General Microbiology - Schlegel, H. 1987

Utilization of inorganic hydrogen donors: aerobic chemolithotrophic bacteria
Oxidation of reduced sulfur compounds

The ability to derive energy by oxidizing reduced sulfur compounds is characteristic of Gram-negative Bacteria with polar flagella, which are grouped in the genus Thiobacillus. Recently, a spirillum with polar flagella (*Thiomicrospira*) was discovered, along with the non-motile thermophilic bacterium *Sulfolobus* (Table 11.3). Most thiobacilli can oxidize various sulfur compounds, yielding sulfate as the end product:

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Many thiobacilli (*T. thiooxidans*, *T. thioparus*, *T. denitrificans*) are obligate chemolithoautotrophs that fix CO₂. Others (*T. novellus*, *T. intermedius*) are also capable of utilizing Organic compounds as sources of energy and carbon.

*T. thiooxidans* produces large amounts of sulfuric acid and tolerates low environmental pH exceptionally well (Cells remain viable even in a 1 N sulfuric acid solution). Such medium acidification is frequently beneficial in practice. To reduce alkalinity, elemental sulfur is added to calcareous soils; the sulfuric acid produced by the thiobacilli then converts calcium carbonate into more soluble calcium sulfate, which is leached from the soil. A similar approach can be used to combat acidophobic pathogens responsible for potato scab.

Table 11.3. Bacteria that oxidize sulfur and its compounds

While the thiobacilli mentioned above live under aerobic conditions, *T. denitrificans* can also use nitrate as a hydrogen acceptor alongside O₂ (Anaerobic Respiration). This bacterium denitrifies nitrate, but lacks the ability to carry out assimilatory reduction of nitrate to ammonia. Consequently, it requires ammonium salts as a nitrogen source.

*Sulfolobus acidocaldarius* and *Caldariella acidophila*¹ inhabit extreme ecosystems. Their natural habitats are hot acidic springs, where the oxidation is driven primarily by hydrogen sulfide of magmatic (volcanic) origin. *S. acidocaldarius* is a thermophilic, facultative chemolithotroph that oxidizes elemental sulfur to sulfuric acid; it grows best at a pH of 2 to 3 and temperatures between 70 and 75°C, yet remains viable even at 90°C.

Stages of sulfur compound oxidation. Studying individual steps of this process is complicated by the fact that hydrogen sulfide and sulfur are also oxidized abbiotically In aqueous solutions, albeit slowly. The most important reactions are illustrated in Fig. 11.1. Yellow elemental sulfur (flowers of sulfur) consists of eight-membered rings (S₈) and is sparingly soluble in Water (0.176 mg/L).

It is believed that electrons released during The oxidation of sulfite to sulfate enter the Respiratory Chain at the level of cytochrome *c*. At least some thiobacilli (*Thiobacillus thioparus*, *T. denitrificans*) can harness The energy released in this oxidation for substrate-level phosphorylation (Fig. 11.1, steps 5 and 6).

¹ *Caldariella* is now classified within the genus *Sulfolobus*. — Ed. note.

Fig. 11.1. KEY STAGES IN the oxidation of sulfur-containing compounds by sulfur-oxidizing bacteria. Enzymes involved in the reactions (circled numbers): 1, sulfide oxidase; 2, thiosulfate-cleaving enzyme (rhodanese); 3, sulfur-oxidizing enzyme; 4, sulfite oxidase; 5, APS reductase; 6, ATP sulfurylase (sulfate adenylyltransferase).

Reactions (1) and (2) are the reverse of dissimilatory sulfate reduction (Fig. 9.3).

Filamentous and other sulfur bacteria. In environments where H₂S accumulates in the sediments of stagnant or slow-moving waters, colorless filamentous sulfur bacteria such as *Beggiatoa*, *Thiothrix*, and *Thioploca* (see Figs. 2.44 and 3.17), as well as large unicellular forms like *Achromatium oxaliferum* and *Thiomicrospira*, can frequently be found on the blackened surface of the mud. *Beggiatoa* was the subject of S. N. Winogradsky's seminal experiments that laid the foundation for METABOLISM/2.html">THE CONCEPT OF chemolithoautotrophy. However, to this day, it has not been possible to isolate any of these "classic sulfur bacteria" in pure culture and thoroughly study their PHYSIOLOGICAL AND BIOCHEMICAL properties¹. The failure to cultivate them in pure laboratory cultures is apparently due to their highly specific nutritional and environmental requirements. Along with hydrogen sulfide, they require molecular oxygen, but tolerate it only at extremely low concentrations (i.e., they are microaerophiles). Maintaining such optimal conditions demands specialized experimental techniques and considerable patience.

¹ Recently, pure cultures of some of these microorganisms, including *Beggiatoa*, have been successfully isolated. Their growth apparently depends on the presence of organic compounds. — Ed. note.

Hydrogen sulfide as The basis of an aphotic ecosystem. All higher heterotrophic organisms rely on biomass generated through Photosynthesis. However, an exception to this rule was discovered several years ago. At great ocean depths, along tectonic spreading centers, hydrothermal vents discharge superheated water reaching temperatures around 350°C from the seafloor. This vent water is rich in dissolved minerals, including H₂S. Upon mixing with cold, oxygenated seawater, these vents support the growth of sulfur- or hydrogen-oxidizing bacteria. These bacteria serve as food for Mollusks, crustaceans, and polychaetes. One remarkable vestimentiferan tubeworm, *Riftia pachyptila*, is exceptionally well adapted to life in these habitats. Lacking both a Mouth and a gut, this animal relies on a specialized organ called a trophosome, which harbors symbiotic H₂S-oxidizing bacteria; its Blood supplies this organ with both hydrogen sulfide and oxygen. Thus, in the pitch-black depths of the ocean near hydrothermal vents, there exists an ecosystem where primary biomass production is driven by chemolithoautotrophy rather than photosynthesis.