MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 5. MICROBIAL METABOLISM

Types of Microbial Metabolism

Microorganisms are characterized by a remarkable diversity of metabolic pathways. This enables them to survive in diverse and often extreme environmental conditions, readily adapt to fluctuating environmental factors, and populate an exceptionally wide habitat range. Nutrients taken up by microorganisms provide them with energy essential for active Biosynthesis and motility, electrons for Electron Transport Chains and the generation of reduced equivalents (NADH₂), and carbon as the primary structural component of The Cell.

The metabolic type of a microorganism is defined by three criteria:

- the energy source;

- the electron source (donor);

- the carbon source.

Based on their energy source, microorganisms are classified into chemo- and phototrophs. Chemotrophs obtain energy through Chemical Reactions, whereas phototrophs harness energy from light conversion.

Depending on THE ORIGIN OF their electron source (donor), microorganisms are divided into organo- and lithotrophs. Organotrophs derive electrons from Organic compounds (such as sugars, organic acids, alcohols, and Amino Acids), whereas lithotrophs utilize Inorganic Compounds (including ammonium, nitrite, sulfide, thiosulfate, tetrathionate, molecular hydrogen, and ferrous iron).

Based on the carbon source, microorganisms are classified into auto- and heterotrophs. Autotrophs utilize inorganic carbon sources (such as carbon dioxide and carbonates), whereas heterotrophs rely on organic sources (including sugars, organic acids, alcohols, and amino acids).

Unlike higher organisms, microorganisms exhibit various combinations of these three characteristics, which ultimately accounts for their vast metabolic diversity (Table 5.2). The compound term describing the metabolic features of a microorganism consists of four roots, such as chemo-litho-auto-trophs:

- the first ROOT indicates the energy source utilized by the microorganism;

- the second indicates the origin of electrons;

- the third indicates the carbon source;

- the fourth originates from the Greek word trophe, meaning nourishment.

Class="center">Table 5.2. Types of Microbial METABOLISM

In scientific literature, the full metabolic designation of microorganisms is not always used. Abbreviated terms (such as chemo-, photo-, organo-, litho-, auto-, or heterotrophs) are employed when it is necessary to emphasize only the energy source, electron donor, or carbon source of specific microorganisms.

Depending on environmental conditions, microorganisms can switch from one metabolic type to another. For example, under anaerobic conditions in the light, purple Bacteria exhibit a photolithotrophic metabolism; in the dark in the presence of oxygen, they display a chemoorganotrophic metabolism; and in the dark in the absence of oxygen, they utilize chemoorganotrophic sulfate or nitrate Respiration. Microorganisms capable of shifting between different metabolic types are referred to as mixotrophs.