⁺. In K⁺-limited medium (0.1 mM), the presence of Cs⁺ (up to 25 mM) in the medium enhanced growth as much as control medium containing 1 mM K⁺. This effect depended on the maintenance of basal levels of intracellular K⁺ by other K⁺ uptake transporters. Higher amounts of K⁺ (1 mM) in the medium eliminated the positive effect of Cs⁺ on growth, and revealed the inhibitory effect of high Cs⁺ on the growth of wild-type E. coli. Cells lacking Kdp, TrkG and TrkH but expressing Kup grew less well when Cs⁺ was increased in the medium. A kdp mutant contained an increased ratio of Cs⁺/K⁺ in the presence of high Cs⁺ in the medium and consequently was strongly inhibited in growth. Taken together, under excess Cs⁺ conditions Kup-mediated Cs⁺ influx sustains cell growth, which is supported by intracellular K⁺ supplied by Kdp.

Restoration of the growth of Escherichia coli under K+-deficient conditions by Cs+ incorporation via the K+ transporter Kup

Biological incorporation of cesium ions (Cs⁺) has recently attracted significant attention in terms of the possible applications for bioremediation of radiocesium and their significant roles in biogeochemical cycling. Although high concentrations of Cs⁺ exhibit cytotoxicity on microorganisms, there are a few reports on the promotive effects of Cs⁺ on microbial growth under K⁺-deficient conditions. However, whether this growth-promoting effect is a common phenomenon remains uncertain, and direct correlation between growth promotion and Cs⁺ uptake abilities has not been confirmed yet. Here, we validated the growth promotive effects of Cs⁺ uptake under K⁺-deficient conditions using an Escherichia coli strain with an inducible expression of the Kup K⁺ transporter that has nonspecific Cs⁺ transport activities (strain kup-IE). The strain kup-IE exhibited superior growth under the Cs⁺-supplemented and K⁺-deficient conditions compared to the wild type and the kup null strains. The intracellular Cs⁺ levels were significantly higher in strain kup-IE than in the other strains, and were well correlated with their growth yields. Furthermore, induction levels of the kup gene, intracellular Cs⁺ concentrations, and the growth stimulation by Cs⁺ also correlated positively. These results clearly demonstrated that Cs⁺ incorporation via Kup transporter restores growth defects of E. coli under K⁺-deficient conditions.

Cesium and strontium tolerant Arthrobacter sp. strain KMSZP6 isolated from a pristine uranium ore deposit

Arthrobacter sp. KMSZP6 isolated from a pristine uranium ore deposit at Domiasiat located in North-East India exhibited noteworthy tolerance for cesium (Cs) and strontium (Sr). The strain displayed a high minimum inhibitory concentration (MIC) of 400 mM for CsCl and for SrCl2. Flow cytometric analysis employing membrane integrity indicators like propidium iodide (PI) and thiazole orange (TO) indicated a greater sensitivity of Arthrobacter cells to cesium than to strontium. On being challenged with 75 mM of Cs, the cells sequestered 9612 mg Cs g(-1) dry weight of cells in 12 h. On being challenged with 75 mM of Sr, the cells sequestered 9989 mg Sr g(-1) dry weight of cells in 18 h. Heat killed cells exhibited limited Cs and Sr binding as compared to live cells highlighting the importance of cell viability for optimal binding. The association of the metals with Arthrobacter sp. KMSZP6 was further substantiated by Field Emission-Scanning Electron Microscopy (FE-SEM) coupled with Energy dispersive X-ray (EDX) spectroscopy. This organism tolerated up to 1 kGy (60)Co-gamma rays without loss of survival. The present report highlights the superior tolerance and binding capacity of the KMSZP6 strain for cesium and strontium over other earlier reported strains and reveals its potential for bioremediation of nuclear waste.

