Institut für Mikrobiologie & Biotechnologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Meckenheimer Allee 168, D-53115 Bonn, Germany1
Physikalisches Institut, Rheinische Friedrich-Wilhelms-Universität Bonn, Nußallee 12, D-53115 Bonn, Germany2
Author for correspondence: Christiane Dahl,b. Tel: +49 228 732119. Fax: +49 228 737576. e-mail: ChDahl@uni-bonn.de
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Keywords: XANES, sulfur K-edge, quantitative analysis, sulfur-oxidizing bacteria, sulfur globules
Abbreviations: XANES, X-ray absorption near edge structure
a Present address: The John Bennett Johnston, Sr Center for Advanced Microstructures & Devices (CAMD), Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806, USA.
b Correspondence on the physical aspects of the work should be sent to Hartwig Modrow (Tel: +49 228 733203. Fax: +49 228 737869. e-mail: Modrow{at}physik.uni-bonn.de).
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
X-ray absorption near edge structure (XANES) spectroscopy at the sulfur K-edge using synchrotron radiation is an excellent tool to study the speciations of sulfur in biological samples (Rompel et al., 1998 ; Pickering et al., 1998
; Prange et al., 1999
, 2001
). The method is non-destructive and measurements can be performed in situ (e.g. aquatic environmental samples and cultured bacteria in liquid media). XANES allows the determination of the valence of an exited atom as well as the electronegativity of neighbouring atoms (Bianconi, 1988
). In addition, the lengths of sulfur chains can be determined at least in a given range (
3 sulfur atoms) (Chauvistré et al., 1997
). Furthermore, XANES spectra can yield information on the type of the chemical bond in the second coordination shell of the excited sulfur atom (e.g. CC single, double or triple bonds). The fact that the local environment of the absorbing atoms is probed implies that XANES spectra are additive, i.e. the spectrum of a mixture of substances A and B can be composed from the separately measured spectra of A and B. This additivity is the basis for the quantitative analysis of XANES spectra which means the decomposition of a sum spectrum into the components it is composed of. To achieve this decomposition, a quality function defined by the difference between experimental data and a linear combination of spectra contained in a basis set can be minimized (e.g. Modrow et al., 2001
).
Using XANES spectroscopy as a fingerprint method, comparing the obtained spectra with those of reference compounds, we recently found evidence for the presence of long sulfur chains (>3 sulfur atoms), most probably terminated by C or H atoms, in different phototrophic sulfur bacteria (Prange et al., 1999 ). In contrast, Pickering et al. (2001)
very recently concluded on the basis of theoretical considerations that the form of sulfur that most resembles the globule sulfur in metabolically different sulfur-accumulating bacteria is simple, solid S8. This conclusion contradicts a number of previous experimental results (Hageage et al., 1970
; Guerrero et al., 1984
; Mas & van Gemerden, 1987
). In the experimental study presented here, we reach a different conclusion which is in good agreement with previous experimental results. The present study aimed at clarifying whether different sulfur species are present in physiologically and phylogenetically diverse sulfur-oxidizing bacteria. A least-square fitting routine was used to determine possible contributions of different reference compounds to the bacterial spectra, thereby allowing a quantitative analysis of sulfur in the bacterial sulfur globules.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Preparation of sulfur globules.
Sulfur globules of Allochromatium vinosum grown photoautotrophically on sulfide were prepared according to the procedure of Schmidt et al. (1971) and Brune (1995)
. The cells were disintegrated by ultrasonification, the sulfur globules were sedimented in a centrifuge and the supernatant was removed with a pipette.
X-ray absorption spectroscopy
Experimental.
