S.F. D'Souza

Uranium and thorium sequestration by a Pseudomonas sp.: Mechanism and chemical characterization

The mechanism and chemical nature of uranium and thorium sequestration by a Pseudomonas strain was investigated by transmission electron microscopy, energy dispersive X-ray (EDX) analysis, FTIR spectroscopy and X-ray diffractometry. Atomic force microscopy (AFM) used in the tapping mode elucidated the morphological changes in bacterial cells following uranium and thorium binding. Transmission electron microscopy revealed intracellular sequestration of uranium and thorium throughout the cell cytoplasm with electron dense microprecipitations of accumulated metals. Energy dispersive X-ray analysis confirmed the cellular deposition of uranium and thorium. EDX and elemental analysis of sorption solution indicated the binding of uranium and thorium by the bacterial biomass via displacement of cellular potassium and calcium. The strong involvement of cellular phosphate, carboxyl and amide groups in radionuclide binding was ascertained by FTIR spectroscopy. X-ray powder diffraction (XRD) analyses confirmed cellular sequestration of crystalline uranium and thorium phosphates. Overall results indicate that a combined ion-exchange-complexation-microprecipitation mechanism could be involved in uranium and thorium sequestration by this bacterium. Atomic force microscopy and topography analysis revealed an undamaged cell surface with an increase in cell length, width and height following radionuclide accumulation. The arithmetic average roughness (R(a)) and root mean square (RMS) roughness (R(q)) values indicated an increase in surface roughness following uranium and thorium sequestration.

Extracellular polysaccharides of a copper-sensitive and a copper-resistant Pseudomonas aeruginosa strain: synthesis, chemical nature and copper binding

Extracellular polysaccharides (EPS) of a copper-sensitive (Cus) and a copper-resistant (Cur) Pseudomonas aeruginosa strain were investigated in terms of their production, chemical nature and response towards copper exposure. The extent of EPS synthesis by the resistant strain (4.78 mg mg−1 cell dry wt.) was considerably higher over its sensitive counterpart (2.78 mg mg−1 dry wt.). FTIR-spectroscopy and gas chromatography revealed that both the polymers were acidic in nature, containing alginate as the major component along with various neutral- and amino-sugars. Acid content in the Cur EPS (480.54 mg g−1) was greater than that in the Cus EPS (442.0 mg g−1). Presence of Cu2+ in the growth medium caused a dramatic stimulation (approximately 4-fold) in EPS synthesis by the Cur strain, while in a similar condition, the Cus failed to exhibit such response. The polymer of the resistant strain showed elevated Cu2+ binding (320 mg g−1 EPS) compared to that of the sensitive type (270 mg g−1). The overall observations show the potential of the Cur EPS for its deployment in metal bioremediation.

Biosorption characteristics of uranium(VI) from aqueous medium onto Catenella repens, a red alga

The biosorption characteristics of uranium(VI) onto Catenella repens (a red alga), were evaluated as a function of pH, biosorbent size, time, biomass dosage, initial uranium concentration and temperature. Within the pH range studied (1.5-7.5), 4.5 was the optimum pH for the uptake of uranium(VI) by C. repens. Reduction in particle size did not increase the biosorption capacity. The metal removal was rapid, with more than 90% of total biosorption taking place in 30 min, and equilibrium was attained in 45 min. The maximum metal loading capacity of the alga was 303 mg/g. Within the temperature range studied (15-55 degrees C), there was no significant change in biosorption, under optimal conditions. Adsorption process could be well defined by both the Langmuir and Freundlich isotherms with r(2) of 0.94 and 0.96, respectively. The kinetic data fitted the pseudo-second-order kinetic model with the r(2) value of 0.99. At a low pH of 2.5, where most of the biomasses show either no or less metal uptake, a good (>15%) metal loading capacity of 25% was achieved. Therefore biosorption characteristics were also evaluated at pH 2.5.

Comparative biochemical and transcriptional profiling of two contrasting varieties of Brassica juncea L. in response to arsenic exposure reveals mechanisms of stress perception and tolerance

The mechanisms of perception of arsenic (As)-induced stress and ensuing tolerance in plants remain unresolved. To obtain an insight into these mechanisms, biochemical and transcriptional profiling of two contrasting genotypes of Brassica juncea was performed. After screening 14 varieties for As tolerance, one tolerant (TPM-1) and one sensitive (TM-4) variety were selected and exposed to arsenate [As(V)] and arsenite [As(III)] for 7 d and 15 d for biochemical analyses. The tolerant variety (TPM-1) demonstrated higher accumulation of As upon exposure to both 500 microM As(V) and 250 microM As(III) [49 microg g(-1) and 37 microg g(-1) dry weight (dw) after 15 d] as well as a better response of thiol metabolism as compared with the responses observed in the sensitive variety (TM-4). Transcriptional profiling of selected genes that are known to be responsive to sulphur depletion and/or metal(loid) stress was conducted in 15-d-old seedlings after 3 h and 6 h exposure to 250 microM As(III). The results showed an up-regulation of sulphate transporters and auxin and jasmonate biosynthesis pathway genes, whereas there was a down-regulation of ethylene biosynthesis and cytokinin-responsive genes in TPM-1 within 6 h of exposure to As(III). This suggested that perception of As-induced stress was presumably mediated through an integrated modulation in hormonal functioning that led to both short- and long-term adaptations to combat the stress. Such a coordinated response of hormones was not seen in the sensitive variety. In conclusion, an early perception of As-induced stress followed by coordinated responses of various pathways was responsible for As tolerance in TPM-1.