Enrichment and isolation of Flavobacterium strains with tolerance to high concentrations of cesium ion

Interest in the interaction of microorganisms with cesium ions (Cs⁺) has arisen, especially in terms of their potent ability for radiocesium bioaccumulation and their important roles in biogeochemical cycling. Although high concentrations of Cs⁺ display toxic effects on microorganisms, there have been only limited reports for Cs⁺-tolerant microorganisms. Here we report enrichment and isolation of Cs⁺-tolerant microorganisms from soil microbiota. Microbial community analysis revealed that bacteria within the phylum Bacteroidetes, especially Flavobacterium spp., dominated in enrichment cultures in the medium supplemented with 50 or 200 mM Cs⁺, while Gammaproteobacteria was dominant in the control enrichment cultures (in the presence of 50 and 200 mM K⁺ instead of Cs⁺). The dominant Flavobacterium sp. was successfully isolated from the enrichment culture and was closely related to Flavobacterium chungbukense with 99.5% identity. Growth experiments clearly demonstrated that the isolate has significantly higher tolerance to Cs⁺ compared to its close relatives, suggesting the Cs⁺-tolerance is a specific trait of this strain, but not a universal trait in the genus Flavobacterium. Measurement of intracellular K⁺ and Cs⁺ concentrations of the Cs⁺-tolerant isolate and its close relatives suggested that the ability to maintain low intracellular Cs⁺ concentration confers the tolerance against high concentrations of external Cs⁺.

Biosorption behavior and mechanism of cesium-137 on Rhodosporidium fluviale strain UA2 isolated from cesium solution

In order to identify a more efficient biosorbent for (137)Cs, we have investigated the biosorption behavior and mechanism of (137)Cs on Rhodosporidium fluviale (R. fluviale) strain UA2, one of the dominant species of a fungal group isolated from a stable cesium solution. We observed that the biosorption of (137)Cs on R. fluviale strain UA2 was a fast and pH-dependent process in the solution composed of R. fluviale strain UA2 (5 g/L) and cesium (1 mg/L). While a Langmuir isotherm equation indicated that the biosorption of (137)Cs was a monolayer adsorption, the biosorption behavior implied that R. fluviale strain UA2 adsorbed cesium ions by electrostatic attraction. The TEM analysis revealed that cesium ions were absorbed into the cytoplasm of R. fluviale strain UA2 across the cell membrane, not merely fixed on the cell surface, which implied that a mechanism of metal uptake contributed largely to the cesium biosorption process. Moreover, PIXE and EPBS analyses showed that ion-exchange was another biosorption mechanism for the cell biosorption of (137)Cs, in which the decreased potassium ions were replaced by cesium ions. All the above results implied that the biosorption of (137)Cs on R. fluviale strain UA2 involved a two-step process. The first step is passive biosorption that cesium ions are adsorbed to cells surface by electrostatic attraction; after that, the second step is active biosorption that cesium ions penetrate the cell membrane and accumulate in the cytoplasm.

Characteristics of cesium accumulation in the filamentous soil bacterium Streptomyces sp. K202

A filamentous soil bacterium, strain K202, was isolated from soil where an edible mushroom (Boletopsis leucomelas) was growing and identified as belonging to the genus Streptomyces on the basis of its morphological characteristics and the presence of LL-2, 6-diaminopimelic acid. We studied the existence states of Cs and its migration from extracellular to intracellular fluid in the mycelia of Streptomyces sp. K202. The results indicated that Cs accumulated in the cells through at least 2 steps: in the first step, Cs(+) was immediately and non-specifically adsorbed on the negatively charged cell surface, and in the second step, this adsorbed Cs(+) was taken up into the cytoplasm, and a part of the Cs entering the cytoplasm was taken up by an energy-dependent transport system(s). Further, we confirmed that a part of the Cs(+) was taken up into the mycelia competitively with K(+), because K(+) uptake into the intact mycelia of the strain was significantly inhibited by the presence of Cs(+) in the culture media. This suggested that part of the Cs is transported by the potassium transport system. Moreover, (133)Cs-NMR spectra and SEM-EDX spectra of the mycelia that accumulated Cs showed the presence of at least 2 intracellular Cs states: Cs(+) trapped by intercellular materials such as polyphosphate and Cs(+) present in a cytoplasmic pool.

Cesium sorption on studtite (UO2O2·4H2O)

One of the mechanisms that may decrease the mobility of cesium released from spent fuel in a high level nuclear waste repository (HLNW) is its sorption onto uranyl-containing alteration phases formed on the spent fuel surface such as studtite (UO2O2·4H2O). The results obtained in this work show that sorption is a very fast process; cesium in solution is sorbed in less than one hour at pH 5. Sorption as a function of initial concentration in solution was also studied between initial cesium concentrations ranging from 7.6×10−9 mol dm−3to 1.0×10−3 mol dm−3. The data have been modelled considering a Freundlich isotherm, withKFandnvalues of 10±1, and 1.4±0.1, respectively (r2=0.998). Sorption is very dependent on ionic strength, suggesting that cesium sorbs onto studtite by forming an outer-sphere complex involving electrostatic interactions. Sorption is observed to be very low at acidic pH, while relatively high at alkaline pH (i.e., almost 60% of the total cesium concentration in solution is sorbed at pH>9). The results point to the importance of sorption processes on uranyl alteration phases on the retention of radionuclides.

