G.R. Palmer

Freeze-Thaw Tolerance and Clues to the Winter Survival of a Soil Community

ABSTRACT Although efforts have been made to sample microorganisms from polar regions and to investigate a few of the properties that facilitate survival at freezing or subzero temperatures, soil communities that overwinter in areas exposed to alternate freezing and thawing caused by Foehn or Chinook winds have been largely overlooked. We designed and constructed a cryocycler to automatically subject soil cultures to alternating freeze-thaw cycles. After 48 freeze-thaw cycles, control Escherichia coli and Pseudomonas chlororaphis isolates were no longer viable. Mixed cultures derived from soil samples collected from a Chinook zone showed that the population complexity and viability were reduced after 48 cycles. However, when bacteria that were still viable after the freeze-thaw treatments were used to obtain selected cultures, these cultures proved to be >1,000-fold more freeze-thaw tolerant than the original consortium. Single-colony isolates obtained from survivors after an additional 48 freeze-thaw cycles were putatively identified by 16S RNA gene fragment sequencing. Five different genera were recognized, and one of the cultures, Chryseobacterium sp. strain C14, inhibited ice recrystallization, a property characteristic of antifreeze proteins that prevents the growth of large, potentially damaging ice crystals at temperatures close to the melting temperature. This strain was also notable since cell-free medium derived from cultures of it appeared to enhance the multiple freeze-thaw survival of another isolate, Enterococcus sp. strain C8. The results of this study and the development of a cryocycler should allow further investigations into the biochemical and soil community adaptations to the rigors of a Chinook environment.

The Effect of Silver Nanoparticles on Seasonal Change in Arctic Tundra Bacterial and Fungal Assemblages

The impact of silver nanoparticles (NPs) and microparticles (MPs) on bacterial and fungal assemblages was studied in soils collected from a low arctic site. Two different concentrations (0.066% and 6.6%) of Ag NPs and Ag MPs were tested in microcosms that were exposed to temperatures mimicking a winter to summer transition. Toxicity was monitored by differential respiration, phospholipid fatty acid analysis, polymerase chain reaction-denaturing gradient gel electrophoresis and DNA sequencing. Notwithstanding the effect of Ag MPs, nanosilver had an obvious, additional impact on the microbial community, underscoring the importance of particle size in toxicity. This impact was evidenced by levels of differential respiration in 0.066% Ag NP-treated soil that were only half that of control soils, a decrease in signature bacterial fatty acids, and changes in both richness and evenness in bacterial and fungal DNA sequence assemblages. Prominent after Ag NP-treatment were Hypocreales fungi, which increased to 70%, from only 1% of fungal sequences under control conditions. Genera within this Order known for their antioxidant properties (Cordyceps/Isaria) dominated the fungal assemblage after NP addition. In contrast, sequences attributed to the nitrogen-fixing Rhizobiales bacteria appeared vulnerable to Ag NP-mediated toxicity. This combination of physiological, biochemical and molecular studies clearly demonstrate that Ag NPs can severely disrupt the natural seasonal progression of tundra assemblages.

Decomposition of Langmuir-Blodgett films to form metal oxide layers

Abstract Ultraviolet/ozone (UVO) treatment is investigated as a processing step in the fabrication of metal oxide thin films from Langmuir-Blodgett (LB) precursor films. UVO processing was found to reduce LB films to metal carbonates at a rate of approximately 1 min per layer. Subsequent thermal treatment resulted in dense, uniform metal oxide thin films. The structure and composition of these films are studied ellipsometry, X-ray diffraction, and Rutherford backscattering spectrometry.

