🌿 Microbial Metabolism

Microbial metabolism is extremely diverse. Some microorganisms live on organic compounds, some use inorganic chemicals, and some capture light energy. This metabolic diversity explains why microbes can survive in so many different ecological conditions and perform so many important agricultural functions.

Broad Metabolic Types

Microorganisms may be viewed broadly by their sources of carbon and energy.

Heterotrophs

These use organic compounds as carbon source.

Autotrophs

These use carbon dioxide as carbon source.

In addition, microbes may differ in energy source:

• light
• organic chemicals
• inorganic chemicals

Heterotrophic Metabolism

Heterotrophs often use carbohydrates, lipids, proteins, and related organic compounds as substrates.

Their metabolism may proceed through:

• fermentation
• aerobic respiration
• anaerobic respiration

Fermentation

Fermentation is the oxidation of organic compounds without an external electron acceptor such as oxygen.

Important features:

• ATP is produced mainly by substrate-level phosphorylation
• reduced organic end products are formed
• energy yield is relatively low

Examples of fermentation types:

• lactic acid fermentation
• alcoholic fermentation
• mixed-acid fermentation

Respiration

Respiration uses an external terminal electron acceptor.

Aerobic Respiration

Oxygen acts as the final electron acceptor.

Anaerobic Respiration

Other compounds act as final electron acceptors, such as:

• nitrate
• sulfate
• carbon dioxide

These pathways are ecologically very important in soils, sediments, and waterlogged systems.

Catabolism of Different Organic Substrates

Microbes can metabolize many compounds besides glucose.

Carbohydrates

Polysaccharides such as starch and cellulose must first be broken down by enzymes into smaller units before uptake and metabolism.

Lipids

Lipids are broken down into glycerol and fatty acids, which can then enter central metabolism.

Proteins

Proteins are hydrolyzed to amino acids, and these amino acids may be deaminated and fed into central metabolic pathways.

Autotrophic Metabolism

Autotrophs use carbon dioxide as carbon source, but they must still obtain energy and reducing power.

They may be:

• photoautotrophs
• chemoautotrophs

Photoautotrophs

These use light energy and carbon dioxide.

Their photosynthesis involves:

• light reaction for ATP and reducing power
• dark reaction for carbon dioxide fixation

The Calvin cycle is a major pathway for carbon dioxide fixation.

Chemoautotrophs or Chemolithotrophs

These obtain energy by oxidizing inorganic compounds.

Important groups include:

• hydrogen bacteria
• sulfur bacteria
• nitrifying bacteria
• iron bacteria

Chemolithotrophs and Agricultural Importance

Chemolithotrophs are important because they participate in:

• sulfur oxidation
• nitrification
• iron transformations
• soil acidity changes

These activities directly affect nutrient availability and soil processes.

Photosynthetic Microorganisms

Photosynthetic microorganisms capture light energy and convert it into chemical energy.

They may differ in:

• pigment systems
• electron donors
• whether oxygen is released

Oxygenic Photosynthesis

Uses water as electron donor and releases oxygen.

Anoxygenic Photosynthesis

Uses substances such as hydrogen sulfide and does not release oxygen.

Why Metabolic Diversity Matters

Microbial metabolism explains why microbes can:

• live in extreme habitats
• drive major biogeochemical cycles
• support plant growth
• decompose residues
• produce useful products

In agriculture, this diversity is the basis of composting, nutrient turnover, fermentation, soil fertility, and biological energy production.

Summary Cheat Sheet

• Microbial metabolism includes great diversity in carbon source, energy source, and pathway type.
• Heterotrophs use organic carbon; autotrophs use carbon dioxide.
• Heterotrophic metabolism includes fermentation and respiration.
• Fermentation gives low energy yield and no external electron acceptor is required.
• Respiration may be aerobic or anaerobic depending on the terminal electron acceptor.
• Photoautotrophs use light; chemolithotrophs use inorganic chemicals for energy.
• Metabolic diversity is fundamental to agricultural processes such as decomposition, nutrient cycling, and soil fertility.

References

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