MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 9. THE ROLE OF MICROORGANISMS IN THE BIOGEOCHEMICAL CYCLING OF SUBSTANCES IN NATURE

Microbial Transformation of Sulfur Compounds

Sulfur is an essential nutrient element for all biological entities. It is a constituent of Certain Amino Acids (Methionine, cystine, Cysteine) as well as B Vitamins (thiamine, biotin). In soil, sulfur occurs in the form of sulfates (CaSO₄ x 2H₂O; Na₂SO₄; K₂SO₄; (NH₄)₂SO₄), sulfides (FeS₂; Na₂S; ZnS, etc.), and Organic compounds. The total reserves of sulfur in soils are insignificant, which is why plants frequently experience a deficiency of this element.

The transformation of sulfur-containing substances comprises two fundamental processes: the Oxidation of reduced sulfur compounds to sulfur and sulfuric acid; and the reduction of organic and inorganic sulfur compounds to hydrogen sulfide (Fig. 9.6).

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Fig. 9.6. The sulfur cycle

Bacteria that oxidize hydrogen sulfide, sulfur, and other sulfur compounds are widely distributed in various types of soils and aquatic environments. The activity of such microorganisms results in The formation of sulfate salts, which are assimilated by plants and microorganisms. The oxidation of reduced forms of sulfur is carried out by many species of autotrophic and heterotrophic microorganisms that belong to various taxonomic groups and differ from one another in both morphological characteristics and the biochemism of their transformations.

Active oxidizers of reduced sulfur compounds include the following groups of microorganisms:

- colorless sulfur-oxidizing bacteria of the genera Achromatium (unicellular, up to 100 μm in length), Beggiatoa (filamentous, a filament may contain up to 50 Cells or separate cells), Hyphomicrobium (cells with prosthecae that may branch; reproduce by budding, with buds forming at the apex of the prostheca), Macromonas (unicellular, up to 3.0-11.0 μm in length), Sulfolobus (single irregular lobed cocci), Thiobacillus (small rod-shaped cells), Thiobacterium (rod-shaped cells), Thiodendron (vibrioid or spirally twisted cells), Thiomicrospira (spiral or vibrioid cells), Thiospira (spiral cells with polar flagella), etc.;

- photosynthetic purple (Amoebobacter, Chromatium, Thiospirillum, etc.) and green sulfur bacteria (Ancalochloris, Chlorobium, Prosthecochloris, etc.), as well as certain cyanobacteria;

- chemoorganoheterotrophs of the genera Bacillus, Pseudomonas, actinomycetes, and Fungi (Aspergillus, Penicillium).

Thiobacilli, which include Representatives of the genera Sulfolobus, Thiobacillus, Thiodendron, and Thiomicrospira, are soil inhabitants. The thiobacilli of the genus Thiobacillus are the most widespread. These bacteria were first isolated from marine mud by A. Nathanson in 1902, and by M. Beijerinck in 1904. Morphologically, they are rod-shaped forms (0.5 x 1.0-4.0 μm). Motile cells possess terminally localized flagella. They obtain energy by oxidizing one or more forms of reduced sulfur compounds: sulfides, sulfur, thiosulfate, polythionate, and thiocyanate. The end product of the oxidation of these compounds is sulfate, although sulfur, sulfite, or polythionates may accumulate in most species. All species are capable of fixing CO₂ and undergoing autotrophic growth. Most species are obligate chemolithoautotrophs (growing at the expense of energy released during the oxidation of inorganic sulfur compounds), while others are also capable of chemoorganotrophic growth. The optimal pH range for their development lies between 2.0 and 8.0, and the Temperature range is 20-43 ^oC. This genus is represented by numerous species (T. thioparus, T. neapolitanus, T. capsulatus, T. denitrificans, T. ferrooxidans, T. thiooxidans, T. novelus, T. acidophilus, etc.) that are quite common in natural environments: sulfur springs, sulfide and sulfur deposits, wastewater Treatment sites, locations where gaseous sulfur compounds (H₂S) are formed, bottom sediments, or anaerobic soils. A necessary condition for their development in these and other environments is the presence of oxidizable sulfur compounds.

The course of the oxidation reactions mediated by sulfur bacteria can be represented by the following equations:

Tetrathionates can be oxidized to sulfuric acid:

The hypothetical chain of reactions for the oxidation of elemental sulfur by bacteria of the genus Thiobacillus can be represented as follows:

Unicellular colorless sulfur bacteria are represented by the genera Achromatium, Thiobacterium, Macromonas, etc. Representatives of these genera possess spherical, oval, rod-shaped, or convoluted cells with a gliding type of motility. For instance, cells of representatives of the genus Achromatium are oval or cylindrical with hemispherical ends (5.0-33.0 x 15.0-100.0 μm). They are Gram-negative aerobes found within or On the surface of bottom sediments in freshwater or saline bodies of Water. Sulfur accumulates within The Cell in the form of globules (A. oxaliferum).

