6.5.1 Autotrophic Nutrition in Bacteria

Introduction

Bacteria can be classified on the basis of methods of obtaining energy and carbon. Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide. This topic focuses on autotrophic nutrition in bacteria.

Definition

Autotrophic Nutrition in Bacteria: Autotrophy means self-nourishing organisms. An autotrophic organism can obtain all the carbon it needs from CO₂ that is present in the atmosphere and that dissolves readily in water.

In simple words: Autotrophic bacteria can make their own food using carbon dioxide.

Core Concept

• Autotrophy means self-nourishing organisms.
• An autotrophic organism can obtain all the carbon it needs from CO₂.
• CO₂ is present in the atmosphere and dissolves readily in water.
• There are two major groups of autotrophs:

Detailed Explanation

1. Photosynthetic Bacteria

Definition: The photosynthetic bacteria contain a unique type of chlorophyll called bacteriochlorophyll. The chlorophyll is incorporated in the membrane of their mesosomes or dispersed in the cytoplasm.

Key Points:

• Photosynthetic bacteria contain bacteriochlorophyll.
• The chlorophyll is incorporated in:
• The membrane of their mesosomes, or
• Dispersed in the cytoplasm.
• Like green plants, photosynthetic bacteria use the energy of sunlight to make carbohydrates from CO₂.
• They use hydrogen sulfide (H₂S) instead of water.

Equation: 2H₂S + O₂ Light→ (CH₂O)_n + H₂O + 2S

• They use H₂S instead of water.
• They produce sulfur (S) as a byproduct.
• They do not produce oxygen.

• Green sulphur bacteria
• Purple sulphur bacteria
• Purple non-sulphur bacteria

2. Chemoautotrophic Bacteria

Definition: Certain colorless bacteria make carbohydrates from inorganic substances. They do not use light energy. They oxidize inorganic substances. The energy produced by this oxidation is then used to make carbohydrates.

Key Points:

• Chemoautotrophic bacteria are colorless.
• They do not use light energy.
• They oxidize inorganic substances.
• The energy produced by this oxidation is used to make carbohydrates.

Example: Sulphur Bacteria

- Sulphur bacteria oxidize sulphur to produce energy.

Equation (Oxidation): 2H₂S + O₂ → 2S + H₂O + Energy

Equation (Carbohydrate Synthesis): 2H₂S + CO₂ Energy→ (CH₂O)_n + H₂O + 2S

• Nitrifying bacteria
• Sulphur bacteria

Comparison of Photosynthetic and Chemoautotrophic Bacteria

Summary Table of Autotrophic Nutrition

Intermediate Level

Photosynthetic Bacteria in Detail

Equation: 2H₂S + O₂ Light→ (CH₂O)_n + H₂O + 2S

Chemoautotrophic Bacteria in Detail

Equations: 2H₂S + O₂ → 2S + H₂O + Energy 2H₂S + CO₂ Energy→ (CH₂O)_n + H₂O + 2S

Comparison of Photosynthetic and Chemoautotrophic Bacteria

Important Points

• 1. Bacteria can be classified on the basis of methods of obtaining energy and carbon.
• 2. Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide.
• 3. Autotrophy means self-nourishing organisms.
• 4. An autotrophic organism can obtain all the carbon it needs from CO₂ that is present in the atmosphere and that dissolves readily in water.
• 5. There are two major groups of autotrophs:
• - Photosynthetic bacteria
• - Chemoautotrophic bacteria
• 6. Photosynthetic Bacteria:
• - The photosynthetic bacteria contain unique type of chlorophyll called bacteriochlorophyll.
• - The chlorophyll is incorporated in the membrane of their mesosomes or dispersed in the cytoplasm.
• - Like green plants, photosynthetic bacteria use the energy of sunlight to make carbohydrates from CO₂.
• - They use hydrogen sulfide (H₂S) instead of water.
• - Equation: 2H₂S + O₂ → (Light) → (CH₂O)ₙ + H₂O + 2S
• - Examples: Green sulphur bacteria, Purple sulphur bacteria, Purple non-sulphur bacteria.
• 7. Chemoautotrophic Bacteria:
• - Certain colorless bacteria make carbohydrates from inorganic substance.
• - They do not use light energy.
• - They oxidize inorganic substance.
• - The energy produced by this oxidation is then used to make carbohydrates.
• - Sulphur bacteria oxidize sulphur to produce energy.
• - Equation (Oxidation): 2H₂S + O₂ → 2S + H₂O + Energy
• - Equation (Carbohydrate Synthesis): 2H₂S + CO₂ → (Energy) → (CH₂O)ₙ + H₂O + 2S
• - Examples: Nitrifying bacteria, Sulphur bacteria.

