The document discusses several types of microorganisms including archaea, which are single-celled prokaryotes that were initially misclassified as bacteria. It describes archaea subgroups like methanogens, halophiles, and thermophiles, which are extremophiles that thrive in harsh environments, as well as eubacteria. The microbes discussed play important roles in ecosystems and some have applications in industry.

1. Microbial Diversity
Course: B.Sc. Microbiology
Sem II
Sub: Bacteriology
Unit III

2. Archaebacteria
• The Archaea constitute a domain or kingdom of single-celled
microorganisms.
• These microbes are prokaryotes, meaning that they have no cell
nucleus or any other membrane-bound organelles in their cells.
• Archaea were initially classified as bacteria, receiving the name
archaebacteria (in the Kingdom Monera), but this classification is
outdated.
• Archaeal cells have unique properties separating them from the
other two domains of life: Bacteria and Eukaryota.
• The Archaea are further divided into four recognized phyla.
Classification is difficult because the majority have not been studied
in the laboratory and have only been detected by analysis of their
nucleic acids in samples from their environment.

3. • Archaea were initially viewed as extremophiles living in harsh
environments, such as hot springs and salt lakes, but they have since
been found in a broad range of habitats, including soils, oceans,
marshlands and the human colon and navel.
• Archaea are particularly numerous in the oceans, and the archaea in
plankton may be one of the most abundant groups of organisms on the
planet.
• Archaea are a major part of Earth's life and may play roles in both the
carbon cycle and the nitrogen cycle.
• No clear examples of archaeal pathogens or parasites are known, but
they are often mutualists or commensals.
• One example is the methanogens that inhabit the human gut and the
ruminant gut, where their vast numbers aid digestion.
• Methanogens are used in biogas production and sewage treatment, and
enzymes from extremophile archaea that can endure high
temperatures and organic solvents are exploited in biotechnology.

4. Methanogens
• Methanogens are microorganisms that produce methane as a metabolic
byproduct in anoxic conditions. They are classified as archaea, a domain
distinct from bacteria.
• They are common in wetlands, where they are responsible for marsh gas,
and in the digestive tracts of animals such as ruminants and humans,
where they are responsible for the methane content of belching in
ruminants and flatulence in humans.
• In marine sediments the biological production of methane, also termed
methanogenesis, is generally confined to where sulfates are depleted,
below the top layers.
• Moreover, the methanogenic archaea populations play an indispensable
role in anaerobic wastewater treatments. Others are extremophiles, found
in environments such as hot springs and submarine hydrothermal vents as
well as in the "solid" rock of the Earth's crust, kilometers below the
surface.
• Not to be confused with methanotrophs which rather consume methane
for their carbon and energy requirements.

5. Halophiles
• Halophiles are organisms that live in high salt
concentrations. They are a type of extremophile organism.
The name comes from the Greek word for "salt-loving".
• While most halophiles are classified into the Archaea
domain, there are also bacterial halophiles and some
eukaryota, such as the alga Dunaliella salina or fungus
Wallemia ichthyophaga.
• Some well-known species give off a red color from
carotenoid compounds, notably bacteriorhodopsin.
• Halophiles can be found anywhere with a concentration of
salt five times greater than the salt concentration of the
ocean, such as the Great Salt Lake in Utah, Owens Lake in
California, the Dead Sea, and in evaporation ponds.

6. Thermophile
• A thermophile is an organism — a type of extremophile — that thrives
at relatively high temperatures, between 41 and 122 °C (106 and
252 °F). Many thermophiles are archaea. Thermophilic eubacteria are
suggested to have been among the earliest bacteria.[
• Thermophiles are found in various geothermally heated regions of the
Earth, such as hot springs like those in Yellowstone National Park (see
image) and deep sea hydrothermal vents, as well as decaying plant
matter, such as peat bogs and compost.
• Unlike other types of bacteria, thermophiles can survive at much
hotter temperatures, whereas other bacteria would be damaged and
sometimes killed if exposed to the same temperatures.
• Professor Zachary Studniberg, from Cambridge University, wrote in his
book 'The Function of Extremophiles' that they are the most unique
organism on the planet in terms of their contribution to modern life.
• As a prerequisite for their survival, thermophiles contain enzymes that
can function at high temperatures. Some of these enzymes are used in
molecular biology (for example, heat-stable DNA polymerases for
PCR), and in washing agents.

7. Eubacteria
• Eubacteria is a bacterium of a large group typically having simple
cells with rigid cell walls and often flagella for movement. The
group comprises the ‘true’ bacteria and cyanobacteria, as distinct
from archaea.
• Bacteria constitute a large domain of prokaryotic microorganisms.
• Typically a few micrometres in length, bacteria have a number of
shapes, ranging from spheres to rods and spirals.
• Bacteria were among the first life forms to appear on Earth, and
are present in most of its habitats.
• Bacteria inhabit soil, water, acidic hot springs, radioactive waste,
and the deep portions of Earth's crust. Bacteria also live in
symbiotic and parasitic relationships with plants and animals. They
are also known to have flourished in manned spacecraft.

8. References
Books:
1. Biology of microorganisms By M. T. Madigan,
J. M. Martinko, D. A. Stahl and D. P. Clark