RESPONSES OF CERTAIN FRESHWATER PLANKTONIC ALGAE TO FLUORIDE1

The effects of dissolved fluoride supplied as NaF at up to 150 p.p.m. F− (7.9 mM) on growth, photosynthesis, dark respiration, enolase activity and fluoride uptake were determined for six phytoplankters: Synechococcus leopoliensis (Racib.) Komarek (Cyanophyta), Oscillatoria limnetica Lemmermann (Cyanophyta), Ankistrodesmus braunii Brun (Chlorophyta), Scenedesmus quadricauda (Turp.) Bréb. (Chlorophyta), Cyclotella meneghiniana Kützing (Bacillariophyta) and Stephanodiscus minutus Grun. ex Cleve et Moll (Bacillariophyta). Growth (determined by absorbance at 660 nm or by cell‐numbers) was unaffected by fluoride at up to 50 p.p.m. (2.6 mM) in all algae except S. leopoliensis, in which growth ceased transiently followed by resumption of growth at reduced rate. These effects showed a threshold at ca. 25 p.p.m. (1.3 mM) F− and increased with increasing F− concentration above this threshold. Photosynthetic O2 evolution in the chlorophytes was unaffected by F− at up to 50 p.p.m., whereas in S. leopoliensis F− above ca. 25 p.p.m. caused a concentration‐dependent inhibition of photosynthesis which was most pronounced at saturating irradiance. Dark O2 uptake was unaffected at up to 50 p.p.m. in chlorophytes but was stimulated in S. leopoliensis. Enolase in clarified cell‐extracts of all six algae was inhibited by F−, with Ki values ranging from 27 to 319 μM. Fluorine (measured by proton‐induced gamma‐ray emission) could not be detected in chlorophytes exposed during growth to up to 50 p.p. m. F−, but was detected in S. leopoliensis, O. limnetica and C. meneghiniana. Fluorine associated with cells of these algae increased as the external F− concentration increased.

RBS analysis of Langmuir-Blodgett films

Abstract Rutherford backscattering spectrometry (RBS) has been used to characterize thin films of yttrium and erbium arachidate deposited on a silicon substrate by a Langmuir-Blodgett technique. RBS can measure film thickness as well as the stoichiometry. Beam effects do not influence these results at moderate beam currents and fluences, although the films visual appearance was very sensitive to beam exposure. Beam-induced hydrogen desorption is also expected, and evidence for this is presented at high doses. RBS has also been used to characterize LB films during thermal decomposition and to analyze the metal-oxide end products (Y 2 O 3 and Er 2 O 3 films). An example is also given where a mixed YBaCu-oxide film is deposited by this method.

Elemental analysis of algae with PIXE and PIGME

Abstract With an external proton beam, layers of algae a few μm thick produced by the simple procedure of filtering fixed amounts of a colony onto millipore filters have been examined. The biological aspects of the investigation dealt with the influence of F− on the growth of the algae; therefore proton inelastic scattering was used in addition to PIXE for the elemental analysis. Since the samples were thin (∼8 μm), the broad resonance structure in the 19 F( p , p ′γ) 19 F yield curve at Ep=2.03 Me V was used to increase the sensitivity. A sensitivity of 10 ppm was reached for these thin specimens, quite adequate to show that the algae take up fluorine from the environment. With beams of a few nanoamperes, the counting rate for all elements was found to decrease linearly with flux, reaching 70% of the initial value after about 1000 s. The concentration of fluorine in the algae was found to be proportional, up to 150 ppm, to the F− ion concentration in the nutrient. For one species of algae, S. leopoliensis, the presence of fluoride ion in the nutrient caused a marked change in the growth pattern. Furthermore, this change was found to be correlated with the concentration in the algae of one essential element, potassium, which the algae seemed to lose and then recover.

