MICROBIOLOGY - M.H. Serhiichuk - 2008

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

Iron Cycle

Iron is an essential cellular component found in small quantities. This element is identified in The Structure of several Enzymes (catalase, Nitrogenase, peroxidase), Cytochromes, iron-porphyrin compounds, and others.

In soil, iron exists in the form of organic and Inorganic Compounds. Plants can assimilate only mineral forms of iron that are soluble in soil Water. The Role of soil microorganisms lies in their ability to transform insoluble iron compounds (unavailable to plants) into water-soluble ones and vice versa. The biological iron cycle is shown in Fig. 9.7.

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Fig. 9.7. Biological iron cycle

During the Mineralization of organic iron compounds, microorganisms attack the organic moiety of the molecule and assimilate it; iron is thus released and, under aerobic conditions, precipitates as its hydrated oxide, meaning it is not utilized by the microbial Cell for metabolic processes. This capacity is exhibited by chemoorganotrophic microorganisms, among which are Bacteria and Fungi.

Certain microorganisms are capable of oxidizing reduced iron compounds. The resulting hydrated iron oxide is deposited On the surface of the microbial cell, frequently forming iron-encrusted sheaths. These include chemoorganotrophs of polluted aquatic environments (bacteria of the genus Blastocaulis) and soils (Representatives of the genera Hyphomicrobium, Seliberia, and others).

Bacteria of the genus Hyphomicrobium are represented by oval or Kidney-shaped Cells (0.3-1.2 x 1.0-3.0 µm) featuring polar prosthecae of varying lengths and a diameter of 0.2-0.3 µm. Reproduction occurs by budding, with the bud forming at the apex of the prostheca. The mature bud, upon Separation from the mother cell, possesses one or more polar or subpolar flagella (Fig. 9.8).

These are aerobes and chemoorganotrophs (H. coagulans, H. vulgare, H. zavarzinii, etc.).

Fig. 9.8. Electron micrograph of Hyphomicrobium sp.: mature cell, hyphae and terminal buds are visible

The genus Seliberia is represented by rod-shaped cells (0.5-0.8 x 1.0-12.0 µm) with a single subpolar flagellum covered by a sheath, or several unsheathed lateral flagella (Fig. 9.9). Polar growth of elongated cells in representatives of this genus, resulting from asymmetric division, is accompanied by The formation of short swarmer cells and long non-motile cells. These are obligate aerobes and heterotrophs (S. stellata).

Fig. 9.9. Electron micrograph of Seliberia sp.: a helically twisted cell and flagellum are visible

Microorganisms are also capable of oxidizing inorganic iron compounds in bogs, drainage pipes, lakes, and other water bodies, resulting in the formation of ochreous sediments. This process is carried out by a group of iron bacteria, which include representatives of the genera Gallionella, Metallogenium, and filamentous bacteria of the genera Leptothrix, Crenothrix, and others.

Representatives of the genus Gallionella possess rod-shaped cells (0.5-0.7 x 0.8-1.8 µm). They secrete colloidal iron hydroxide (ferrihydrite) with the formation of stalks, the shape and structure of which serve as a primary diagnostic feature. Cells are located apically at the end of the stalk, perpendicular to its axis. They move via a single, polar-located flagellum. They are aerobes, microaerophiles, and chemolithoautotrophs. Fe²⁺ serves exclusively as the electron donor. The Cell yield is 1 g of dry biomass per 150 g of oxidized ferrous iron. These are among the most important iron bacteria, producing significant amounts of ferrihydrite in water bodies and water supply systems (G. ferruginea).

The genus Metallogenium is represented by spherical cells (0.2-1.5 µm) lacking a Cell wall. These are mycoplasma-like organisms that proliferate directly or through repeated budding to form a cluster of rounded "elementary bodies," the germination of which yields one or more flexible filaments. The life cycle stages include cocci and cocci with filaments that taper and radiate outward from the center, coated with oxides. The filaments (or arhae) are approximately 0.02-0.3 µm thick. Microcolonies (or coenobia) possess one or more central cocci and thin arhae radiating from the center in a "spider-like" pattern, making them easily identifiable at this stage. These are chemoorganotrophs or parasites of fungal mycelium, and aerobes (M. personatum).

Bacteria of the genus Leptothrix form filaments with oxide-encrusted sheaths. The filaments are non-motile, but motile single cells bearing flagella may be produced. The cells are rod-shaped (0.6-1.4 x 1.0-12.0 µm), forming sheath-enclosed chains or existing in a free state. The sheaths are impregnated or coated with iron and manganese oxides (Fig. 9.10). Empty sheaths devoid of cells are frequently observed. Regarding their type of Nutrition, they are chemoorganotrophs with a respiratory type of METABOLISM. The Temperature range is from 10 ^oC to 35 ^oC, with an optimum around 25 ^oC. The optimal pH range is 6.5-7.5. The type species is L. ochracea. Bacteria of this genus oxidize ferrous iron (FeCO₃) to ferric iron Fe₂(CO₃)₃, with its subsequent transformation into Fe(OH)₃:

Fig. 9.10. Leptothrix pseudo-ochracea:

Representatives of the genus Crenothrix have cells ranging from cylindrical to disc-shaped. Cell Division occurs through the formation of transverse septa, yielding filaments up to 1 cm in length enclosed in sheaths. The filaments are frequently attached to a solid substratum (Fig. 9.11). The sheath surrounding the filament is quite thin, potentially colorless at the apex or encrusted with iron (or manganese) oxide at the base. Thickening is observed at the ends of the filaments, within which spherical reproductive cells—"macrogonidia"—are formed; these slip out of the sheath and develop into new sheathed filaments. In the thickened Regions of the filament, cell division may occur in both longitudinal and transverse planes.

Fig. 9.11. Crenothrix polyspora

The small cubic cells ("microgonidia") formed in this process become rounded, slip out, and give rise to new filaments. Some chains of cells exhibit a gliding type of motility. Bacteria of the genus Crenothrix are found in stagnant and running waters containing low concentrations of Fe ²+ and methane. The typical species is C. polyspora.

Obligate chemolithoautotrophic bacteria capable of deriving energy through the Oxidation of ferrous iron and utilizing carbon from CO₂ have also been described. Such bacteria include the sulfur-oxidizing bacterium Thiobacillus ferrooxidans, which drives reactions According to the following scheme:

The oxidation of ferrous iron forms yield relatively little energy, requiring bacteria to transform substantial amounts of iron to support their growth. For instance, to produce 1 g of biomass, these bacteria must oxidize about 500 g of ferrous sulfate.

The ability to accumulate iron oxides has also been established in certain phototrophic bacteria (e.g., cyanobacteria), filamentous green bacteria, and some Algae.

It is believed that chemolithotrophic and A number of chemoorganoheterotrophic microorganisms, which drive the transformation of iron in nature, participate in the accumulation of iron oxides, thereby contributing to the formation of iron deposits in bogs, lakes, and other water bodies.