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Micro-organisms are very tiny living things. They are so small
that you need a microscope to see them.
Viruses, bacteria and fungus are all types of micro-organism.
Micro-organisms that cause diseases are often called germs.
Micro-organisms are all around us, in the air, in our bodies and
in water. Some micro-organisms are harmful to us, but others
are helpful to us.
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The study of microorganisms is know as
Microbiology.
There are four types of microorganisms:
1) bacteria
2) algae
3) fungus
4) protozoa
Bacteria, one-celled organisms visible only
through a microscope. Bacteria live all around
us and within us. The air is filled with
bacteria, and they have even entered outer
space in spacecraft. Bacteria live in the
deepest parts of the ocean and deep within
Earth. They are in the soil, in our food, and
on plants and animals. Even our bodies are
home to many different kinds of bacteria.
Our lives are closely intertwined with theirs,
and the health of our planet depends very
much on their activities.
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Bacterial cells are so small that scientists measure
them in units called micrometers (µm). One
micrometer equals a millionth of a meter
(0.0000001 m or about 0.000039 in), and an
average bacterium is about one micrometer long.
Hundreds of thousands of bacteria would fit on a
rounded dot made by a pencil.
Bacteria lack a true nucleus, a feature that
distinguishes them from plant and animal cells. In
plants and animals the saclike nucleus carries
genetic material in the form of deoxyribonucleic
acid (DNA). Bacteria also have DNA but it floats
within the cell, usually in a loop or coil. A tough but
resilient protective shell surrounds the bacterial
cell.
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Biologists classify all life forms as either
prokaryotes or eukaryotes. Prokaryotes are
simple, single-celled organisms like bacteria.
They lack a defined nucleus of the sort found
in plant and animal cells. More complex
organisms, including all plants and animals,
whose cells have a nucleus, belong to the
group called eukaryotes. The word prokaryote
comes from Greek words meaning “before
nucleus”; eukaryote comes from Greek words
for “true nucleus.” The study of bacteria is
called bacteriology, a branch of microbiology
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Bacteria play a key role in making soil fertile.
They convert nitrogen in Earth’s atmosphere into
the nitrogen compound ammonia, which plants
need to grow. Bacteria are the only organisms
able to carry out this biochemical process known
as nitrogen fixation. The bacteria able to fix
atmospheric nitrogen usually live in association
with plants, often integrated into the plant
tissue. Bacteria in the genus Rhizobium, for
example, form nodules (knobs) on the roots of
beans and other plants in the legume family.
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Algae, diverse group of simple, plant like
organisms. Like plants, most algae use the
energy of sunlight to make their own food, a
process called photosynthesis. However, algae
lack the roots, leaves, and other structures
typical of true plants. Algae are the most
important photosynthesizing organisms on
Earth. They capture more of the sun’s energy
and produce more oxygen (a by-product of
photosynthesis) than all plants combined. Algae
form the foundation of most aquatic food
webs, which support an abundance of animals.
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Algae vary greatly in size and grow in many
diverse habitats. Microscopic algae, called
phytoplankton, float or swim in lakes and oceans.
Phytoplankton are so small that 1000 individuals
could fit on the head of a pin (see Plankton).
The largest forms of algae are seaweeds that
stretch 100 m (300 ft) from the ocean bottom
to the water’s surface. Although most algae
grow in fresh water or seawater, they also grow
on soil, trees, and animals, and even under or
inside porous rocks, such as sandstone and
limestone. Algae tolerate a wide range of
temperatures and can be found growing in hot
springs, on snow banks, or deep within polar ice.
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Fungus, any member of a diverse group of organisms that—
unlike plants and animals—obtain food by absorbing nutrients
from an external source. The fossil record suggests that
fungi were present 550 million years ago and may have
evolved even earlier. Today thousands of different types of
fungi grow on and absorb food from substances such as soil,
wood, decaying organic matter, or living plants and other
organisms. They range from tiny, single-celled organisms
invisible to the naked eye to some of the largest living
multicellular organisms. In Michigan for example, the
underground portion of an individual Armillaria mushroom, a
type of fungus, extends more than 12 hectares (30 acres).
Other fungi are among the longest-lived organisms on Earth—
some lichens, a living partnership of a fungus and an alga, are
thought to be more than 4,500 years old.
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Common fungi include mushrooms, puffballs, truffles,
yeasts, and most mildews, as well as various plant and
animal pathogens (disease agents), such as plant rusts
and smuts. Some experts estimate that there are 1.5
million fungus species, of which approximately 100,000
have been identified. The unique characteristics of
fungi led scientists to classify these important
organisms into a separate kingdom, Kingdom Fungi (also
known as Mycenae). Certain fungus-like organisms,
such as downy mildews, water moulds (also known as
oomycetes), and slime moulds, once classified as fungi,
are now placed in the Kingdom Protista.
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Protozoa, collective name for animal-like, single-celled
organisms, some of which may form colonies. In the
classification followed in this the protozoa are placed in the
kingdom Protista with other single-celled organisms that have
membrane-enclosed nuclei. Protozoa have little or no
differentiation into tissue systems. Several phyla are commonly
recognized. They include flagellated Zoomastigina, many species
of which live as parasites in plants and animals; the amoeboid
Sarcodina, which includes the Foraminifera and Radiolaria both
important components of the plankton; ciliated Ciliophora, many
with specialized structures suggesting the mouth and anus of
higher organisms; Cnidosporidia, parasites of invertebrates,
fish, and a few reptiles and amphibians; and Sporozoa, many
species of which are parasites of animals (including humans).
More than 20,000 species are known, including such familiar
forms as paramecium and amoeba
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Most species are found in such aquatic
habitats as oceans, lakes, rivers, and ponds.
They vary in length from 2 to 70 micrometers.
Protozoa obtain their food by ingesting
bacteria, waste products of other organisms,
algae, or other protozoa. Most species are
motile, either by whip like structures called
flagella, hair like structures called cilia, or
amoeboid motion, a streaming type of
movement involving the formation of pseudo
pods (foot like extensions).
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Virus (life science), infectious agent found in virtually all
life forms, including humans, animals, plants, fungi, and
bacteria. Viruses consist of genetic material—either
deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—
surrounded by a protective coating of protein, called a
capsid, with or without an outer lipid envelope. Viruses are
between 20 and 100 times smaller than bacteria and hence
are too small to be seen by light microscopy. Viruses vary
in size from the largest poxviruses of about 450
nanometres (about 0.000014 in) in length to the smallest
polioviruses of about 30 nanometres (about 0.000001 in).
Viruses are not considered free-living, since they cannot
reproduce outside of a living cell; they have evolved to
transmit their genetic information from one cell to another
for the purpose of replication.