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172 Wilfred E. Kleinjan et al. noted that not all sulfur compound oxidizing bacteria are capable of all the reactions listed in this table. With respect to the carbon source, the sulfur compound oxidizing bacte- ria can be classified as heterotrophs, autotrophs, or mixotrophs. Autotrophic bacteria use CO2 as the major carbon source for production of organic mole- cules. Heterotrophic bacteria use organic material as carbon source. Obligate autotrophs strictly need CO2 as carbon source and facultative autotrophs (or mixotrophs) can also grow heterotrophically. Heterotrophic sulfur com- pound oxidizing microorganisms need a nutrient feed containing organic substrates (e.g., meat or yeast extracts [11]). Sulfur compound oxidizing bacteria can also be classified according to the nature of the hydrogen source. Lithotrophic bacteria use inorganic substrates (e.g., NH3, H2S) as a source for hydrogen atoms. Organotrophic bacteria obtain hydrogen from organic molecules. In most organisms lithotrophy is linked to autotrophy, which means that for instance obligate autotrophic Thiobacillus bacteria are also called chemolithotrophic. Elemental sulfur is often observed as an intermediate product in the oxi- dation of sulfide to sulfate in sulfur compound oxidizing bacteria. It can be present in considerable concentrations but will eventually be further oxi- dized 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. The optimum pH of growth of the bacteria differs per species. Neutral and acidiphilic sulfur compound oxidizing bacteria have been long known, mostly isolated from marine or freshwater sediments (e.g., [2, 5]). From highly alkaline salt lakes a number of alkaliphilic sulfur compound oxidizing bacteria have been isolated, both phototrophic [8] and chemotrophic [10, 16]. The technological relevance of alkaliphilic sulfur compound oxidizers is discussed later in this chapter. 2 Colloidal Stability of Sulfur Particles Sulfur particles formed by sulfur compound oxidizing bacteria have particle sizes that fall in the range of the colloidal domain (approximately up to 1.0 mm). The internally stored sulfur particles produced by four different chemotrophic Beggiatoas as well as by a phototrophic Allochromatium bac- terium have a particle diameter around 250 nm (both observed with electron microscopy) [17, 18] but the diameter of internally stored sulfur globules can reach up to 1 mm. Extracellularly stored sulfur globules produced by Thiobacillus bacteria in a sulfide-oxidizing reactor initially have approxi- mately the same diameter (observed with electron microscopy and size dis- tribution measured with Single Particle Optical Sizing) [19]. The balance between attractive and repulsive forces governs stability of colloidal sulfur particles. The most important forces are the van der Waals attraction and electrostatic repulsion, on which the DLVO-theory of colloidalPDF Image | Topics in Current Chemistry
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