Autotrophic Bacteria 

Bacteria are among the simplest and most diverse microorganisms on Earth, showing remarkable adaptability in their modes of nutrition. Among them, autotrophic bacteria stand out as organisms capable of synthesising their own food from inorganic substances. Unlike heterotrophic bacteria, which depend on organic material for nutrition, autotrophic bacteria utilise simple inorganic compounds, such as carbon dioxide (CO₂), water (H₂O), and minerals, to synthesise organic compounds.

1.0What are Autotrophic Bacteria?

Autotrophic bacteria are microorganisms that synthesise their own food using inorganic compounds as a source of energy. They convert carbon dioxide into organic compounds through either light energy (photosynthesis) or chemical energy (chemosynthesis).

Depending on the source of energy they use, autotrophic bacteria are broadly classified into:

1. Photoautotrophic bacteria – use light energy.
2. Chemoautotrophic bacteria – use energy from chemical reactions.

These two groups represent distinct survival strategies that enable autotrophic bacteria to thrive in diverse environments—from sunlit aquatic surfaces to dark ocean floors and even volcanic vents.

2.0Types of Autotrophic Bacteria

1. Photoautotrophic Bacteria

• Photoautotrophic bacteria (or photosynthetic bacteria) obtain energy from sunlight and use it to convert CO₂ and water into carbohydrates through a process similar to photosynthesis in plants.
• They contain photosynthetic pigments, but unlike plants and algae, many do not release oxygen during the process. Depending on whether oxygen is produced or not, photoautotrophic bacteria can be divided into two categories:

(a) Oxygenic Photoautotrophic Bacteria

These bacteria carry out oxygenic photosynthesis, releasing oxygen as a byproduct.

• They contain the pigment chlorophyll a, similar to that found in higher plants.
• Water (H₂O) acts as the hydrogen donor.
• Light energy splits water molecules, releasing oxygen gas.

Example:

• Cyanobacteria (Blue-green algae) such as Anabaena, Nostoc, and Oscillatoria.
• Cyanobacteria are often found in aquatic habitats and moist soils, contributing to oxygen production and nitrogen fixation.

(b) Anoxygenic Photoautotrophic Bacteria

These bacteria perform photosynthesis without producing oxygen.

• They lack chlorophyll a and instead contain pigments like bacteriochlorophyll and bacteriopurpurin.
• Instead of water, they use hydrogen sulfide (H₂S) or organic compounds as hydrogen donors.

Examples:

• Purple sulfur bacteria (Chromatium)
• Green sulfur bacteria (Chlorobium)

2. Chemoautotrophic Bacteria

• Chemoautotrophic bacteria (or chemosynthetic bacteria) do not rely on sunlight for energy. Instead, they obtain energy from the oxidation of inorganic substances, such as ammonia (NH₃), nitrites (NO₂⁻), hydrogen sulfide (H₂S), iron (Fe²⁺), or hydrogen gas (H₂).
• This energy is then used to convert CO₂ into organic compounds via the Calvin cycle, as in photoautotrophs.
• These bacteria play a vital ecological role in recycling nutrients and maintaining the biogeochemical balance in ecosystems.

Major Groups of Chemoautotrophic Bacteria:

3.0Characteristics of Autotrophic Bacteria

1. Self-Sufficient Nutrition:
   They can synthesise organic food molecules from inorganic sources like CO₂ and H₂O.
2. Presence of Enzymes and Pigments:
   Photoautotrophs possess pigments such as chlorophyll, bacteriochlorophyll, or carotenoids, while chemoautotrophs possess enzymes for the oxidation of inorganic compounds.
3. No Dependence on Other Organisms:
   Autotrophic bacteria do not rely on external organic food sources.
4. Energy Utilisation:
5. • Photoautotrophs utilise light energy.
   • Chemoautotrophs use chemical energy released during oxidation.
5. Ecological Importance:
   They act as primary producers and play key roles in nutrient cycling (e.g., nitrogen and sulfur cycles).
6. Adaptation to Harsh Environments:
   Some autotrophic bacteria can survive in extreme conditions such as hot springs, deep-sea vents, and acidic soils.

4.0Importance of Autotrophic Bacteria in Nature

• Maintain ecosystem balance by producing food from inorganic substances.
• Fix atmospheric nitrogen, converting it into forms usable by plants.
• Oxidize harmful compounds, thereby detoxifying environments.
• Support food webs as primary producers in both aquatic and terrestrial ecosystems.
• Regulate global biogeochemical cycles, especially carbon, nitrogen, and sulfur cycles.

5.0Differences Between Photoautotrophic and Chemoautotrophic Bacteria