Remember

• - Autotrophy = self-nourishing.
• - Autotrophic bacteria = can make their own food.
• - Two types: Photosynthetic bacteria, Chemoautotrophic bacteria.
• - Photosynthetic bacteria = use sunlight; bacteriochlorophyll; use H₂S; produce sulfur.
• - Chemoautotrophic bacteria = colorless; oxidize inorganic substances; use energy to make carbohydrates.
• - Both = do not produce oxygen.
• - Examples: Green sulphur bacteria, Purple sulphur bacteria, Nitrifying bacteria, Sulphur bacteria.

Common Mistakes

• 1. Confusing photosynthetic and chemoautotrophic bacteria – Photosynthetic = use sunlight; Chemoautotrophic = oxidize inorganic substances.
• 2. Thinking photosynthetic bacteria produce oxygen – Photosynthetic bacteria do not produce oxygen.
• 3. Thinking photosynthetic bacteria use water – Photosynthetic bacteria use H₂S instead of water.
• 4. Thinking chemoautotrophic bacteria use light – Chemoautotrophic bacteria do not use light.
• 5. Confusing bacteriochlorophyll and chlorophyll – Bacteriochlorophyll is found in photosynthetic bacteria; chlorophyll is found in plants.
• 6. Thinking all autotrophic bacteria are photosynthetic – Autotrophic bacteria include both photosynthetic and chemoautotrophic bacteria.
• 7. Forgetting the byproduct – Photosynthetic and chemoautotrophic bacteria produce sulfur as a byproduct.
• 8. Confusing the examples – Green sulphur bacteria, Purple sulphur bacteria, Nitrifying bacteria, Sulphur bacteria.
• 9. Thinking chemoautotrophic bacteria are colored – Chemoautotrophic bacteria are colorless.
• 10. Forgetting the equations – Equations for photosynthesis and oxidation.
• 11. Thinking autotrophic bacteria are harmful – Autotrophic bacteria are important in nutrient cycling.
• 12. Confusing the terms "autotrophic" and "heterotrophic" – Autotrophic = self-nourishing; Heterotrophic = depend on external organic sources.

Quick Revision

• Bacteria can be classified on the basis of methods of obtaining energy and carbon.
• Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide.
• Autotrophy means self-nourishing organisms.
• An autotrophic organism can obtain all the carbon it needs from CO₂ that is present in the atmosphere and that dissolves readily in water.
• There are two major groups of autotrophs: photosynthetic and chemoautotrophic.
• Photosynthetic Bacteria:
• The photosynthetic bacteria contain unique type of chlorophyll called bacteriochlorophyll.
• The chlorophyll is incorporated in the membrane of their mesosomes or dispersed in the cytoplasm.
• Like green plants, photosynthetic bacteria use the energy of sunlight to make carbohydrates from CO₂.
• They use hydrogen sulfide (H₂S) instead of water.
• 2H₂S + O₂ → (Light) → (CH₂O)ₙ + H₂O + 2S
• Examples of photosynthetic bacteria are green sulphur bacteria, purple sulphur bacteria, purple non-sulphur bacteria.
• Chemoautotrophic Bacteria:
• Certain colorless bacteria make carbohydrates from inorganic substance.
• They do not use light energy.
• They oxidize inorganic substance.
• The energy produced by this oxidation is then used to make carbohydrates.
• Sulphur bacteria oxidize sulphur to produce energy.
• 2H₂S + O₂ → 2S + H₂O + Energy
• The energy thus produced is used by bacteria to make carbohydrate (CH₂O)ₙ.
• 2H₂S + CO₂ → (Energy) → (CH₂O)ₙ + H₂O + 2S
• Examples of chemoautotrophic bacteria are Nitrifying bacteria and Sulphur bacteria.

60-Second Revision

• Autotrophy = self-nourishing.
• Autotrophic bacteria = can make their own food.
• Two types: Photosynthetic bacteria, Chemoautotrophic bacteria.
• Photosynthetic bacteria = use sunlight; bacteriochlorophyll; use H₂S; produce sulfur.
• Chemoautotrophic bacteria = colorless; oxidize inorganic substances; use energy to make carbohydrates.
• Both = do not produce oxygen.
• Examples: Green sulphur bacteria, Purple sulphur bacteria, Nitrifying bacteria, Sulphur bacteria.
• Warning: Photosynthetic bacteria do not produce oxygen. They use H₂S instead of water. Chemoautotrophic bacteria are colorless and do not use light. Both produce sulfur as a byproduct. Examples: Green sulphur bacteria, Purple sulphur bacteria, Nitrifying bacteria, Sulphur bacteria. Do not confuse autotrophic and heterotrophic bacteria.