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Biologically Produced Sulfur 175 Fig. 4 The effect of salt concentration on the adsorption of charged polymers at a sur- face (especially Thiobacillus sp.). This section is about the physico-chemical properties of those biologically produced sulfur globules stored intracellu- larly and extracellularly. Research by Vetter [23] on externally excreted sulfur globules suggested that sulfur is stored as an energy reservoir rather than as a way of detoxifi- cation of excess hydrogen sulfide. As long as dissolved sulfide is available, sulfur is stored in globules. When sulfide is depleted, the stored sulfur is ox- idized. The identification of the nature of elemental sulfur in globules produced by sulfur compound oxidizing bacteria is not straightforward. Several stud- ies have been performed with several different techniques but unfortunately the interpretation of the results sometimes seems to be contradictory. Although several studies propose models for the nature of sulfur globules produced by all sulfur compound oxidizing bacteria [24–26] it is safer to distinguish between sulfur globules produced by different bacteria because there are indications that not all sulfur globules are comparable [27, 28]. To give a clear overview of what is known about the nature of sulfur in globules, studies on sulfur globules from each organism are discussed separately. 3.1 Intracellularly Stored Sulfur Several sulfide-oxidizing bacteria contain globules inside the cell membrane. The globules are membrane-limited and extracytoplasmic [17, 29]. In that way these globules could also be called extracellular, however we will call them intracellular because the globules are stored within the cell membrane. In 1887 Winogradsky had already stated that living cells would contain globules of amorphous sulfur while in dead cells the sulfur would crystallize (ref. in [1]). X-ray diffraction (XRD) on intracellularly produced sulfur glob- ules (among which Allochromatium sp.) has shown a difference between the isolated sulfur structure of fresh globules in the wet-state and the dried sul- fur globules. The XRD pattern for fresh globules resembled the pattern of liquid sulfur whereas the pattern for dried globules and globules that were kept in a wet-state for several days, resembled the pattern of crystalline or- thorhombic sulfur [1, 30]. This indicated that the structure of fresh sulfurPDF Image | Topics in Current Chemistry
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