Purple Sulfur Bacteria

Purple Sulfur Bacteria

·       Example – Chromatium, Allochromatium, Thiocapsa, Thiocystis, Thiospirillum, Ectothiorhodospira and Lamprocystis.

·       Number of Species – Over 100 described species were identified within the group belonging mainly to the family Chromatiaceae (Class: Gammaproteobacteria) and Ectothiorhodospiraceae.

·       Habitat – Anaerobic aquatic environments, especially:

ü  Stagnant Water Bodies: Lakes, ponds, and lagoons, especially those with decaying organic matter.

ü  Sulfur Springs: Environments where Hydrogen sulfide is geothermally produced which includes Sulfide-rich stagnant waters, lagoons, lakes, and hot springs.

ü  Salt Marshes and Microbial Mats: In the deeper, anoxic layers where sulfide is present.

ü  Meromictic Lakes: Permanently stratified lakes where the deeper layer (hypolimnion) is anoxic and may contain hydrogen sulfide.

·       Mode of Nutrition – Photoautotrophic (use light as energy, CO₂ as carbon source). Some can also be photoheterotrophic (use organic carbon sources).

·       Mode of Reproduction – Asexual reproduction via Binary fission.

·       Mode of Respiration – Primarily, Anaerobic (Obligate or Facultative anaerobes) but some can tolerate low Oxygen (Microaerophilic). They mainly rely on Sulfur compounds.

·       Type of Photosynthesis – Anoxygenic photosynthesis (no oxygen produced). Uses Bacteriochlorophyll a or b (absorbing infrared/near-IR light).

·       Pigments

a)     Bacteriochlorophyll (mostly Bacteriochlorophyll a or b) - For light absorption.

b)     Carotenoids - Give purple, red, orange or brown colorus. Examples: Spirilloxanthin, Rhodopinal and Okenone.

·       Electron Donor – Mainly Hydrogen sulfide (H₂S) act as an Electron donor. Sometimes, Hydrogen sulfide (H₂S) oxidized to Thiosulfate (S₂O₃²⁻) or Elemental Sulfur (S⁰) and act as an Electron donor.

·       By-products

a)     Elemental sulfur (S⁰) (stored inside or outside cells as globules).

b)     Sulfate (SO₄²⁻) (if sulfur is further oxidized).

·       Photosystem – Only Photosystem I (PS I) is used.

 

·       Ecological role

ü  Primary producers in anoxic aquatic environments. They convert light energy into chemical energy, forming the base of food webs in these unique ecosystems.

ü  Sulfur cycling: Play a role in Sulfur cycling by oxidizing Hydrogen sulfide to Sulfur/Sulfate.

ü  Carbon fixation: Contribute to organic matter production in anaerobic zones.

ü  Biological Indicator: Indicators of Polluted or Eutrophic environments rich in Sulfide.

ü  Symbiotic relationships: Found in microbial mats with other bacteria.