THE MECHANISMS OF FLUORIDE TOXICITY AND FLUORIDE RESISTANCE IN SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYCEAE)1

Fluoride was supplied as dissolved NaF at concentrations ranging from 0.26 to 7.9 mM (5–150 ppm) to three freshwater microalgae: Synechococcus leopoliensis (Racib.) Komarek (Cyanophyta), Oscillatoria limnetica Lemmermann (Cyanophyta) and Chlorella pyrenoidosa Chick (Chlorophyta). Growth of C. pyrenoidosa was unaffected by fluoride, and uptake of fluoride by this organism was not detectable. Growth of the cyanophytes was temporarily inhibited by NaF. The duration of this growth lag increased markedly as the pH was lowered at constant external fluoride concentration. In S. leopoliensis, fluoride uptake and inhibition of photosynthesis by NaF increased in the same way as did the growth lag in response to pH. Growth‐inhibitory NaF treatments decreased the ATP level in cells of S. leopoliensis by 75% and also abolished phosphate uptake. Cells of S. leopoliensis in which fluoride‐resistance was induced by prior growth in non‐growth‐inhibitory levels of NaF accumulated much less fluoride than did normal (“sensitive”) cells, and also did not respond to fluride by reduction of the ATP pool. It is suggested (1) that fluoride enters sensitive cells of S. leopoliensis principally as undissociated HF; (2) that its major inhibitory effect in these cells is the reduction in cellular ATP; (3)that fluoride‐resistant cells accumulate less fluoride by developing incresed permeability to the fluoride anion.

Hydrogen ingress into oxidized Zr-2.5Nb

Abstract Deuterium (D) distributions in D2O-corroded Zr-2.5 wt% Nb samples have been measured for three different corrosion temperatures: 763, 673 and 573 K. The oxide thickness and details in the D profile for 763 K show considerable variation due to changes in surface preparation. Nevertheless, the D concentration in the central part of the oxide is within 0.002−0.005 D Zr atomic ratio. At lower temperatures − 673 K and below — the D concentration is somewhat higher: ∼ 0.01 D Zr . These concentrations are less than the 2–5% level observed with Zircaloy-2 after corrosion at 763 K. D distributions in O2-oxidized Zr-2.5 wt% Nb have been measured following a subsequent exposure to either D2 or D2O exposure at 573 K. D ingress into the metal is enhanced with D2 compared to D2O, supporting the expectation that reducing conditions (D2) may lead to a degradation of the protective oxide. Evidence is presented that two different diffusing species are responsible for the different D distributions measured in the oxide for the two types of exposure.

Hafnium diffusion in Zircaloy-2 and Zr-2.5 wt.% Nb: A Rutherford backscattering study

Rutherford backscattering has been applied to the determination of the temperature dependence of Hf diffusion in the commercial alloys Zr−2.5 wt.% Nb and Zircaloy-2. The diffusion anneals were made, under vacuum, in the temperature interval 830–1101 K; the specimens were well annealed with large (7–20 μm) grains. The diffusion coefficients, D, are of the same order of magnitude as self-diffusion values (10−17 to 10−21 m2/s) over the comparable temperature range; also they display broadly similar temperature dependences. These results indicate that diffusion in the alloys, like that in pure Zr, is controlled by the presence of trace Fe in solid solution. The D values for Zircaloy-2 are generally higher than those for Zr-2.5 wt.% Nb, but the difference is relatively small. The small differences in substitutional diffusion among Zircaloy-2, Zr-2.5 wt.% Nb and Zr may be attributable to “compositional and microstructural effects”.

Below-surface analysis of inclusions with PIXE and PIGE

Abstract The composition of fluid inclusions in host minerals holds much information about the chemical environment of mineral formation. When solid inclusions are exposed through polishing, their content can readily be investigated with an electron or proton probe. However, with an electron probe, only the daughter minerals or the residue material left when a fluid inclusion is opened can be analyzed since electrons with energies of tens of keV cannot penetrate to the unexposed inclusion. On the other hand, proton beams of a few MeV can penetrate a few tens of μm of material and still be able to excite characteristic radiation. This phenomenon has been exploited for the analysis of subsurface inclusions. Ideally, standard petrographie sections are polished so that inclusions, targetted for analysis, are brought to within 10 μm of the surface. The overlying matrix reduces the sensitivity of PIXE for the elements of low Z such as Na and Al because of the attenuation of the X-rays. However, these elements, as well as elements of even lower Z , which cannot be analyzed with the electron probe, can readily be detected with PIGE at good sensitivity.