XANES spectra were recorded at beamline BN3 using synchrotron radiation from the electron stretcher accelerator (ELSA) of the Institute of Physics at the University of Bonn (Althoff et al., 1990 ). The storage ring was operated at an energy of 2·3 GeV with electron currents between 20 and 100 mA. The synchrotron radiation was monochromatized by a modified Lemonnier-type double-crystal X-ray monochromator (Lemonnier et al., 1978
) equipped with InSb (111) crystals. The monochromatic flux rate per second on the sample was about 109 photons (at 50 mA). Measurements were performed in transmission mode with ionization chambers (60 mbar air inside), measuring the beam intensities in front of and behind the sample. For energy calibration of the spectra, the spectrum of ZnSO4 was used as a secondary standard setting the maximum of the first resonance (white line) to an energy of 2481·4 eV. According to the step width, this value is reproducible to ±0·1 eV. Further details of the experimental procedure have been published previously (Winter et al., 1995
; Chauvistré et al., 1997
). Spectra were scanned with step widths of 0·6 eV in the pre-edge region between 2450 and 2460 eV, 0·09 eV between 2460 and 2490 eV, the main region of interest, and 0·2 eV between 2490 and 2510 eV. Typical integration times were 5001000 ms per point. A linear background determined in the pre-edge region was subtracted from the raw data to correct the absorption from higher shells and from supporting materials. Spectra were normalized at 2490 eV, where the variation of the absorption cross-section is already very small. In some cases, spectra were normalized to the maximum of the white line for a better comparison e.g. of energy positions.
MINUIT fitting of spectra.
The interactive fitting and plotting package Mn-Fit 4.04/15 was used (available at: http://www-zeus.physik.uni-bonn.de/~brock/mn_fit.html). Mn-Fit 4.04/15 uses the function minimization tool MINUIT which is part of the CERNlib available at CERN (http://wwwinfo.cern.ch/asdoc/minuit/node2.html) to fit histograms or data. MINUIT is a tool to find the minimum value of a multi-parameter function, and it analyses the shape of the function around the minimum. In our case, a chi-square function was used to compute the best fit parameter values. For a set of reference spectra, MINUIT finds a linear combination of these spectra which reproduces the XANES spectra with the highest probability. Non-statistical errors may occur from an incomplete set of reference spectra or from reference spectra not monitoring exactly the local sulfur environment. The error of the percentage contributions of sulfur (Tables 1, 2
and 3
) can be estimated to be ±10%.
|
|
|
![]() |
RESULTS AND DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Verification of the quantitative analysis
In order to verify our approach, we performed analysis of a defined mixture composed of cystine (47%), cysteine (24%), reduced glutathione (22%) and oxidized glutathione (7%). Such a mixture presents a demanding example because the white lines of these substances are quite similar in shape and overlap considerably (Fig. 1). The results of the quantitative analysis are given in Table 1
and Fig. 2(a)
. Evidently, the results are independent of the energy range the fitting is performed on, which indicates that stable background correction and normalization procedures have been employed. This is also immediately clear when looking at Fig. 2(a)
, as the fits reproduce the shape of the experimental data equally well for each energy range used in the fitting routine. It should be stressed that this is true even if we restrict ourselves exclusively to the electronic information provided by the white line. In Fig. 2(a)
it is also apparent that experimental data and the fit using the longest energy range run in parallel to each other, which again shows the stability of the background subtraction procedures.
We would like to strengthen further the case for the stability and reliability of our results against the background of the previously mentioned paper dealing with X-ray absorption spectroscopy on sulfur bacteria by Pickering et al. (2001) . In this paper, it is erroneously assumed that there is no difference between the S K-edge XANES spectra of carefully ground cyclooctasulfur and polymeric sulfur, but that the observed differences between these spectra are due to experimental artefacts. If this were true, it should not be possible to perform a correct quantitative analysis of a mixture of these two compounds. To test this presumption, we analysed two independently prepared defined mixtures of cyclooctasulfur (25%) and polymeric sulfur (75%). MINUIT fits of the spectra obtained covering two different energy ranges were performed (Fig. 2b
, c
, Table 2
) yielding correct results with errors of only
5%. Pickering et al. (2001)
assume that the reduced white line of cyclooctasulfur is induced by pinhole effects. As we find that the white line of the spectrum of polymeric sulfur is comparatively much more intense (Fig. 1a
) one would have to assume that pinhole effects do not significantly influence the spectrum in this case. Based on this hypothesis, pinhole effects should be reduced in a mixture of cyclooctasulfur and polymeric sulfur. Consequently, the intensity of the white line of such a mixture would have to be increased, thus resulting in artificially high amounts of polymeric sulfur after quantitative analysis. To accommodate this increase, the contribution of polymeric sulfur would have to be overestimated by the fitting routine. However, we do not find evidence for such an effect (Table 2
). From Fig. 2(b
, c
) it is evident that the fitting of the white line alone is again sufficient to extract the relative contributions of the different references. This is especially important as possible thickness or pinhole effects affect resonances of different intensity to a different extent (Parrat et al., 1957
; Stern & Kim, 1981
). Consequently, the stability of the fitting results with respect to the energy range of the fit supports the conclusion that we are not dealing with experimental artefacts. Again, including an increased energy range in the fitting routine does not affect the fit significantly and the long-range fit runs in parallel to the experimental data in both cases, proving yet again the stability of the background treatment. Fig. 2(d)
shows the residuum between the experimental spectra (spectrum sample 2 minus spectrum sample 1) displayed in Fig. 2(b
, c
), suggesting an explanation for the slightly different results. It is evident from this figure that, both in the area of the white line and the shape resonances, the difference between the spectra exceeds the noise level significantly. We attribute this fact, which we also encountered with other reference mixtures, to slight inhomogeneities of the distribution of the various compounds in the sample which remain even after a careful mixing procedure.
