6.5 Modes of Nutrition in Bacteria | Prokaryotes | Biology 11

This section outlines the diverse metabolic strategies bacteria use to obtain energy and carbon, as well as their different respiratory processes.

6.5.1 Autotrophic Nutrition in Bacteria

Autotrophy refers to self-nourishing organisms that can produce their own food. They obtain the carbon they need from atmospheric carbon dioxide ( $C O_{2}$ ).

Photosynthetic Bacteria

Use light energy to synthesize carbohydrates from $C O_{2}$ .
Contain a unique pigment called bacteriochlorophyll, located in mesosomes or dispersed in the cytoplasm.
Unlike plants, they often use hydrogen sulfide ( $H_{2} S$ ) instead of water ( $H_{2} O$ ) as an electron donor, and therefore do not produce oxygen. This is called anoxygenic photosynthesis.
Equation: $2 H_{2} S + C O_{2} Light Chlorophyll (C H_{2} O)_{n} + H_{2} O + 2 S$
Examples: Green sulphur bacteria, purple sulphur bacteria.

Chemoautotrophic Bacteria

Synthesize carbohydrates without using light energy.
They derive energy by oxidizing inorganic substances like hydrogen sulfide or ammonia.
This chemical energy is then used to fix $C O_{2}$ into organic molecules.
Oxidation for Energy: $2 H_{2} S + O_{2} \to 2 S + H_{2} O + Energy$
Carbohydrate Synthesis: $2 H_{2} S + C O_{2} Energy (C H_{2} O)_{n} + H_{2} O + 2 S$
Examples: Nitrifying bacteria, Sulphur bacteria.

6.5.2 Heterotrophic Nutrition in Bacteria

Heterotrophic bacteria obtain carbon and energy by consuming pre-existing organic compounds from other organisms.

Saprotrophic Bacteria

Obtain nutrition from dead and decaying organic matter.
Act as decomposers in ecosystems.
They secrete extracellular enzymes to break down complex organic molecules into simpler forms, which are then absorbed.
They play a vital role in nutrient cycling.
Examples: Pseudomonas, Azotobacter.

Parasitic Bacteria

Obtain nutrients and energy from a living host organism, often causing harm or disease (pathogenic).
Have evolved mechanisms to invade host tissues and evade the immune system.
Obligate Parasites: Completely dependent on the host to complete their life cycle. Example: Rickettsia.
Facultative Parasites: Can survive independently but can also act as parasites when a host is available. Example: Escherichia coli.
Other examples: Streptococcus pneumoniae, Salmonella typhi, Mycobacterium tuberculosis.

Respiration in Bacteria

Bacteria exhibit various modes of respiration based on their oxygen requirements.

Aerobic Bacteria: Require oxygen for respiration. Example: Pseudomonas.
Anaerobic Bacteria: Grow in the absence of oxygen. Example: Spirochaeta.
Facultative Bacteria: Can grow with or without oxygen, switching between aerobic respiration and fermentation. Example: E. coli.
Microaerophilic Bacteria: Require a low concentration of oxygen for growth, as high concentrations can be toxic. Example: Campylobacter.

Comparison of Photosynthesis: Cyanobacteria vs. Other Photosynthetic Bacteria

Feature	Cyanobacteria (Blue-Green Algae)	Other Photosynthetic Bacteria (e.g., Purple & Green Bacteria)
Photosynthetic Pigments	Chlorophyll-a, Chlorophyll-b, and phycobiliproteins (phycocyanin, phycoerythrin). Captures a broad light spectrum.	Bacteriochlorophylls. Absorb light in the infrared and near-infrared regions.
Cellular Location	Photosynthesis occurs in thylakoids, often stacked into grana, similar to plant chloroplasts.	Pigments are embedded in invaginations of the cell membrane called chromatophores. They lack thylakoids.
Oxygen Production	Oxygenic Photosynthesis: Water ( $H_{2} O$ ) is the electron donor, releasing oxygen ( $O_{2}$ ) as a byproduct.	Anoxygenic Photosynthesis: Do not produce oxygen. Use substances like $H_{2} S$ as electron donors.
Significance	Major contributors to atmospheric oxygen.	Adapted to anaerobic niches; do not significantly contribute to atmospheric oxygen.

