Photosynthetic Bacteria

Introduction

Photosynthetic bacteria are among Earth’s oldest life forms, dating back over 3 billion years. These microorganisms revolutionized the planet by inventing photosynthesis—long before plants existed. Unlike the familiar oxygenic photosynthesis of trees and algae, many photosynthetic bacteria perform anoxygenic photosynthesis, using hydrogen sulfide (H₂S) or organic molecules instead of water. Today, they thrive in environments from ocean depths to hot springs, playing critical roles in ecology, industry, and even medicine.

Classification of Photosynthetic Bacteria

Photosynthetic bacteria are divided into oxygenic (O₂-producing) and anoxygenic (non-O₂-producing) groups based on their pigments and electron donors.

1. Oxygenic Photosynthetic Bacteria

Cyanobacteria (Blue-Green Algae)

l Pigments: Chlorophyll *a* and phycobilins (phycocyanin, phycoerythrin).

l Habitat: Freshwater, oceans, deserts, and extreme environments.

Examples:

l Prochlorococcus (smallest and most abundant photosynthetic organism; produces ~20% of Earth’s oxygen).

l Spirulina (used as a protein-rich superfood).

l Anabaena (forms nitrogen-fixing symbiotic relationships with plants).

2. Anoxygenic Photosynthetic Bacteria

These bacteria do not split water and thus do not produce oxygen. Instead, they use alternative electron donors like H₂S, H₂, or organic compounds.

A. Purple Bacteria

l Pigments: Bacteriochlorophyll *a* or *b* (absorb infrared light).

Types:

l Purple Sulfur Bacteria (e.g., Chromatium): Use H₂S, depositing sulfur granules.

l Purple Non-Sulfur Bacteria (e.g., Rhodobacter): Prefer organic molecules (e.g., succinate).

Habitat: Sulfur-rich mud, stagnant ponds, and hot springs.

B. Green Bacteria

l Pigments: Bacteriochlorophyll *c*, *d*, or *e*.

Types:

l Green Sulfur Bacteria (e.g., Chlorobium): Strictly anaerobic, use H₂S.

l Green Non-Sulfur Bacteria (e.g., Chloroflexus): Filamentous, thermophilic.

l Habitat: Deep aquatic zones (e.g., Black Sea) and hot springs.

C. Heliobacteria

l Pigments: Bacteriochlorophyll *g* (unique to this group).

l Example: Heliobacterium modesticaldum (found in waterlogged soils).

l Note: The only known Gram-positive photosynthetic bacteria.

How Photosynthetic Bacteria Harvest Light

1. Light Absorption Mechanisms

l Chlorophylls: Primary pigments in cyanobacteria (similar to plants).

l Bacteriochlorophylls: Found in anoxygenic bacteria; absorb longer wavelengths (infrared).

l Accessory Pigments:

l Phycobilisomes (in cyanobacteria) enhance light capture.

l Carotenoids protect against photooxidation.

2. Photosynthetic Structures

Bacterial Group	Photosynthetic Apparatus
Cyanobacteria	Thylakoid membranes (like chloroplasts)
Purple Bacteria	Intracytoplasmic membranes (chromatophores)
Green Sulfur Bacteria	Chlorosomes (antenna complexes)

 

Ecological and Industrial Significance

1. Oxygen Production and the Great Oxidation Event

Cyanobacteria were responsible for the Great Oxidation Event (~2.4 billion years ago), transforming Earth’s atmosphere from anaerobic to oxygen-rich.

 

Today, marine cyanobacteria (e.g., Prochlorococcus) contribute significantly to global oxygen cycles.

 

2. Nitrogen Fixation

Some cyanobacteria (e.g., Nostoc, Anabaena) form heterocysts to fix atmospheric nitrogen (N₂ → NH₃), enriching soils and supporting agriculture.

 

3. Bioremediation and Biofuels

l Oil Spill Cleanup: Purple non-sulfur bacteria (e.g., Rhodopseudomonas) degrade hydrocarbons.

l Hydrogen Production: Engineered cyanobacteria generate H₂ gas as a clean energy source.

 

l Carbon Capture: Cyanobacteria sequester CO₂, potentially mitigating climate change.

4. Medical and Commercial Uses

l Spirulina: A sustainable protein supplement.

l Antibiotics & Toxins: Some species produce bioactive compounds (e.g., Lyngbya yields anticancer agents).

l Biofertilizers: Cyanobacteria reduce dependency on synthetic fertilizers.

Challenges and Threats

1. Harmful Algal Blooms (HABs)

Toxin-producing cyanobacteria (e.g., Microcystis, Anabaena) cause:

l Animal deaths (e.g., livestock drinking contaminated water).

l Human health risks (e.g., liver damage from microcystins).

Triggered by nutrient pollution (eutrophication) and warming waters.

2. Antibiotic Resistance

Some photosynthetic bacteria harbor resistance genes, posing risks if transferred to pathogens.

Conclusion

Photosynthetic bacteria are not just relics of early life—they are keystones of modern ecosystems and pioneers of green technology. From filling our atmosphere with oxygen to inspiring sustainable energy solutions, these microbes continue to shape our world in extraordinary ways. As research advances, their potential in medicine, agriculture, and environmental science remains vast and untapped.