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    These are the bacteria that can use sulfate to accept the electrons during anaerobic conditions. These are the anaerobic bacteria that reduce sulfate into hydrogen sulfide.


    The characteristics of the sulfur-reducing bacteria are as follows:

    • These bacteria are present in sediments, sea waters, and the area where the rich source of decay organic matter is present.
    • They are present in anaerobic conditions.
    • They produce a rotten egg smell by producing H2S gas.
    • They produce metal sulfide when they react with hydrogen sulfide.
    • These metal sulfides are insoluble and brown to red in color.
    • These bacteria produce metal sulfides that lead to the dark color of sludge.

    Why bacteria use sulfate?
    These bacteria grow in anaerobic conditions. They can’t use oxygen as electron terminal acceptor. They reduce inorganic sulfate into hydrogen sulfide. Sulfur accepts the electron from ETC.  Some microbes assimilate sulfur to fulfill their sulfur requirement. They may use organic sulfur to fulfill their requirement.

    SO4                        SO3-2                             H2S

    This is done by sulfate reductase enzymes

    Classification of sulfate-reducing bacteria:

    These are of two types.

    • Organotrophic
    • Lithotrophic
    • Phototrophic
    • Chemotrophic


    These are the bacteria that oxidize organic compounds such as carbohydrates, organic alcohols and acids, and aromatic hydrocarbons. They obtain electron acceptor from the oxidation organic compounds.


    These bacteria use inorganic compounds for the acceptance of electron. They oxidize the molecular hydrogen and use them as the electron acceptor. They live in anaerobic conditions and compete with methanogens. They can use inorganic iron as the electron acceptor.


    These bacteria can prepare their own food by photosynthesis. They can use light energy and fix the CO2 into food.  They have bacterial chlorophyll that can convert light energy into glucose. They are commonly green sulfur bacteria.


    These bacteria can’t prepare their own food. They use the chemical to get energy and use organic carbon for its requirement. They don’t have bacterial chlorophyll to convert light energy into glucose. They are purple sulfur bacteria.


    Purple sulfur bacteria and green sulfur bacteria.


    These are the group of anaerobic bacteria that are capable of photosynthesis. They may be Microaerophilic bacteria. These bacteria are divided into two groups i.e. the Chromatiaceae and the Ectothiorhodospiraceae. They are different in their internal membranes structure and presence of extracellular or intracellular sulfur granules.

    The characteristics of purple sulfur bacteria are as follows.

    • They are anaerobic or Microaerophilic bacteria that is they use either no or a small amount of air.
    • Unlike cyanobacteria, they use reduced sulfur to produce sulfur.

    Reduced sulfur                     S

    • They sulfur oxidize into sulfur granules that are present inside or outside the cell.
    • They are present in stagnant water and hot springs.
    • Hydrogen sulfide forms the bloom of purple sulfur bacteria.
    • They have bacterial chlorophyll with carotenoids that give them various colors such as purple, pink etc.
    • Their photosynthesis occurs in the cell membrane where photosynthetic pigments are present in the form of vesicles and stacks.




    These bacteria use reduce sulfur as electron donor as they can’t use oxygen. They produced hydrogen sulfide that produces rotten egg smell. They are photosynthetic in nature as they can fix the inorganic carbon into glucose. They can use light energy to get energy and fix CO2

    CO2 + H2S                C6H12O6 + S

    They produce granules that can be observed under 1000X microscope. They give purple to pink color to the water.  They grow in dark places.


    They are mainly photoautotrophic that is they can synthesize or prepare its own food. But some are chemoautotrophic that they use chemical to get energy. They may be photoheterotrophic that can’t fix carbon into its food but get energy from light. They are strict anaerobic bacteria but some of them are good fermenters.

    Ecological importance:

    The ecological importance of purple sulfur bacteria is as:

    • They take part in nutrient cycles.
    • They play an important role in primary production as they can fix the carbon.
    • They recycle the phosphorous cycle for heterotrophic bacteria to use.
    • These bacteria can use as food for other organism.
    • These bacteria release excess light as heat energy.


    These bacteria can reduce the harmful waste from the environment. Purple sulfur bacteria lower the amount of hydrogen sulfide, methane and toxic compound present in water lagoons.

    Hydrogen sulfide use as a source of sulfur for the purple sulfur bacteria while methane use as the source of carbon for these bacteria. Thus, purple sulfur bacteria lower the odor and toxicity of water.

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