6.5.1 Autotrophic Nutrition in Bacteria

Autotrophy refers to self-nourishing organisms that can produce their own food. They obtain the carbon they need from atmospheric carbon dioxide (

C O_{2}

Use light energy to synthesize carbohydrates from

C O_{2}

Contain a unique pigment called bacteriochlorophyll, located in mesosomes or dispersed in the cytoplasm.

Unlike plants, they often use hydrogen sulfide (

H_{2} S

) instead of water (

H_{2} O

) as an electron donor, and therefore do not produce oxygen. This is called anoxygenic photosynthesis.

Equation:

2 H_{2} S + C O_{2} Light Chlorophyll (C H_{2} O)_{n} + H_{2} O + 2 S

Examples: Green sulphur bacteria, purple sulphur bacteria.

Synthesize carbohydrates without using light energy.

They derive energy by oxidizing inorganic substances like hydrogen sulfide or ammonia.

This chemical energy is then used to fix

C O_{2}

into organic molecules.

Oxidation for Energy:

2 H_{2} S + O_{2} \to 2 S + H_{2} O + Energy

Carbohydrate Synthesis:

2 H_{2} S + C O_{2} Energy (C H_{2} O)_{n} + H_{2} O + 2 S

Examples: Nitrifying bacteria, Sulphur bacteria.

6.5.2 Heterotrophic Nutrition in Bacteria

Heterotrophic bacteria obtain carbon and energy by consuming pre-existing organic compounds from other organisms.

Obtain nutrition from dead and decaying organic matter.

Act as decomposers in ecosystems.

They secrete extracellular enzymes to break down complex organic molecules into simpler forms, which are then absorbed.

They play a vital role in nutrient cycling.

Examples: Pseudomonas, Azotobacter.

Obtain nutrients and energy from a living host organism, often causing harm or disease (pathogenic).

Have evolved mechanisms to invade host tissues and evade the immune system.

Obligate Parasites: Completely dependent on the host to complete their life cycle. Example: Rickettsia.

Facultative Parasites: Can survive independently but can also act as parasites when a host is available. Example: Escherichia coli.

Other examples: Streptococcus pneumoniae, Salmonella typhi, Mycobacterium tuberculosis.

Respiration in Bacteria

Bacteria exhibit various modes of respiration based on their oxygen requirements.

Aerobic Bacteria: Require oxygen for respiration. Example: Pseudomonas.

Anaerobic Bacteria: Grow in the absence of oxygen. Example: Spirochaeta.

Facultative Bacteria: Can grow with or without oxygen, switching between aerobic respiration and fermentation. Example: E. coli.

Microaerophilic Bacteria: Require a low concentration of oxygen for growth, as high concentrations can be toxic. Example: Campylobacter.

Comparison of Photosynthesis: Cyanobacteria vs. Other Photosynthetic Bacteria

Feature	Cyanobacteria (Blue-Green Algae)	Other Photosynthetic Bacteria (e.g., Purple & Green Bacteria)
Photosynthetic Pigments	Chlorophyll-a, Chlorophyll-b, and phycobiliproteins (phycocyanin, phycoerythrin). Captures a broad light spectrum.	Bacteriochlorophylls. Absorb light in the infrared and near-infrared regions.
Cellular Location	Photosynthesis occurs in thylakoids, often stacked into grana, similar to plant chloroplasts.	Pigments are embedded in invaginations of the cell membrane called chromatophores. They lack thylakoids.
Oxygen Production	Oxygenic Photosynthesis: Water ( $H_{2} O$ ) is the electron donor, releasing oxygen ( $O_{2}$ ) as a byproduct.	Anoxygenic Photosynthesis: Do not produce oxygen. Use substances like $H_{2} S$ as electron donors.
Significance	Major contributors to atmospheric oxygen.	Adapted to anaerobic niches; do not significantly contribute to atmospheric oxygen.