Evaporation Driven Self-Assembly of a Colloidal Dispersion during Spray Drying: Volume Fraction Dependent Morphological Transition

Morphological transition of droplets during evaporation driven self-assembly of colloidal dispersion of alumina particles has been investigated. It was found that a sphere to doughnut-like transition of the droplet morphology takes place even when the rate of drying remains moderate and is not extremely fast. Further, it has been seen that such transition is strongly dependent on the volume fraction of the colloids in the droplets. The transition proceeds via buckling of the initial spherical droplets, which occurs when the capillary forces driving the deformation overcomes the interparticle electrostatic forces. However, the transition is hindered and the buckling probability is reduced due to the inherent spatial constraint when the colloid volume fraction is increased. Mesoscopic structures of the assembled grains have been investigated by scanning electron microscopy, small-angle neutron scattering, and dynamic light scattering techniques. Interestingly, it has been observed that the functionality of photoluminescence spectrum of the dried nanoporous grains depends somewhat on the grain morphology.

Biosorption of uranium(VI) by Aspergillus fumigatus

Aspergillus fumigatus removed uranium(VI) very rapidly and reached equilibrium within 1 h of contact of biomass with the aqueous metal solution. Biosorption data fitted to Langmuir model of isotherm and a maximum loading capacity of 423 mg U g−1 dry wt was obtained. Distribution coefficient as high as 10,000 (mg U g−1)/(mg U ml−1) at a residual metal ion concentration of 19 mg l−1 indicates its usefulness in removal of uranium(VI) from dilute waste streams. Optimum biosorption was seen at pH 5.0 and was independent of temperature (5–50 °C ). Initial metal ion concentration significantly influenced uptake capacity which brought down % (w/w) uranium(VI) removal from 90 at 200 mg U l−1 to 35 at 1000 mg U l−1. Presence of 0.84 mmol Fe2+, Fe3+, Ca2+ and Zn2+ had no effect on uranium(VI) biosorption unlike Al3+ (0.84 mM) which was inhibitory.

Immobilization and stabilization of biomaterials for biosensor applications.

Biosensors are finding applications in a variety of analytical fields. A biosensor basically consists of a transducer in conjunction with a biologically active molecule that converts a biochemical signal into a quantifiable electric response. The specificity of the biosensor depends on the selection of the biomaterial. Enzymes, antibodies, DNA, receptors, organelles, microorganisms as well as animal and plant cells or tissues have been used as biologic sensing materials. Advances in biochemistry, molecularbiology, and immunochemistry are expected to lead to a rapid expansion in the range of biologic recognition elements to be used in the field of biosensors. Biomaterials that are stable and function even in highly acidic, alkaline, hydrophobic, or oxidizing environments as well as stable to high temperature and immune to toxic substrates in the processing stream will play an important role. Techniques for immobilization of the biomaterials have played a significant role in the biosensor field. Immobilization not only brings about the intimate contact of the biologic catalysts with the transducer, but also helps in the stabilization of the biologic system, thus enhancing its operational and storage stability. A number of techniques have been developed in our laboratory for the immobilization of enzymes, multienzyme systems, cells, and enzymecell conjugates. Some of these aspects that are of significance in biosensor applications have been highlighted.

Adaptive response of Haloferax mediterranei to low concentrations of NaCl (<20%) in the growth medium

Abstract Halobacteria require 20–25% NaCl for optimal growth and lyse when the salt concentration falls below 10%. The response of Haloferax mediterranei cells to low concentrations of NaCl (< 20%) in the medium was studied. The cells adapted to and grew in concentrations of NaCl as low as 10% and survived in concentrations lower than 5%. The cells synthesised a red pigment, bacterioruberin, in response to stress caused by a low concentration of NaCl (< 20%).

Immobilization of yeast cells by adhesion to glass surface using polyethylenimine

SummaryA method has been described for immobilization of yeast cells by adsorption to glass surface using polyethylenimine for coating cells or glass or both. The immobilized yeast cells were found to be viable and adhered strongly as a monolayer, which could not be desorbed by washing with running tap water or under extreme conditions of pH and ionic strength or during repeated uses in sucrose solutions.

Entrapment of live microbial cells in electropolymerized polyaniline and their use as urea biosensor.

The lyophilized biomass of bacterium Brevibacterium ammoniagenes was immobilized in polystyrene sulphonate-polyaniline (PSS-PANI) conducting polymer on a Pt twin wire electrode by potentiostatic electropolymerization. The bacterial cells retained their viability as well as urease activity under entrapped state, as confirmed with bacterial live-dead fluorescent assay and enzymatic assays. The entrapped cells were visualized using scanning electron microscope. The immobilized cells were used as a source of unpurified urease to develop a conductometric urea biosensor. The catalytic action of urease in the sensor released ammonia, thereby causing an increase in the pH of the microenvironment. The pH dependant change in the resistivity of the polymer was used as the basis of sensing mechanism. The sensor response was linear over a range of 0-75 mM urea with a sensitivity of 0.125 mM(-1). The sensor could be reused for 12-15 independent measurements and was quite stable in dry as well as buffered storage condition at 4 degrees C for at least 7 days.