Sulfur as sulfur rings (cyclooctasulfur)
In Fig. 3, the S K-edge XANES spectra of Thiomargarita namibiensis are shown together with their accompanying MINUIT fits. Both species appear to thrive best under microoxic or anoxic conditions, although atmospheric oxygen conditions are not toxic (Schulz et al., 1999
). They both store sulfur in globules inside the cells. B. alba stores sulfur in special protein envelopes (Strohl et al., 1981
, 1982
) while the sulfur is present in the thin outer layer of the cytoplasm in T. namibiensis, which oxidizes sulfide to sulfur while reducing nitrate (Schulz et al., 1999
). For Beggiatoa species it is clear that the sulfur is located not in the cytoplasm but in the periplasm of these Gram-negative organisms, while the exact subcellular localization in T. namibiensis has not been determined (Larkin & Strohl, 1983
; Schulz et al., 1999
). The sulfur in the globules of both T. namibiensis and B. alba appears to consist mainly of cyclooctasulfur. A small minority of longer sulfur chains is also present. This can be deduced from the fact that MINUIT is able to fit the white lines of spectra quite well with only two reference compounds: cyclooctasulfur and polymeric sulfur (Table 3
). Sulfate peaks in the spectra (2481·4 eV) and sulfate as indicated by the MINUIT result from salt water in the case of T. namibiensis (Fig. 3
, B
). In the case of the B. alba strain used (which is not able to oxidize sulfur to sulfate) sulfate results from the medium (Fig. 3
, C
). This medium consists of a saturated calcium sulfate solution (Beggiatoa glides fastened on to the salt crystals) and it was not possible to remove sulfate completely. Our findings support the assumption that these organisms oxidize sulfide to elemental sulfur (cyclooctasulfur) stored in sulfur globules. This could be part of a detoxification strategy in the case of Beggiatoa to cope with sulfide in coastal sediments, their natural environment. Thus, one can assume that the sulfur in other species of these or closely related genera is also deposited as cyclooctasulfur. Our results and assumptions are supported by a recent Raman spectroscopic analysis of environmental samples of Beggiatoa spp. Raman spectroscopy indicated the same speciation of sulfur (S8 rings) in Beggiatoa spp. and also in environmental Thioploca spp. samples and in sulfur-depositing endosymbionts of a vesicomyid clam (Pasteris et al., 2001
).
Sulfur as polythionates
The chemotrophic sulfur oxidizer Acidithiobacillus ferrooxidans is able to grow at extremely low pH values, to oxidize Fe2+ ions or to grow on reduced sulfur compounds (Kelly & Wood, 2000 ). When there is a lot of substrate, very small extracellular sulfur globules may be formed and later oxidized to sulfate (Steudel et al., 1987
). In Fig. 4
, the S K-edge XANES spectrum of A. ferrooxidans and accompanying MINUIT fit is shown. The increased noise level of the spectrum of A. ferrooxidans is due to the weak absorption of this sample, which in turn results from the low concentration of small sulfur globules in the culture investigated. We found that sulfur in the sulfur globules of A. ferrooxidans appears as sulfur chains which are most probably terminated by
groups (polythionates), by fitting the whole spectrum on the basis of polymeric sulfur (
53%), tetrathionate (
23%), reduced glutathione (
10%) and thiosulfate (
8%) (Table 3
). The latter might be a leftover from thiosulfate in the medium. The sulfonate group of the tetrathionate is indicated by the clear signal at 2480·1 eV (Fig. 1b
), the typical energy position for sulfonate sulfur. These results support the assumption that polythionates are the dominant sulfur species in sulfur globules of A. ferrooxidans. Our finding is in good accordance with previous conclusions based on HPLC analyses of Steudel et al. (1987
), who found polythionates with long sulfur chains (
, n=3...22) in cultures of A. ferrooxidans. However, the simple sulfur globule model proposed by Steudel et al. (1987)
suggests the presence of polythionates and mainly cyclooctasulfur. MINUIT fitting of the spectrum did not indicate S8 rings at all (Table 3
), so this model has to be at least partly revised. From our data, we conclude that the sulfur in the globules of A. ferrooxidans mainly consists of polythionates. No significant contribution of cyclooctasulfur (which can be formed in the medium by chemical processes) was found, but its presence cannot be ruled out completely.