Biosorption of stable cesium by chemically modified biomass of Sargassum glaucescens and Cystoseira indica in a continuous flow system

Pretreatment of biosorbents have been suggested to modify the surface characteristics which could improve biosorption process. Stable cesium biosorption was studied in continuous fixed-bed column by chemically modified biosorbents. Two kinds of brown algae (Sargassum glaucescens and Cystoseira indica) were treated with chemical agents including formaldehyde (FA), glutaraldehyde (GA), potassium hexacyanoferrate (HCF), FA and HCF, and GA and HCF. The highest biosorption capacity (BC) was obtained from C. indica treated with FA (63.5 mg Cs/g biomass) and S. glaucescens treated with FA and HCF (62 mg Cs/g biomass). To study the effect of the best treatments on the BC, the concentration of each treatment agent was decreased. With decreasing FA agent for C. indica treatment, the BC dropped. Treatment of 1g S. glaucescens biomass with 2.2g FA and then 0.18 g HCF resulted in the highest BC (73.08 mg Cs/g dry biomass) which was 35.8 times higher than intact S. glaucescens.

Trehalose-producing enzymes MTSase and MTHase in Anabaena 7120 under NaCl stress

Salt tolerance, a multigenic trait, necessitates knowledge about biosynthesis and function of candidate gene(s) at the cellular level. Among the osmolytes, trehalose biosynthesis in cyanobacteria facing NaCl stress is little understood. Anabaena 7120 filaments exposed to 150 mM: NaCl fragmented and recovered on transfer to -NaCl medium with the increased heterocysts frequency (7%) over the control (4%). Cells failed to retain Na+ beyond a threshold [2.19 mM/cm3 (PCV)]. Whereas NaCl-stressed cells exhibited a marginal rise in K+ (1.1-fold) only at 30 h, for Na+ it was 130-fold at 48 h over cells in control. A time-course study (0-54 h) revealed reduction in intracellular Na+ beyond 48 h [0.80 mM/cm3 (PCV)] suggestive of ion efflux. The NaCl-stressed cells showed differential expression of maltooligosyltrehalose synthase (MTSase; EC 5.4.99.15) and maltooligosyltrehalose trehalohydrolase (MTHase; EC 3.2.1.141) depending on the time and the extent of intracellular Na+ buildup.

Auxotrophic mutant of the cyanobacterium Nostoc muscorum showing absolute requirement of Cs+ or Rb+ for diazotrophy and autotrophy

Caesium-resistant (Cs(+)-R) mutant clones of the cyanobacterium Nostoc muscorum were characterized for diazotrophic growth in a medium devoid of Cs(+) or Rb(+) or both. Cs(+)-R phenotype suffered severe genetic damage of a pleiotropic nature affecting diazotrophic growth, chlorophyll a content, nitrogenase activity and photosynthetic O(2) evolution. Mutation leading to development of Cs(+)-R phenotype could be overcome by availability of Cs(+)/Rb(+). Parent and mutant strains were similar with respect to their Cs(+)/Rb(+) uptake. Available data suggests operation of an efficient coupling of the two incompatible reactions viz. oxygenic photosynthesis and oxygen sensitive N(2) fixation in this cyanobacterium.

The effects of K+ growth conditions on the accumulation of cesium by the bacterium Thermus sp. TibetanG6

The accumulation of cesium by the bacterium Thermus sp. TibetanG6 was examined under different K+ growth conditions. The effects of external pH and Na+ on the accumulation of cesium were also studied, and the mechanism involved was discussed. K+ regimes played an important role in the accumulation of cesium by the strain TibetanG6. The quantity of cesium accumulated (24 h) was much higher in K+-deficient regime than that in K+-sufficient regime. The pH and Na+ had different effects on the accumulation of cesium in the two K+ regimes. IR spectra analyses indicated that the biosorption is a process of homeostasis with cesium initially accumulated on the cell wall.

