PHOTOSYNTHETIC BACTERIA - AN INTRODUCTION

GENERAL CHARACTERISTICS OF PHOTOSYNTHETIC BACTERIA

• Photosynthetic Bacteria are Prokaryotic organisms capable of converting Light energy into Chemical energy through Photosynthesis. Unlike plants, many of them do not produce Oxygen (except Cyanobacteria) during this process.
• Cellular Structures: Photosynthetic Bacteria may be unicellular, filamentous, or colonial. Some can form specialized cells like Heterocysts (for nitrogen fixation in cyanobacteria) or Chlorosomes (light-harvesting structures in green bacteria).
• Habitat - Photosynthetic bacteria are found in a wide array of habitats, including aquatic (freshwater and marine), terrestrial (soil), and extreme environments (hot springs, salt lakes).
• Biochemical Composition - Photosynthetic Bacteria contain a high Protein content (40 – 60 %), with pigments and vitamins making up a significant portion of their cellular dry weight.
• Mode of Nutrition - Many exhibit metabolic flexibility, capable of different modes of nutrition (e.g., Photoautotrophy, Photoheterotrophy, and even Chemoheterotrophy in some cases).
• Mode of Reproduction - Mainly reproduce asexually through Binary fission, although some can exchange Genetic material through Horizontal Gene Transfer (Conjugation, Transformation and Transduction).
• Mode of Respiration – Microaerophilic or Anaerobic.
• Photosynthesis - Photosynthetic Bacteria perform either Oxygenic photosynthesis or Anoxygenic photosynthesis.

a)     Oxygenic Photosynthesis: Similar to Plants and Algae, Cyanobacteria use Water as an Electron donor, releasing oxygen (O₂) as a by-product.

b)     Anoxygenic Photosynthesis: Other Photosynthetic bacteria use compounds like Hydrogen sulfide (H₂S), Ferrous Iron (Fe²⁺), Elemental sulfur (S⁰), thiosulfate (S₂O₃²⁻), or Organic matter as Electron donors and do not produce Oxygen.

• Photosynthetic Membranes: Photosynthetic Bacteria have specialized internal membrane systems (e.g., chromatophores, lamellae, or chlorosomes) to house pigments.
• Photosynthetic Pigments

a)     Chlorophylls: Some Photosynthetic bacteria, particularly Cyanobacteria (Oxygenic photosynthesis) have pigments similar to those in plants, like Chlorophyll - a.

b)     Bacteriochlorophylls: In Anoxygenic photosynthetic bacteria, these pigments absorb light in the far-red region of the spectrum, and they vary among groups (e.g., Bacteriochlorophyll a, b, c, d, e). Bacteriochlorophylls are present in the Plasma membrane of Photosynthetic bacteria.

c)      Accessory pigments: Carotenoids or Phycobilins are also present.

• Energy Production - Photosynthetic bacteria generate ATP and reduce NADP+ to NADPH using light energy, which are then used in carbon fixation pathways (e.g., Calvin cycle).
• Carbon Fixation - Most use the reverse Krebs cycle (reductive TCA cycle) or the Calvin-Benson cycle for CO₂ fixation.
• Ecological Role - They are essential Primary producers, forming the base of many food webs and playing a critical role in Biogeochemical cycles, including carbon, sulfur and nitrogen cycling.

HISTORY OF PHOTOSYNTHETIC BACTERIA

• Early 19^th Century: Purple bacteria were thought to have photosynthetic machinery because they moved towards light (phototaxis) and their growth was stimulated by light.
• Sergi Winogradsky discovered that some Purple bacteria use Hydrogen sulfide (H₂S) and convert it to Sulfate, depositing Sulfur inside their cells.
• 1875 - Discovery of Purple Sulfur Bacteria by Ferdinand Cohn in Sulfur springs.
• 1889 - Hans Molisch demonstrated Photosynthesis in Anoxygenic Purple bacteria.
• 1901 - Cornelis van Niel proposed that all photosynthesis involves light-driven CO₂ fixation using electron donors, generalizing the process.
• 1930 - Van Niel clarified differences between Oxygenic and Anoxygenic Photosynthesis. He defined Anoxygenic Photosynthesis as a mode of metabolism in Purple and Green bacteria. He showed these bacteria use Hydrogen donors like H₂S or Organic compounds instead of Water.
• 1975 - Parson and Cogdell isolated functional complexes from photosynthetic bacteria. The reaction centre from the purple non-sulphur bacterium, Rhodopseudomonas sphaeroides, contains four molecules of chlorophyll and two molecules of bacteriopheophytin (like b chlorophyll but the Mg replaced by two H+), one or two molecules of ubiquinone and one atom of ferrous iron together with three polypeptides of apparent molecular weight in the region of 28, 24 and 21 KD.

 TYPES OF PHOTOSYNTHETIC BACTERIA

1) Cyanobacteria (Blue – Green Algae)

2) Purple Sulfur Bacteria

3) Purple Non-Sulfur Bacteria

4) Green Sulfur Bacteria

5) Green Non-Sulfur Bacteria

6) Helicobacteria