|
|
|
Analysis of isolated sulfur globules from Allochromatium vinosum
Measurements of isolated sulfur globules from anaerobically grown Allochromatium vinosum (globules were isolated 2·5 h after feeding with sulfide) showed completely different spectra (Fig. 6, A
) as compared to those of intact cells (Fig. 6
, B
). Sulfur was found mainly as cyclooctasulfur (
92%) and a minority as sulfate (
7%), which is found in contrast to the chain sulfur structure for intact cells of A. vinosum (Table 3
). During extraction of the globules from the cells, the integrity of the cells was destroyed and oxygen was present (Schmidt et al., 1971
), which might have led to spontaneous sulfur ring formation. This shows clearly the necessity of in situ measurements and a non-destructive analysis of the cells, as is the case when XANES spectroscopy is used.
Conclusions
The spectra and the corresponding quantitative analyses presented in this paper showed clear differences in the speciation of sulfur in the sulfur globules of different metabolic groups of sulfur-oxidizing bacteria, reflecting their ecological and physiological properties. In situ XANES measurements revealed at least three different sulfur species: sulfur rings (cyclooctasulfur) in the microaerobic chemotrophic sulfur-oxidizing bacteria B. alba and T. namibiensis, polythionates in the aerobic chemotroph A. ferrooxidans, and sulfur chains in the anaerobically grown phototrophic sulfur bacteria.
Many purple bacteria of the family Chromatiaceae are able to grow chemolithoautotrophically under micro- to semioxic conditions using reduced sulfur compounds as electron donors and oxygen as electron acceptor (Pfennig & Trüper, 1992 ). In the future, it will be important to find out whether the sulfur globules formed under these conditions resemble those formed by phototrophically grown cells (sulfur chains) or those formed by chemolithotrophic sulfur bacteria. Such studies would also help to clarify which factors (environmental conditions, metabolic pathways, phylogenetic affiliation of the organisms) govern the speciation of the stored sulfur. Sulfur of isolated sulfur globules from anaerobically grown A. vinosum was found as cyclooctasulfur, indicating the necessity of methods like XANES that can be applied to avoid destruction of the original sulfur environment.
![]() |
ACKNOWLEDGEMENTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bartsch, R. G., Newton, G. L., Sherill, C. & Fahey, R. C. (1996). Glutathione amide and its perthiol in anaerobic sulfur bacteria. J Bacteriol 178, 4742-4746.[Abstract]
Bianconi, A. (1988). XANES-spectroscopy. In X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES , pp. 573-662. Edited by D. C. Koningsberger & R. C. Prins. New York:Wiley.
Brune, D. C. (1995). Isolation and characterization of sulfur globules proteins from Chromatium vinosum and Thiocapsa roseopersicina. Arch Microbiol 163, 391-399.[Medline]
Chauvistré, R., Hormes, J., Hartmann, E., Etzenbach, N., Hosch, R. & Hahn, J. (1997). Sulfur K-shell photoabsorption spectroscopy of the sulfanes R-Sn-R, n=24. Chem Phys 223, 293-302.
Cohn, F. (1875). Untersuchungen über Bakterien. Beitr Biol Pflanz 1, 141-207.
Cramer, C. (1870). In Chemisch-Physikalische Beschreibung der Thermen von Baden in der Schweiz. Edited by C. Müller. Baden: C. Müller.
