The Sulfur Oxidizing Bacteria

The sulfur-oxidizing bacteria are Gram-negative rods or spirals, which sometimes grow in filaments. They obtain energy by oxidizing reduced sulfur compounds, including hydrogen sulfide, elemental sulfur, and thiosulfate. Molecular oxygen serves as a terminal electron acceptor, generating sulfuric acid. These bacteria play an important role in the sulfur cycle. ■ sulfur cycle, p. 777

Filamentous Sulfur Oxidizers

Species of the filamentous sulfur oxidizers Beggiatoa and Thiothrix live in sulfur springs, in sewage-polluted waters, and on the surface of marine and freshwater sediments. They accumulate sulfur, depositing it as intracellular granules. Members of the genera Beggiatoa and Thiothrix differ in the nature of their filamentous growth (figure 11.10). The filaments of Beggiatoa species move by gliding motility, the mechanism of which is poorly understood. The filaments may flex or twist to form a tuft. In contrast, the filaments of Thiothrix species are immobile; they fasten at one end to rocks or other solid surfaces. Often they attach to other cells, causing the filaments to form a characteristic rosette arrangement. Progeny cells detach from the ends of these filaments and use gliding motility to disperse to new locations, where they form new filaments. Overgrowth of these filamentous organisms in sewage at treatment facilities causes a problem called bulking. Because the masses of filamentous organisms do not settle easily, bulking interferes with the separation of the solid sludge from the liquid effluent. ■ sewage treatment, p. 786

Unicellular Sulfur Oxidizers

Thiobacillus species are found in both terrestrial and aquatic habitats, where their ability to oxidize metal sulfides is responsible for a process called bioleaching. In this process, insoluble metal sulfides are oxidized, producing sulfuric acid while converting the metal to a soluble form; some species can produce enough acid to lower the pH to 1.0. Bioleaching can cause severe environmental problems. For example, the strip mining of coal exposes metal sulfides, which can then be oxidized by Thiobacillus species to produce sulfuric acid. The resulting runoff can acidify nearby streams, killing trees, fish, and other wildlife (figure 11.11). The runoff may also contain toxic metals made soluble by the bacteria. Under controlled conditions, however, bioleaching can enhance the recovery of metals. For example, gold can be extracted from deposits of gold sulfide. The metabolic activities of Thiobacillus species can also be used to prevent acid rain, which results from the burning of sulfur-containing coals and oils. After allowing the bacteria to oxidize the sulfur to sulfate, the latter can be extracted from the fuel. One species of Thiobacillus, T. fer-rooxidans, can oxidize iron as well as sulfur.

280 Chapter 11 The Diversity of Prokaryotic Organisms

280 Chapter 11 The Diversity of Prokaryotic Organisms

Sulfur Granules Filamentous Bacteria
10 mm

10 mm

Figure 11.10 Filamentous Sulfur Bacteria Phase-contrast photomicrographs. (a) Multicellular filament of a Beggiatoa species. (b) Multicellular filaments of a Thiothrix species, forming a rosette arrangement.

10 mm

Figure 11.10 Filamentous Sulfur Bacteria Phase-contrast photomicrographs. (a) Multicellular filament of a Beggiatoa species. (b) Multicellular filaments of a Thiothrix species, forming a rosette arrangement.

The Nitrifiers

Nitrifiers are a diverse group of Gram-negative bacteria that obtain energy by oxidizing inorganic nitrogen compounds such as ammonia or nitrite. These bacteria are of particular interest to farmers who fertilize their crops with ammonium nitrogen, a form of nitrogen that is retained by soils because its positive charge enables it to adhere to negatively charged soil particles. The potency and longevity of the fertilizer are affected by nitrifying bacteria converting the ammonia to nitrate. While plants use this form of nitrogen more readily, it is rapidly leached from soils. Nitrifying bacteria are also an important consideration in disposal of sewage or other wastes with a high ammonia concentration. As nitrifying bacteria oxidize nitrogen compounds, they consume O2. Because of this, waters polluted with nitrogen-containing wastes can quickly become anaerobic.

The nitrifiers encompass two metabolically distinct groups of bacteria that typically grow in close association. Together, they can oxidize ammonia to form nitrate. The ammonia oxidizers,

Sulfur Oxidizing Bacterium
Figure 11.11 Acid Drainage from a Mine Sulfur-oxidizing bacteria oxidize exposed metal sulfides, generating sulfuric acid.The yellow-red color is due to insoluble iron oxides.

which include the genera Nitrosomonas and Nitrosococcus, convert ammonia to nitrite. The nitrite oxidizers, which include the genera Nitrobacter and Nitrococcus, then convert nitrite to nitrate. The latter group is particularly important in preventing the buildup of nitrite in soils, which is toxic and can leach into groundwater. The oxidation of ammonia to nitrate is called nitrification and is an important part of the nitrogen cycle. ■ nitrogen cycle, p. 775

The Hydrogen-Oxidizing Bacteria

Members of the Gram-negative genera Aquifex and Hydro-genobacter are among the few hydrogen-oxidizing bacteria that are obligate chemolithotrophs. These related organisms are thermophilic and typically inhabit hot springs. Some Aquifex species have a maximum growth temperature of 95°C, the highest of any members of the Domain Bacteria. The hydrogen-oxidizing bacteria are deeply branching in the phylogenetic tree, meaning that according to 16S rRNA studies, they were one of the earliest bacterial forms to exist on earth. The fact that they require O2 seems contradictory to the presumed evolutionary position, but in fact, the low amount they require might have been available early on in certain niches due to photochemical processes that split water.

A wide range of aerobic chemoorganotrophs can also oxidize hydrogen gas. These organisms switch between energy sources as conditions dictate.

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  • mathias
    What type of soil contain sulphur oxidizing bacteria?
    12 months ago
  • aman
    Do sulfur oxidizers require oxygen?
    7 months ago
  • Adelchi Conti
    Can nitrate be used by aerobic sulfur oxidizers?
    4 months ago
  • May
    What is nitrogen oxidizing bacteria?
    3 months ago

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