# 6.5 Modes of Nutrition in Bacteria 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 ($\mathrm{CO_2}$). ### Photosynthetic Bacteria - Use light energy to synthesize carbohydrates from $\mathrm{CO_2}$. - Contain a unique pigment called **bacteriochlorophyll**, located in mesosomes or dispersed in the cytoplasm. - Unlike plants, they often use hydrogen sulfide ($\mathrm{H_2S}$) instead of water ($\mathrm{H_2O}$) as an electron donor, and therefore do not produce oxygen. This is called *anoxygenic photosynthesis*. - **Equation:** $2\mathrm{H_2S} + \mathrm{CO_2} \xrightarrow[\text{Chlorophyll}]{\text{Light}} (\mathrm{CH_2O})_n + \mathrm{H_2O} + 2\mathrm{S}$ - *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 $\mathrm{CO_2}$ into organic molecules. - **Oxidation for Energy:** $2\mathrm{H_2S} + \mathrm{O_2} \rightarrow 2\mathrm{S} + \mathrm{H_2O} + \text{Energy}$ - **Carbohydrate Synthesis:** $2\mathrm{H_2S} + \mathrm{CO_2} \xrightarrow{\text{Energy}} (\mathrm{CH_2O})_n + \mathrm{H_2O} + 2\mathrm{S}$ - *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 ($\mathrm{H_2O}$) is the electron donor, releasing oxygen ($\mathrm{O_2}$) as a byproduct. | **Anoxygenic Photosynthesis:** Do not produce oxygen. Use substances like $\mathrm{H_2S}$ as electron donors. | | **Significance** | Major contributors to atmospheric oxygen. | Adapted to anaerobic niches; do not significantly contribute to atmospheric oxygen. |