Donohue, J. (1974). The Structures of the Elements. New York: Wiley.
Guerrero, R., Mas, J. & Pedrós-Alió, C. (1984). Buoyant density changes due to intracellular content of sulfur in Chromatium warmingii and Chromatium vinosum. Arch Microbiol 137, 350-356.
Hageage, G. J.Jr, Eanes, E. D. & Gherna, R. L. (1970). X-ray diffraction studies of the sulfur globules accumulated by Chromatium species. J Bacteriol 101, 464-469.[Medline]
Imhoff, J. F. (1992). The family Ectothiorhodospiraceae. In The Prokaryotes , pp. 3222-3229. Edited by A. Balows, H. G. Trüper, M. Dworkin, W. Harder & K. H. Schleifer. New York:Springer.
Imhoff, J. F., Süling, J. & Petri, R. (1998). Phylogenetic and taxonomic reclassification of Chromatium species and related purple sulfur bacteria. Int J Syst Bacteriol 48, 1129-1143.
Kelly, D. P. & Wood, A. P. (2000). Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermothiobacillus gen. nov. Int J Syst Evol Microbiol 50, 511-516.[Abstract]
Larkin, J. M. & Strohl, W. R. (1983). Beggiatoa, Thiothrix and Thioploca. Annu Rev Microbiol 37, 341-367.[Medline]
Lemonnier, M., Collet, O., Depautex, C., Esteva, J.-M. & Raoux, D. (1978). High vacuum two crystal soft X-ray monochromator. Nucl Instr Methods 152, 109-111.
Mas, J. & van Gemerden, H. (1987). Influence of sulfur accumulation and composition of sulfur globule on cell volume and buoyant density of Chromatium vinosum. Arch Microbiol 146, 362-369.
Modrow, H., Visel, F., Zimmer, R. & Hormes, J. (2001). Monitoring thermal oxidation of sulfur crosslinks in SBR-elastomers by quantitative analysis of sulfur K-edge XANES-spectra. Rubber Chem Technol 74, 281-294.
Parrat, L. G., Hempstead, C. F. & Jossem, E. L. (1957). Thickness effect in absorption spectra near absorption edges. Phys Rev 105, 1228-1232.
Pasteris, J. D., Freeman, J. J., Goffredi, S. K. & Buck, K. R. (2001). Raman spectroscopic and laser scanning confocal microscopic analysis of sulfur in living sulfur-precipitating marine bacteria. Chem Geol 180, 3-18.
Pattaragulwanit, K., Brune, D. C., Trüper, H. G. & Dahl, C. (1998). Molecular genetic evidence for extracytoplasmatic localization of sulfur globules in Chromatium vinosum. Arch Microbiol 169, 434-444.[Medline]
Pfennig, N. & Trüper, H. G. (1992). The family Chromatiaceae. In The Prokaryotes , pp. 3584-3592. Edited by A. Balows, H. G. Trüper, M. Dworkin, W. Harder & K. H. Schleifer. New York:Springer.
Pickering, I. J., Prince, R. C., Divers, T. & George, G. N. (1998). Sulfur K-edge X-ray absorption spectroscopy for determining the chemical speciation of sulfur in biological systems. FEBS Lett 441, 11-14.[Medline]
Pickering, I. J., George, G. N., Yu, E. Y., Brune, D. C., Tuschak, C., Overmann, J., Beatty, J. T. & Prince, R. C. (2001). Analysis of sulfur biochemistry of sulfur bacteria using X-ray absorption spectroscopy. Biochemistry 40, 8138-8145.[Medline]
Prange, A., Arzberger, I., Engemann, C., Modrow, H., Schumann, O., Trüper, H. G., Steudel, R., Dahl, C. & Hormes, J. (1999). In situ analysis of sulfur in the sulfur globules of phototrophic sulfur bacteria by X-ray absorption near edge spectroscopy. Biochim Biophys Acta 1428, 446-454.[Medline]
Prange, A., Kühlsen, N., Birzele, B., Arzberger, I., Hormes, J., Antes, S. & Köhler, P. (2001). Sulfur in wheat gluten: in situ analysis by X-ray absorption near edge structure (XANES) spectroscopy. Eur Food Res Technol 212, 570-575.
