Purple And Green Photosynthetic Bacteria

Photosynthesis is commonly associated with plants, algae, and cyanobacteria. However, some bacteria can also perform photosynthesis using unique mechanisms. These are known as purple and green photosynthetic bacteria. Unlike plants, they do not produce oxygen during photosynthesis and can thrive in environments where oxygen is limited.

In this topic, we will explore the characteristics, types, and ecological significance of purple and green photosynthetic bacteria.

What Are Purple and Green Photosynthetic Bacteria?

Purple and green photosynthetic bacteria are anoxygenic phototrophic bacteria, meaning they use sunlight to produce energy without releasing oxygen. They differ from plants and cyanobacteria, which perform oxygenic photosynthesis by splitting water molecules.

Instead of water, these bacteria use alternative electron donors such as hydrogen sulfide (H₂S), sulfur, or organic compounds, making them vital in sulfur and carbon cycling.

These bacteria are classified into two main groups:

1. Purple photosynthetic bacteria

2. Green photosynthetic bacteria

Each group has distinct characteristics, pigments, and habitats.

1. Purple Photosynthetic Bacteria

Purple photosynthetic bacteria are divided into:

• Purple sulfur bacteria (PSB)

• Purple non-sulfur bacteria (PNSB)

Characteristics of Purple Photosynthetic Bacteria

• Contain bacteriochlorophyll a or b, giving them their purple, red, or brown color.

• Found in anaerobic or low-oxygen environments, such as lakes, sulfur springs, and deep sediments.

• Use hydrogen sulfide (H₂S) or organic compounds instead of water for photosynthesis.

Purple Sulfur Bacteria (PSB)

Purple sulfur bacteria belong to the Gammaproteobacteria class. They thrive in sulfur-rich environments like hot springs and stagnant water.

Examples of Purple Sulfur Bacteria

• Chromatium – Found in sulfur springs and deep sediments.

• Thiocapsa – Capable of surviving in diverse environments.

• Ectothiorhodospira – Lives in alkaline lakes and produces elemental sulfur.

Purple Non-Sulfur Bacteria (PNSB)

Unlike PSB, purple non-sulfur bacteria use organic compounds (instead of sulfur) as electron donors. They belong to the Alphaproteobacteria class.

Examples of Purple Non-Sulfur Bacteria

• Rhodospirillum – A flexible bacterium that can switch between photosynthesis and respiration.

• Rhodobacter – Commonly found in freshwater and marine environments.

• Rhodopseudomonas – Important for bioremediation and nitrogen fixation.

2. Green Photosynthetic Bacteria

Green photosynthetic bacteria are also classified into two groups:

• Green sulfur bacteria (GSB)

• Green non-sulfur bacteria (GNSB)

Characteristics of Green Photosynthetic Bacteria

• Contain bacteriochlorophyll c, d, or e, giving them a green or brownish color.

• Prefer anoxic (oxygen-free) environments, such as deep-sea hydrothermal vents and sulfur springs.

• Use sulfur compounds or organic molecules instead of water for photosynthesis.

Green Sulfur Bacteria (GSB)

Green sulfur bacteria belong to the Chlorobi phylum and are strict anaerobes. They rely on hydrogen sulfide (H₂S) or elemental sulfur for photosynthesis.

Examples of Green Sulfur Bacteria

• Chlorobium – Plays a role in sulfur cycling in aquatic ecosystems.

• Prosthecochloris – Found in marine sediments and sulfur-rich waters.

• Pelodictyon – Forms green bacterial mats in low-light environments.

Green Non-Sulfur Bacteria (GNSB)

Green non-sulfur bacteria belong to the Chloroflexi phylum and are more flexible in their metabolism. They can use both organic compounds and light energy for growth.

Examples of Green Non-Sulfur Bacteria

• Chloroflexus – Thrives in hot springs and can switch between photosynthesis and respiration.

• Roseiflexus – Found in microbial mats and tolerates fluctuating oxygen levels.

Differences Between Purple and Green Photosynthetic Bacteria

Feature	Purple Photosynthetic Bacteria	Green Photosynthetic Bacteria
Pigments	Bacteriochlorophyll a & b	Bacteriochlorophyll c, d & e
Electron Donors	Hydrogen sulfide (PSB), organic compounds (PNSB)	Hydrogen sulfide (GSB), organic compounds (GNSB)
Preferred Habitat	Shallow water, sediments	Deep water, extreme environments
Oxygen Requirement	Mostly anaerobic	Strictly anaerobic

Ecological Importance of Photosynthetic Bacteria

1. Role in Sulfur and Carbon Cycling

Purple and green bacteria help in decomposing organic matter and recycling sulfur. They convert hydrogen sulfide (H₂S) into elemental sulfur or sulfate, which other organisms can use.

2. Contribution to Primary Production

These bacteria perform photosynthesis in low-light environments where plants and algae cannot survive. They contribute to energy flow in deep-sea and underground ecosystems.

3. Biotechnological Applications

• Bioremediation: Some species remove toxic metals and pollutants from contaminated water.

• Biohydrogen Production: Certain bacteria produce hydrogen gas, a potential clean energy source.

• Agriculture: They improve soil fertility by fixing nitrogen and decomposing organic material.

Purple and green photosynthetic bacteria are unique microorganisms that play a vital role in ecosystems. Unlike plants, they do not release oxygen but instead use alternative electron donors like sulfur or organic compounds. These bacteria contribute to nutrient cycling, primary production in extreme environments, and have valuable applications in biotechnology and environmental management. Understanding their functions can help scientists develop sustainable solutions for energy production, pollution control, and agriculture.