Acute toxicity of trivalent thallium compounds to Daphnia magna

Daphnia magna was used to evaluate the aquatic toxicity of Tl(III) compounds including Tl(III) nitrate, Tl(III) chloride, and Tl(III) acetate. The results clearly show that Tl(III) is extremely toxic to daphnids. The 48-h LC50 values for Tl(III) nitrate, Tl(III) chloride, and Tl(III) acetate are 24, 61, and 203 microg/L, respectively. Tl(III) is much more toxic than Tl(I) and many other metals such as Cd(II), Cu(II), and Ni(II); it is similar to the toxicity that of Hg(II). The formation of Tl(III)-complexes would significantly reduce Tl(III) toxicity.

Biosorption of cesium by native and chemically modified biomass of marine algae: introduce the new biosorbents for biotechnology applications

Biosorption batch experiments were conducted to determine the cesium binding ability of native biomass and chemically modified biosorbents derived from marine algae, namely ferrocyanide algal sorbents type 1 and type 2 (FASs1 and FASs2). The applicability of the Langmuir and Freundlich isotherms for representation of the experimental data was investigated. The cesium sorption performances of the various types of sorbents were compared using the maximum capacities (qmax values) obtained from fitting the Langmuir isotherm to the values calculated from the sorption experiments, which FASs type 1 and type 2 showed better sorption performances for cesium. FASs1 and FASs2 derived from formaldehyde and glutaraldehyde crosslinked Padina australis exhibited lower sorption capacities than those prepared from the non-crosslinked one. Most of the cesium ions were bound to FASs1, derived from Sargassum glaucescens and P. australis, in < 2 min and equilibrium reached within the first 30 min of contact. Biosorption of cesium by FASs1 derived from P. australis and Cystoseria indica was constantly occurred at a wide range of pH, between 1 and 10, and the highest removal took place at pH 4. The presence of sodium and potassium at 0.5 and 1mM did not inhibit cesium biosorption by algae biomass. The maximum cesium uptake was acquired using the large particles of FAS2 originated from S. glaucescens (2-4 mm). Desorption of cesium from the metal-laden FASs1 (from P. australis, S. glaucescens and Dictyota indica) was completely achieved applying 0.5 and 1 M NaOH and KOH, although the cesium sorption capacity of the biosorbents (from C. indica and S. glaucescens) decreased by 46-51% after 9 sorption-desorption cycles.

Bioremediation of Heavy Metal Pollution Exploiting Constituents, Metabolites and Metabolic Pathways of Livings. A Review

Removal of heavy metals from the soil and water or their remediation from the waste streams "at source" has been a long-term challenge. During the recent era of environmental protection, the use of microorganisms for the recovery of metals from waste streams as well as employment of plants for landfill applications has generated growing attention. Many studies have demonstrated that both prokaryotes and eukaryotes have the ability to remove metals from contaminated water or waste streams. They sequester metals from soils and sediments or solubilize them to aid their extraction. The proposed microbial processes for bioremediation of toxic metals and radionuclides from waste streams employ living cells and non-living biomass or biopolymers as biosorbents. Microbial biotransformation of metals or metalloids results in an alteration of their oxidation state or in their alkylation and subsequent precipitation or volatilization. Specific metabolic pathways leading to precipitation of heavy metals as metal sulfides, phosphates or carbonates possess significance for possible biotechnology application. Moreover, the possibility of altering the properties of living species used in heavy metal remediation or constructing chimeric organisms possessing desirable features using genetic engineering is now under study in many laboratories. The encouraging evidence as to the usefulness of living organisms and their constituents as well as metabolic pathways for the remediation of metal contamination is reviewed here. A review with 243 references.

Health impacts of large releases of radionuclides. Roles of micro-organisms in the environmental fate of radionuclides

Micro-organisms play important roles in the environmental fate of radionuclides in both aquatic and terrestrial ecosystems, with a multiplicity of physico-chemical and biological mechanisms effecting changes in mobility and speciation. Physico-chemical mechanisms of removal, which may be encompassed by the general term 'biosorption', include adsorption, ion exchange and entrapment. These are features of living and dead organisms as well as their derived products. In living cells biosorptive processes can be directly and indirectly influenced by metabolism, and may be reversible and affected by changing environmental conditions. Metabolism-dependent mechanisms of radionuclide immobilization include metal precipitation as sulfides, sequestration by metal-binding proteins and peptides, and transport and intracellular compartmentation. Chemical transformations of radionuclide species, particularly by reduction, can result in immobilization. Microbial processes involved in solubilization include autotrophic and heterotrophic leaching, complexation by siderophores and other metabolites, and chemical transformations. Such mechanisms are important components of natural biogeochemical cycles for radionuclides and should be considered in any analyses of environmental radionuclide contamination. Several micro-organism-based biotechnologies, e.g. those based on biosorption or precipitation, are of potential use for the treatment of radionuclide contamination.

