MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 5. MICROBIAL METABOLISM

TERMINAL ELECTRON ACCEPTORS IN MICROORGANISMS

In addition to an energy source, carbon, and an electron donor, metabolic processes in microorganisms require an electron acceptor. For the vast majority of living organisms, molecular oxygen serves as the optimal and sole possible acceptor. During aerobic Respiration, it accepts electrons from the Cell/36.html">Respiratory Chain and is reduced to Water. Aerobic (or obligate aerobic) microorganisms are those that utilize oxygen in their METABOLISM as a terminal electron acceptor during respiration. Aerobic microorganisms grow in environments with a high oxidation-reduction potential (ORP).

When certain Enzymes come into contact with molecular oxygen in living organisms, toxic compounds are formed. For instance, the Oxidation of reduced Flavoproteins and iron-sulfur Proteins by oxygen leads to the generation of hydrogen peroxide (Н₂О₂) and the superoxide radical (О₂^-). These compounds are toxic to The Cell because, by interacting with unsaturated fatty acid residues of Membrane Lipids, they damage cell membranes. Organisms living in an oxygen-rich atmosphere possess the enzymes superoxide dismutase, catalase, and peroxidase, which neutralize the superoxide radical and hydrogen peroxide (Fig. 5.2).

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Fig. 5.2. Reactions Catalyzed by superoxide dismutase, catalase, and peroxidase

Microaerophilic microorganisms also use oxygen as a terminal electron acceptor, but they only grow at low partial pressures of oxygen (no more than 0.2 atm).

Molecular oxygen is not strictly essential for the survival of many microorganisms. In anaerobes, The Role of the terminal electron acceptor can be performed by various organic and Inorganic Compounds. Anaerobic microorganisms are divided into:

- facultative anaerobes;

- obligate anaerobes;

- aerotolerant anaerobes.

Facultative anaerobic microorganisms can thrive both in the presence and absence of oxygen. When molecular oxygen is present in the environment, they utilize it as a terminal electron acceptor in respiration. However, if oxygen is absent, facultative anaerobes easily switch to anaerobic processes, such as nitrate respiration or Fermentation. Their Cells contain superoxide dismutase and catalase.

Obligate anaerobic microorganisms do not utilize oxygen in their metabolism as a terminal electron acceptor during respiration. They can only grow in environments with low ORP values. Their cells lack catalase, superoxide dismutase, and peroxidase, which is why they perish in an oxygen atmosphere.

Aerotolerant anaerobes are microorganisms that do not use oxygen as a terminal electron acceptor, yet possess certain defense mechanisms against hydrogen peroxide and the superoxide radical, allowing them to exist under aerobic conditions. For example, aerotolerant anaerobic lactic acid Bacteria use peroxidase instead of catalase, which reduces peroxide to water using NADH₂.

Anaerobic microorganisms exhibit the same Types of Metabolism as aerobes (see Table 5.2), but depending on The Nature of the terminal electron acceptor, metabolic processes are categorized into:

- Anaerobic respiration;

- fermentation;

- methanogenesis.

In anaerobic respiration, the terminal electron acceptor is primarily represented by oxidized inorganic compounds: nitrate nitrogen (nitrate respiration), sulfate sulfur (sulfate respiration), carbon dioxide (carbon respiration), iron(III) (iron respiration), etc. Anaerobic respiration is most characteristic of anaerobic chemoorganoheterotrophs and chemolithoautotrophs.

During fermentation, the electron acceptors are organic substances (Pyruvate, acetaldehyde, oxaloacetic acid, etc.) generated As a result of The oxidation of an organic electron donor. Fermentation is typical of obligate and facultative anaerobic chemoorganoheterotrophs.

Methanogenesis is a unique and very ancient type of Anaerobic Metabolism that shares features of both Respiration and Fermentation, yet fundamentally differs from both processes. Methanogenesis is carried out by obligate anaerobic chemolithoautotrophs and chemoorganoautotrophs.

Most phototrophic prokaryotes perform anoxygenic Photosynthesis (without O₂ evolution) under anaerobic conditions. In these processes, the electron acceptors are oxidized bacteriochlorophyll and NAD⁺. The electron Donors include light-excited bacteriochlorophyll of the photoreaction center, as well as exogenous electron donors (sulfide, thiosulfate, tetrathionate, molecular sulfur, and molecular hydrogen in photolithotrophs, and organic substances in photoorganotrophs).