Sulfur globules

Prange, A., Arzberger, I., Engemann, C., Modrow, H., Schumann, 0.,Trtiper, H. G., Steudel, R., Dahl, C., and Hormes, J. (1999). In situ analysis of sulfur in the sulfur globules of pho-totrophic sulfur bacteria by X-ray absorption near edge spectroscopy. Biochim. Biophys. Acta 1428,446 154. 

In general, sulfide is oxidized via sulfite to sulfate, while thiosulfate can either be oxidized only to tetrathionate or may be split into sulfide and sulfite. Both compounds are then further oxidized to sulfate. During anaerobic sulfide or thiosulfate oxidation, elemental sulfur appears as sulfur globules inside or outside the cells. Sulfur metabolism in Anoxyphotobacteria has two main functions ... 

During anaerobic sulfide oxidation only the members of Chromatiaceae store the elemental sulfur inside their cells, while the Ectothiorhodospiraceae, Chlorobiaceae and Chloroflexaceae deposit elemental sulfur outside their cells (see 4). For a long time it was assumed that the sulfur globules contain the orthorhombic cycloocta sulfur consisting of Sa-rings (1). 

The above-mentioned findings on the structure of sulfur globules are more or less confirmed by Mas and van Gemerden (20) concerning the structure of the globules of Chromatium. The authors assumed that the sulfur globules could consist of a concentrical array of layers of sulfanes, arranged parallel to... 

Only the intracellular sulfur globules of Chromatiaceae proved to be surrounded by a protein layer consisting of a single protein with a molecular weight of 13,500 aaltons (21). The existence of such a protein layer could not clearly be proved for the extracellularly stored sulfur globules of Chlorobiaceae and Ectothiorhodospiraceae (18). 

Figure 5. Thick sulfur-asphalt film from a Thermopave mix and large sulfur globules resulting from coagulation...

In cultures of sulfur bacteria the reactions at Eqs. (20)-(22) are catalyzed by appropriate enzymes. The sulfane monosulfonate formed in the reaction at Eq. (22) is unstable and decomposes to polythionate anions and elemental sulfur which are the components of Raffo and 8elmi sols as shown above see Eqs. (10) and (11). These reactions take place, for example, if cultures of Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans) are incubated (mixed) with tetra- or pentathionate at 30 °C in air. After some time all polythionate anions with up to 17 sulfur atoms could be detected by ion-pair chromatography in the filtrated aqueous phase while the sulfur globules excreted extracellularly were isolated and extracted by C82. According to a HPLC analysis this extract contained 8s and small amounts of the homocycles 85, 87, and 812 [53]. 

Keywords Sulfur bacteria Colloidal stability Sulfur globules Hydrogen sulfide Biotechnology... 

Organism Energy Carbon source Sulfur globules pH of growth Ref... 

Elemental sulfur is often observed as an intermediate product in the oxidation of sulfide to sulfate in sulfur compound oxidizing bacteria. It can be present in considerable concentrations but will eventually be further oxidized to sulfate. The elemental sulfur is stored in sulfur globules, which some bacteria deposit inside the cell membrane and others outside the cell membrane. Later in this chapter the properties of these sulfur globules are discussed. 

In the last 30 years a significant amount of research has been done on sulfur globules that are stored intracellularly (especially sulfur globules of AL vinosum and Beggiatoa alba) and that are excreted outside the cell membrane... 

Research by Vetter [23] on externally excreted sulfur globules suggested that sulfur is stored as an energy reservoir rather than as a way of detoxification of excess hydrogen sulfide. As long as dissolved sulfide is available, sulfur is stored in globules. When sulfide is depleted, the stored sulfur is oxidized. 

The sulfur globules of the chemotrophic bacteria B. alba have also been studied quite extensively. The density of the globules has not been determined but energy-dispersive X-ray microanalysis showed that the globules consisted almost entirely of sulfur [9]. XANES [28] and Raman spectroscopy [26] confirmed that the sulfur globules of B. alba consist of Ss sulfur rings. 

Sulfur globules produced by bacteria excreting the globules outside the cell membrane have also been studied. Especially for biotechnological applications these sulfur compound oxidizing bacteria are interesting since they allow an easy separation of the sulfur from the bacteria. In Fig. 5 sulfur globules can be seen which have been excreted by a Thiobacillus bacterium. 

Steudel proposed a number of models for the composition of bacterial sulfur globules in which he did not always distinguish between intracellular-ly and extracellularly stored sulfur. In a model proposed for sulfur globules excreted by Acidithiobacillus ferrooxidans [39] the globules consist of a sulfur nucleus (mainly Ss rings and small amounts of other sulfur rings) and... 

Fig. 6 Vesicle model proposed by Steudel for sulfur globules excreted by Ac. ferrooxi-dans. Vesicles are composed of long chain polythionates ( 03S-Sn-S03 ) in which small amounts of sulfur rings can be present (after [24])...

It should be noted that polythionates are only stable at low pH [33] and it is therefore unlikely that the polythionate vesicle model is appHcable to sulfur globules produced by bacteria growing at another pH Aan the acidic conditions at which Ac. ferrooxidans grows [5] (see Fig. 6). 

As stated above, the presence of proteins on the surface of sulfur globules stored intracellularly has been demonstrated [8, 37]. These well-defined proteins act as a membrane between the cytoplasm and the intracellular sulfur particle. It is not known whether the proteins associated with the sulfur particles excreted by Thiobacillus bacteria are well-defined proteins synthesized by the bacterium or if they are originating from organic compounds already present in the liquid reactor system. 

Stored extracellularly or intracellularly. Van Gemerden however suggested that the location of the electron acceptor in the HS oxidation step determines whether sulfur is stored extracellularly or intracellularly [46] as is explained in Fig. 8. This is supported by research of Then and Triiper [8] on sulfide oxidation in Halorhodospira abdelmalekii, excreting sulfur globules extracellularly. They showed the cytochrome c-551 to have a catalytic effect on the oxidation of sulfide and to be located on the outside of the cell membrane. In Al. vinosum cytochrome c-551 is located in the periplasmic space, the space between the outer cell wall and the cytoplasmic membrane, which is also the location of storage of sulfur globules [47]. 

Prange A, Chausvistre R, Modrow H, Hormes J, Truper HG, Dahl C (2002) Quantitative speciation of sulfur in bacterial sulfur globules X-ray absorption spectroscopy reveals at least three different species of sulfur. Microbiology 148 267-276... 

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