A domain is a taxonomic level above kingdom. 1.Domain Bacteria (Eubacteria) Account for most of the prokaryotes, with every major mode of nutrition and metabolism represented among the thousands of known species. (“True” bacteria) 2.Domain Archaea (Archaebacteria) The term archeabacteria refers to the antiquity of the group’s origin from the earliest cells (Gr. archaio, “ancient”)  The archaea inhabit extreme environments and differ from bacteria in many key structural, biochemical, and physiological characteristics.

1. Eubacteria Eubacteria are true bacteria. The characteristic feature is the presence of rigid cell wall and if present a motile flagellum that aids in locomotion. These organisms are characterized based on their nutrition and their shapes. 2. Archaebacteria Archaebacteria are microbes that live in extreme and harsh conditions, they are known as extremophiles. These bacteria lack cell wall, their cell membrane is made up of different lipids, and their ribosomes are similar to that of eukaryotes. Archaebacteria are of three major groups of bacteria based on their habitat i.e., thermophiles, halophiles and methanogens. 3. Cyanobacteria Cyanobacteria (Blue-green algae). They have chlorophyll similar to plants and hence they are photosynthetic autotrophs. The marine and terrestrial and they may be unicellular, colonial or filamentous. The colonies are surrounded by gelatinous sheath. They can also fix atmospheric nitrogen.

 Nutrition here refers to how an organism obtains energy and a carbon source from the environment to build the organic molecules of cells.

 Methanogens obtain energy by using CO 2 to oxidize H 2 replacing methane as a waste.  They live in swamps and marshes where other microbes have consumed all the oxygen  Other methanogens live in the anaerobic guts of herbivorous animals, playing an important role in their nutrition.

 Extreme thermophiles thrive in hot environments.  The optimum temperatures for most thermophiles are 60 o C- 80 o C.  Sulfolobus oxidizes sulfur in hot sulfur springs in Yellowstone National Park.  Another sulfur- metabolizing thermophile lives at 105 o C water near deep-sea hydrothermal vents.

 Use inorganic compounds as source of energy (instead of sun)  Inorganic = compounds containing sulfur, iron, nitrogen (not carbon!)  Use energy to change carbon dioxide into organic food molecules  Live in harsh environments (e.g. hot sulfur vents on ocean floor)

 Extreme halophiles live in such saline places as the Great Salt Lake and the Dead Sea.  Some species merely tolerate elevated salinity; others require an extremely salty environment to grow.  Colonies of halophiles form a purple-red scum from bacteriorhodopsin, a photosynthetic pigment very similar to the visual pigment in the human retina.

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Nostoc Oscillatoria

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