PHOTOSYNTHESIS IN MICROORGANISMS AND PHOTOSYNTHETIC PIGMENTS

 PHOTOSYNTHESIS IN MICROORGANISMS AND PHOTOSYNTHETIC PIGMENTS

• The process by which light energy is trapped and converted to chemical energy is called Photosynthesis.
• Microorganisms derive energy not only from the oxidation of inorganic and organic compounds but also from light energy, which they capture and use to synthesize ATP and reduce power (e.g., NADPH).
• Essentially, photosynthesis is the conversion of light energy from the sun into chemical energy. The chemical energy is then used to convert CO₂ from the atmosphere to more reduced carbon compounds, primarily sugars. This synthesis of sugars by using carbon atoms from CO₂ gas is also called Carbon fixation.
• In Prokaryotes, Photosynthesis takes place in the Plasma membrane.
• In Eukaryotes, Photosynthesis takes place in the Chloroplast.

DIVERSITY OF PHOTOSYNTHETIC ORGANISMS

• Eukaryotic organism - a) Plant; b) Multicellular Algae (Green algae, Red algae, and Brown algae), and c) Unicellular Protists (Euglenoids, Dinoflagellates, and Diatoms).
• Prokaryotic organisms - a) Cyanobacteria; b) Green sulfur bacteria; c) Green non-sulfur bacteria; d) Halobacterium sp. (Archaea); e) Purple sulfur bacteria; f) Purple non-sulfur bacteria and Prochloron sp.

PHOTOSYNTHETIC PIGMENTS

• Pigments are chemical compounds that reflect only certain wavelengths of visible light. This makes them appear "colorful".
• Flowers, corals, and even animal skin contain pigments which give them their colors.

1) Chlorophyll 

• Chlorophyll is a Green photosynthetic pigment. Three types: Chlorophyll – a, Chlorophyll – b and Chlorophyll – c.
• Chlorophyll – a absorbs light of wavelength 430 nm (blue) to 662 nm (red). The Chlorophyll – a is present in Plant, Algae and Cyanobacteria.
• Chlorophyll – b absorbs light of wavelength 453 nm to 642 nm. Chlorophyll – b is present only in Plants, Chloroxybacteria and Green algae (Chlorophyta).
• Chlorophyll – c is present only in Diatoms (Bacillariophyta), Dinoflagellates, Golden algae (Chrysophyta) and Brown algae (Phaeophyta).
• Bacteriochlorophyll is a type of Chlorophyll present only in Bacteria. They absorbs maximally between 800 and 925 nm. 

2) Carotenoids

• Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color.
• Carotenoids absorb light maximally between 460 nm and 550 nm.
• Completely hydrophobic (fat soluble) and exist in lipid membranes.
• Carotenoids are classified into 2 types. They are (i) Carotenes and (ii) Xanthophylls.
• Carotenes are present in the entire photosynthetic microorganism.
• Xanthophylls (Fucoxanthin) are present only in Brown algae, Golden algae and Diatoms.   

3) Phycobilins or Phycobiliproteins

• Phycobilins are water-soluble pigments, and are therefore found in the Cytoplasm, or in the Stroma of the chloroplast.
• They occur only in Cyanobacteria (Blue – Green Algae; contains blue pigment Phycocyanin, maximum absorption at 620 nm to 640 nm) and Rhodophyta (Red Algae; contains red pigment Phycoerythrin, maximum absorption at 550 nm).

4) Accessory Pigments

• Carotenoids and Phycobiliproteins are often called Accessory pigments because of their role in photosynthesis.
• Carotenoids and Phycobiliproteins cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called Accessory pigments.
• One very visible Accessory pigment is Fucoxanthin (a type of Xanthophylls) the brown pigment which colors kelps and other Brown algae, Golden algae as well as the Diatoms.
• Accessory pigments also protect microorganisms from intense sunlight, which could oxidize and damage the photosynthetic apparatus.