MICROBIOLOGY Textbook - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.3. MICROBIAL RESPIRATION

9.3.2. Anaerobic Respiration

Under anaerobic conditions, i.e., in the absence of molecular oxygen, certain microorganisms transfer hydrogen from an organic substrate to oxidized mineral compounds (such as nitrates, sulfates, and carbonates) that readily release oxygen while being reduced to lower oxidation states. The ability of microorganisms to use nitrates, sulfates, and carbonates as electron acceptors enables them to achieve complete substrate oxidation and thereby generate more energy than through Fermentation.

Nitrate reduction. The process of reducing nitrates to molecular gas is known as denitrification. In this process, Bacteria first reduce nitrate to nitrite using the molybdenum-containing enzyme nitrate reductase.

Some bacteria (e.g., Escherichia coli) can reduce nitrate only as far as nitrite. Other facultatively anaerobic bacteria, notably Paracoccus denitrificans, are capable of reducing it all the way to dinitrogen gas:

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During denitrification, bound nitrogen is removed from soil and Water, releasing nitrogen gas into the atmosphere. Denitrifying bacteria possess a complete Respiratory Chain; the Synthesis of the Enzymes required for denitrification is induced solely under anaerobic conditions. In many denitrifiers, this induction occurs only in the presence of nitrate. Denitrifying bacteria can grow using not only nitrate, but also nitrite, and sometimes even nitrous oxide, as an electron acceptor.

Denitrification is of great ecological importance. Although it leads to a depletion of soil nitrogen, it is the only natural process by which bound nitrogen is converted back into free nitrogen. This is critical for the preservation of life on Earth. Highly soluble nitrate ions present in soil are washed away by water and carried into the oceans. Without denitrification, the Earth's nitrogen reserves, including atmospheric nitrogen, would gradually become concentrated in the oceans, making life impossible across the greater part of the landmass.

Sulfate reduction. The reduction of sulfate to hydrogen sulfide is carried out by sulfate-reducing bacteria. The most widespread sulfate-reducing bacteria include Desulfovibrio desulfuricans, D. vulgaris (vibrios), and Desulfotomaculum nigricans (spore-forming rods). These obligately anaerobic microorganisms oxidize Organic compounds and molecular hydrogen using sulfate:

In addition to sulfates, sulfur compounds such as thiosulfate, sulfite, and elemental sulfur can also serve as TERMINAL ELECTRON ACCEPTORS.

The process of energy generation via sulfate reduction consists of three stages: the detachment of electrons from an energy-yielding substrate; their transfer along the respiratory chain; and their attachment to terminal electron acceptor substances.

The activity of sulfate-reducing bacteria is particularly pronounced in bottom sediments of ponds, in swamps, and along marine coasts. Tell-tale signs of organic matter mineralization in marine shallows include the odor of hydrogen sulfide and the black coloration of silt, caused by the presence of large amounts of ferrous sulfide.

Carbonate reduction. During the anaerobic decomposition of organic matter by methanogenic microorganisms, methane is produced as a metabolic end product. Its formation occurs in ecosystems such as tundras, wetlands (hence another name for methane, "marsh gas"), rice paddies, lake and pond sediments, and sewage Treatment plants. Under anaerobic conditions, various bacteria ferment organic compounds through a series of intermediate steps into acetate, СO₂, and Н₂. These metabolic products are subsequently utilized by methanogenic bacteria:

Methanogenic bacteria are characterized as anaerobic, hydrogen-oxidizing autotrophs:

Based on their Morphology, methanogenic bacteria are classified into rod-shaped forms (Methanobacterium), coccoid forms (Methanococcus), sarcina-like forms (Methanosarcina), and spirillar forms (Methanospirillum). Methanogenic bacteria are strict anaerobes that lack the enzymes catalase and superoxide dismutase.