Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses the chemical reactions that occur within microorganisms to sustain life. These reactions allow microbes to obtain energy, grow, and reproduce by converting nutrients into cellular components and waste products.

• Metabolism is divided into two main processes: catabolism (breaking down molecules to release energy) and anabolism (using energy to build cellular components).

• Microbes display remarkable metabolic diversity, enabling them to thrive in a wide range of environments.

Metabolic Diversity Among Organisms

Classification Based on Energy and Carbon Sources

Microorganisms are classified by how they obtain energy and carbon for growth. The two main energy sources are light and chemicals, while the two main carbon sources are carbon dioxide (CO2) and organic compounds.

• Phototrophs: Use light as their energy source.

• Chemotrophs: Use chemicals as their energy source.

• Autotrophs: Use CO2 as their principal carbon source.

• Heterotrophs: Use organic compounds as their carbon source.

Phototrophs

Phototrophs harness light energy to produce ATP, which is then used for cellular processes. They are further divided based on their carbon source and whether they produce oxygen during photosynthesis.

• Photoautotrophs: Use light energy and CO2 to synthesize organic compounds via the Calvin-Benson cycle.

• Oxygenic photoautotrophs: Produce oxygen as a byproduct (e.g., cyanobacteria, algae, plants).

• Anoxygenic photoautotrophs: Do not produce oxygen (e.g., green and purple sulfur bacteria).

• Photoheterotrophs: Use light for energy but require organic compounds as a carbon source; typically anoxygenic.

Example: Rhodobacter species are photoheterotrophic bacteria found in aquatic environments.

Chemoautotrophs

Chemoautotrophs obtain energy by oxidizing inorganic chemicals (such as ammonia, nitrite, sulfur, or iron) and use CO2 as their carbon source. The energy derived from chemical reactions is used in the Calvin-Benson cycle to fix CO2 into organic molecules.

• Common in environments lacking light, such as deep-sea vents.

• Play important roles in biogeochemical cycles (e.g., nitrifying bacteria in the nitrogen cycle).

Example: Thiobacillus species oxidize sulfur compounds for energy.

Chemoheterotrophs

Chemoheterotrophs obtain both energy and carbon from organic compounds. This group includes most medically and economically important microorganisms, such as bacteria, fungi, and protozoa.

• Energy is generated through processes like fermentation and aerobic or anaerobic respiration.

• Includes all animals, most fungi, and many bacteria.

Example: Escherichia coli is a chemoheterotrophic bacterium commonly found in the human gut.

Summary Table: Nutritional Classification of Organisms

Additional info: The Calvin-Benson cycle is the primary pathway for carbon fixation in autotrophic organisms. Oxygenic photosynthesis is responsible for producing the majority of atmospheric oxygen.