Influence of altered plasma membrane fatty acid composition on cesium transport characteristics and toxicity in Saccharomyces cerevisiae

The influence of altered plasma membrane fatty acid composition on cesium uptake and toxicity was investigated in Saccharomyces cerevisiae. Detailed kinetic studies revealed that both the Vmax and Km values for Cs+ transport increased (by approximately twofold in the latter case) when S. cerevisiae was grown in medium supplemented with the polyunsaturated fatty acid linoleate. In addition, Cs+ uptake by linoleate-enriched cells was considerably less sensitive to the competitive effects of other monovalent cations (K+, Rb+, and NH4+) than that by unsupplemented cells. Stimulation of Cs+ uptake in the presence of certain K+ and Rb+ concentrations was only evident in linoleate-enriched S. cerevisiae. At 100 mM CsCl, the initial rate of Cs+ uptake was greater in linoleate-supplemented cells than in unsupplemented cells and this was reflected in a more rapid displacement of cellular K+. However, little difference in net Cs+ accumulation between linoleate-supplemented and unsupplemented cells was evident during prolonged incubation in buffer or during growth. Thus, Cs+ toxicity was similar in linoleate-supplemented and unsupplemented cells. The results were consistent with the Cs+ (K+) transport mechanism adopting an altered conformational state in linoleate-enriched S. cerevisiae.

The influence of pH and external K+ concentration on caesium toxicity and accumulation in Escherichia coli and Bacillus subtilis

Toxicity screening of Escherichia coli NCIB 9484 and Bacillus subtilis 007, NCIB 168 and NCIB 1650 has shown Cs+ to be the most toxic Group 1 metal cation. However, toxicity and accumulation of Cs+ by the bacteria was affected by two main external factors; pH and the presence of other monovalent cations, particularly K+. Over the pH range 6-9 both E. coli and B. subtilis showed increasing sensitivity towards caesium as the pH was raised. The presence of K+ and Na+ in the laboratory media used lowered caesium toxicity and lowered accumulation of the metal. In order to assess accurately Cs+ toxicity towards the bacterial strains it was therefore necessary to define the K+:Cs+ ratio in the external medium. The minimum inhibitory K+:Cs+ concentration ratio for the Bacillus strains tested was in the range 1:2-1:3 while E. coli had a minimum inhibitory K+:Cs+ concentration ratio of 1:6.

Caesium accumulation by microorganisms: uptake mechanisms, cation competition, compartmentalization and toxicity

The continued release of caesium radioisotopes into the environment has led to a resurgence of interest in microbe-Cs interactions. Caesium exists almost exclusively as the monovalent cation Cs+ in the natural environment. Although Cs+ is a weak Lewis acid that exhibits a low tendency to form complexes with ligands, its chemical similarity to the biologically essential alkali cation K+ facilitates high levels of metabolism-dependent intracellular accumulation. Microbial Cs+ (K+) uptake is generally mediated by monovalent cation transport systems located on the plasma membrane. These differ widely in specificity for alkali cations and consequently microorganisms display large differences in their ability to accumulate Cs+; Cs+ appears to have an equal or greater affinity than K+ for transport in certain microorganisms. Microbial Cs+ accumulation is markedly influenced by the presence of external cations, e.g. K+, Na+, NH4+ and H+, and is generally accompanied by an approximate stoichiometric exchange for intracellular K+. However, stimulation of growth of K(+)-starved microbial cultures by Cs+ is limited and it has been proposed that it is not the presence of Cs+ in cells that is growth inhibitory but rather the resulting loss of K+. Increased microbial tolerance to Cs+ may result from sequestration of Cs+ in vacuoles or changes in the activity and/or specificity of transport systems mediating Cs+ uptake. The precise intracellular target(s) for Cs(+)-induced toxicity has yet to be clearly defined, although certain internal structures, e.g. ribosomes, become unstable in the presence of Cs+ and Cs+ is known to substitute poorly for K+ in the activation of many K(+)-requiring enzymes.