Sulfur-oxidizing bacteria whose cells form colorless filaments include representatives of the genera Beggiatoa, Thioploca, Thiothrix, and others—inhabitants of sulfur springs and polluted waters that oxidize hydrogen sulfide to elemental sulfur, which is deposited inside the cell. Representatives of the genus Beggiatoa can exist as single cells (1.0-200 x 2.0-10 μm) or form filaments comprising 50 or more cells. The filaments do not form sheaths. In strains with a small thickness (~ 7.0 μm), the cells are cylindrical; in strains with a large thickness (up to 200 μm), they are disc-shaped, and their diameter exceeds their length (~ 10 μm). Filaments can break apart to form single cells or pairs of cells (hormogonia), which grow to form a new filament. Hormogonia and filaments are capable of gliding motility. When growing in the presence of hydrogen sulfide (or occasionally thiosulfate), sulfur inclusions are revealed within the cells. Among them are aerobes and microaerophiles, chemoorgano-

trophs, and facultative autotrophs. Certain strains are capable of mixotrophic growth. They grow within a temperature range from 0 ^oC to 40 ^oC. The type and only species of the genus is B. alba. Multicellular filaments of representatives of the genus Thiothrix attach to aquatic substrates by means of specialized holdfasts. The filaments contain rod-shaped cells with a diameter of 1.0-1.5 μm. Gonidia are capable of gliding motility. These are mixotrophs that require low-molecular-weight Organic compounds as well as a source of inorganic reduced sulfur for their growth. Cells growing in the presence of reduced inorganic sulfur compounds in the medium contain sulfur inclusions. The type (and only) species is T. nivea.

The oxidation of sulfides and sulfur by these bacteria proceeds According to the following scheme:

Extreme bacteria that metabolize elemental sulfur include representatives of the genus Sulfolobus. These are irregular lobed cocci with a diameter of 0.8-2.0 μm, growing exclusively under aerobic conditions at a low Ionic strength of the medium. Under microaerophilic conditions, they can utilize Fe³⁺ or MoO₄^2- as electron acceptors. The pH range for their development varies from 1.0 to 6.0, and the temperature range is from 55 ^oC to 87 ^oC. These are bacteria capable of both lithotrophic and organotrophic growth. Lithotrophically, they grow through the oxidation of sulfide or tetrathionate with the formation of sulfuric acid. S⁰ may or may not be oxidized. Bacteria of the genus Sulfolobus are found in solfataric areas characterized by an acidic environment and elevated temperature, both in soils and in aquatic environments (S. acidocaldarius).

Many chemoorganoheterotrophic microorganisms (including certain species of the genera Bacillus, Pseudomonas, actinomycetes, and fungi) are also capable of oxidizing sulfur. Chemoorganoheterotrophic microorganisms oxidize sulfur in the presence of organic compounds in the environment. Although sulfur oxidation is an exothermic process for these bacteria, chemoorganoheterotrophs do not utilize this energy. Instead, such transformations serve as a secondary process alongside their primary metabolic pathway.

Microorganisms are also capable of reducing inorganic sulfur compounds. These processes take place in poorly aerated soils (such as flooded soils) and water bodies (lagoons) and are referred to as desulfofication. Sulfate-reducing bacteria encompass representatives of two genera: endospore-forming Desulfotomaculum and asporogenous Desulfovibrio. These are obligate anaerobes and heterotrophs in which sulfate reduction occurs via coupled oxidation

of organic compounds, serving a vital energetic function for them. As carbon sources, they can utilize CARBOHYDRATES, organic acids, fats, petroleum Hydrocarbons, naphthalene, and certain nitrogen-containing compounds. Metal sewage pipes undergo severe corrosion and become unusable under METABOLISM/18.html">The Influence of these bacteria.

Representatives of the genus Desulfotomaculum feature rod-shaped cells (0.5-2.0 × 2.0-9.0 μm) and form heat-resistant endospores. As a rule, these are motile forms with peritrichous or polar flagellation. They reduce sulfate and, in some cases, sulfite or thiosulfate to hydrogen sulfide. Organic substrates are oxidized either incompletely (to acetate) or completely to CO₂. Species that utilize H₂ can be heterotrophs requiring acetate as a carbon source, or autotrophs. The optimal pH range is 6.6-7.4, and optimal temperatures are 25-40 ^oC (for mesophilic species) and 40-65 ^oC (for thermophilic species).

Species of the genus Desulfotomaculum are found in flooded soils (such as rice paddies), as well as in anoxic freshwater and marine sediments, and the digestive tracts of animals (D. nigrificans, D. geothermicum, D. kuznetsovii, D. ruminis, etc.).

The genus Desulfovibrio is represented by spiral or vibrioid cells (0.5-1.3 × 0.8-5.0 μm). Motile cells possess one or more polarly localized flagella. The optimal pH range is 6.6-7.5, and the optimal temperature is 25-40 ^oC. These are bacteria that reduce sulfate, and frequently sulfite or thiosulfate, to H₂S, and they are also capable of reducing sulfur. Their habitats include anoxic freshwater and marine sediments, oilfield formation waters, industrial water systems, and the digestive tracts of animals (D. desulfuricans, D. africanus, D. vulgaris, etc.).

The anaerobic oxidation of organic substrates by anaerobic sulfate reducers is incomplete and is accompanied by the formation of acetic acid as the end product:

Some characteristics of sulfate-reducing bacteria are summarized in Table 9.2.

Sulfate-reducing bacteria can cause significant damage by degrading Materials that are vulnerable to H₂S attack. Hydrogen sulfide also exhibits toxic properties; its accumulation in soils or water bodies leads to the death of both PLANT AND ANIMAL organisms. At the same time, sulfate-reducing bacteria play a crucial role in geochemical processes. They generate hydrogen sulfide, which participates in the formation of sulfur deposits. Subsequent oxidation of hydrogen sulfide by sulfur bacteria produces commercially valuable sulfur deposits. Sulfate-reducing bacteria are involved in the genesis of deposits containing not only sulfur but also sulfide ores.

Table 9.2. Selected characteristics of genera of sulfate-reducing bacteria

*does not always correlate with Gram-staining results. For example, Desulfotomaculum stains Gram-negative, but its cell wall is of the gram-positive type (lacking an outer membrane); "+" indicates a positive trait; "-" indicates a negative trait.