Pott, A. S. & Dahl, C. (1998). Sirohaem sulfite reductase and other proteins encoded by genes at the dsr locus of Chromatium vinosum are involved in the oxidation of intracellular sulfur. Microbiology 144, 1881-1894.[Abstract]
Rompel, A., Cinco, R. M., Latimer, M. J. & 7 other authors (1998). Sulfur K-edge X-ray absorption spectroscopy: a spectroscopic tool to examine the redox state of S-containing metabolites in vivo. Proc Natl Acad Sci USA 95, 61226127.
Schmidt, G. L., Nicolson, G. L. & Kamen, M. D. (1971). Composition of the sulfur particle of Chromatium strain D. J Bacteriol 105, 1137-1141.[Medline]
Schulz, H., Brinkhoff, T., Ferdelman, T. G., Marine, M. H., Teske, A. & Jørgensen, B. B. (1999). Dense populations of a giant sulfur bacterium in Namibian shelf sediments. Science 284, 493-495.
Siefert, E. & Pfennig, N. (1987). Convenient method to prepare neutral sulfide solution for cultivation of phototrophic sulfur bacteria. Arch Microbiol 139, 100-101.
Stern, E. A. & Kim, K. (1981). Thickness effect on the extended-X-ray-absorption-fine-structure amplitude. Phys Rev B 23, 3781-3787.
Steudel, R. (1989). On the nature of the elemental sulfur (S0) reduced by sulfur-oxidizing bacteria a model for S0 globules. In Autotrophic Bacteria , pp. 289-303. Edited by H. G. Schlegel & B. Bowien. Madison, WI:Science Technology Publishers.
Steudel, R. (1996). Das gelbe Element und seine erstaunliche Vielseitigkeit. Chemie in unserer Zeit 30, 226-234.
Steudel, R. & Albertsen, A. (1999). The chemistry of aqueous sulfur sols models for bacterial sulfur globules? In Biochemical Principles and Mechanisms of Biosynthesis and Biodegradation of Polymers , pp. 17-26. Edited by A. Steinbüchel. Weinheim:Wiley-VCH.
Steudel, R., Holdt, G., Göbel, T. & Hazeu, W. (1987). Chromatographic separation of higher polythionates Sn (n=3...22) and their detection in cultures of Thiobacillus ferrooxidans: molecular composition of bacterial sulfur secretions. Angew Chem Int Ed Engl 26, 151-153.
Steudel, R., Holdt, G., Visscher, P. T. & van Gemerden, H. (1990). Search for polythionates in cultures of Chromatium vinosum after sulfide incubation. Arch Microbiol 153, 432-437.
Strohl, W. R., Geffers, I. & Larkin, J. M. (1981). Structure of the sulfur inclusion envelopes from four Beggiatoas. Curr Microbiol 6, 75-79.
Strohl, W. R., Howard, K. S. & Larkin, J. M. (1982). Ultrastructure of Beggiatoa alba strain B15LD. J Gen Microbiol 128, 73-84.
Sze, K. H., Brion, C. E., Tronc, M., Bodeur, S. & Hitchcock, A. P. (1988). Inner and valence shell electronic excitation of dimethyl sulfoxide by electron energy loss and photoabsorption spectroscopies. Chem Phys 121, 279-297.
Then, J. & Trüper, H. G. (1983). Sulfide oxidation in Ectothiorhodospira abdelmalekii. Evidence for the catalytic role of cytochrome c-551. Arch Microbiol 135, 254-258.
Then, J. & Trüper, H. G. (1984). Utilization of sulfide and elemental sulfur by Ectothiorhodospira halochloris. Arch Microbiol 139, 295-298.
Trüper, H. G. & Hathaway, J. C. (1967). Orthorhombic sulfur formed by photosynthetic sulphur bacteria. Nature 215, 435-436.[Medline]
Winogradsky, S. N. (1887). Über Schwefelbakterien. Bot Ztg 45, 489-508.
Winter, I., Hormes, J. & Hiller, M. (1995). Thermal ageing of electrically conducting polymers: XANES measurements of polythiophenes. Nucl Instr Methods B97, 287-291.
Received 18 June 2001;
revised 17 September 2001;
accepted 26 September 2001.
HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
INT J SYST EVOL MICROBIOL | MICROBIOLOGY | J GEN VIROL |
J MED MICROBIOL | ALL SGM JOURNALS |