Discussing Bacteria, Protozoa, Fungi, Algae and Archaea
What is a Unicellular Organism?
Essentially, unicellular organisms are living
organisms that exist as single cells. Examples include such bacteria as Salmonella and protozoa like Entamoeba coli. Being single
celled organisms, various types possess different structures and characteristics that
allow them to survive.
According to a report that was released in 2012 from the
University of Potsdam in German, it was stated that the ocean is home to an
estimated 2.9×1029 unicellular organisms (about 20,000 species).
Here, it is
worth noting that this figure only represents the number of single-celled
organisms in the ocean and not on land, which in other words means that the
total number is much higher.
Although there are a vast number that exist on earth, they are divided into the following groups:
- Fungi (unicellular)
- Algae (unicellular)
Despite their diversity, they share a number of basic characteristics.
* They are alive and share a number of
characteristic with all living things such as:
Organization - Unicellular organisms
possess various structures that make it possible for them to survive. These
structures are contained within the cell (in the cytoplasm) and include such
structures as the endoplasmic reticulum and genetic material among others.
Growth - Given that they are
living things, unicellular organisms increase in size.
Reproduction - Unicellular organisms
also reproduce, which allows them to form other organisms that are like
themselves. The genetic material possessed by these microorganisms divides
allowing each of the daughter cells to get an exact copy of the genetic
material that was contained in the original cell.
Response to external
environment - Unicellular organisms also respond to various conditions such
as change in temperature, light as well as touch. It is this ability to respond
to environmental changes that make it possible for unicellular organisms to
find food and continue surviving.
* Considering that single-celled
(unicellular) organisms have the characteristics of living things, we cannot
include viruses here. This is due to the fact that viruses are not considered
living things despite the fact that they have genetic material and various
characteristics of living organisms.
There are a number of characteristics that
distinguish viruses from other unicellular organisms, these include:
- Viruses do not
grow/increase in size once they are formed
- Viruses don't take in
energy like other unicellular organisms.
- They depend on the host
cell to reproduce (they are unable to reproduce on their own).
See more on viruses under the microscope.
Bacteria (single - bacterium) are some of the
most abundant unicellular organisms in the world. For instance, according to
the National Academy of Sciences, a single human body is estimated to have 100
trillion individual bacterial cells. They are prokaryotic cells, which means
that they are simple, unicellular organisms that lack a nucleus and
membrane-bound organelles (they have small ribosome).
For a majority of
bacteria (prokaryotic cells) the DNA is contained in a nucleoid in form of a
large loop of circular chromosome. Different types of bacteria will have such
structures as the flagellum, pili, a biofilm, cell wall and a capsule among
Although a good number of bacteria can and do cause both human and
animal diseases, some bacteria are very important. For instance, whereas
bacteria like actinomycetes are used to produce antibiotics that are valuable in treating given diseases, others like Lactobacillus bulgaricus and Streptococcus
thermophilus are used in yogurts.
Other types of bacteria have also been shown
to be beneficial in such industries as agriculture and the food industry.
There are different types of bacteria based on shape, nutrition, gaseous requirement as well as the cell wall.
There are three types of bacteria based on
shape. The different shapes include:
Spherical Shaped (Coccus) Bacteria
The cocci bacteria are spherical in shape (or
ovoid). While they may occur as single cells, the cocci bacteria can also
remain attached to others.
There are different types of attached bacteria
Diplococci bacteria - Diplococci occur in
pairs (two) and includes such bacteria as Nisseria gonorrhoeae
Streptococci bacteria - Streptococci bacteria
occur as chains with a number of bacteria being attached to each other in a
chain like manner. A good example of streptococci bacteria includes the species
Streptococcus canis and Streptococcus bovis.
Staphylococci bacteria - This type of bacteria
occurs in a cluster. An example of staphylococci bacteria is Staphylococcus
aureus. Some of the bacteria that exist in clusters only exist in clusters of
four and are known as tetrads (such as the species Micrococcus).
Spirilli are Gram-negative bacteria that tend to
be spiral in shape. They belong to the family Spirillaceae and include the
species Spirillum winogradskyi and Spirillum volutans.
While the spirilli are spiral in shape, vibrio
bacteria such as Vibrio cholerae are comma shaped.
Classification Based on Nutrition
Bacteria are also grouped based on nutrition. Essentially, there are two broad categories which include:
Autotrophic bacteria are the type of bacteria that can
synthesize their food from inorganic substances (substances that do not contain
carbon). For this type of bacteria, carbon dioxide is used to obtain carbon.
There are two main types of autotrophic bacteria.
Photoautotrophs - Photoautotrophic
bacteria are the type of bacteria that have a photosynthetic pigment (purple
pigment, green pigment etc). These pigments are used to synthesize food
(carbohydrate) in the presence of sunlight through a process known as
Examples of photoautotrophic bacteria include:
See more on Autotrophs
Chemosynthetic bacteria - Unlike photoautotrophic
bacteria, chemosynthetic bacteria can synthesize food from given inorganic
chemicals in the absence of sunlight. As such, they do not require light energy
to synthesize food.
Chemosynthetic bacteria are also divided into:
- Nitrifying bacteria such as
Nitrosomonas which obtain energy by oxidizing ammonia
- Sulphomonas bacteria like
Thiobacillus which obtain energy through the oxidization of hydrogen sulphide
- Ferrromonas bacteria or
iron bacteria like Leptothrix. These bacteria obtain energy from the oxidation
of ferrous compounds.
- Hydromonas bacteria - For
hydromonas bacteria like Bacillus pantotrouphs, energy is obtain from hydrogen conversion
Heterotrophic bacteria are the type of bacteria
that obtain energy from organic compounds. This means that they are unable to
make their own food, and thus use ready-made foods as their source of energy.
Heterotrophic bacteria are also divided into:
Saprophytic bacteria - This includes bacteria
like Acetobacter that obtain energy from dead and decaying organic substances
like leaves, meat and humus. These bacteria are able to secrete enzymes that
are used for fermentation or putrefaction during digestion.
Parasitic bacteria - Bacillus anthracis and
Vibrio cholerae are examples of parasitic bacteria. As such, they obtain energy
from the tissues of living things. Whereas some may be harmless, some of these
bacteria can cause serious diseases to the host.
Symbiotic bacteria - Symbiotic bacteria like
Bacillus azotobacter and Rhizobium are able to establish a symbiotic
relationship with the host. As such, they are beneficial to the host and do not
See more on Heterotrophs
Bacteria Based on Gaseous Requirement
Bacteria are also classified according to
gaseous requirements. Whereas some bacteria need oxygen to survive, others do
Aerobes - Aerobes (aerobic bacteria) are the type of bacteria that can
only live and reproduce in the presence of oxygen. There are two groups of
aerobes which include the obligate aerobes and the microaerophiles. For
obligate microbes like Mycobacteria, high concentration of oxygen (like in room
air) is required for life. However, microaerophiles like Campylobacter only
require a lower concentration of oxygen (about 5 percent of oxygen) to survive.
Anaerobes - Unlike aerobes, anaerobes are the type of bacteria that do not
need oxygen for survival. Anaerobes include obligate anaerobes like Clostridium
that are unable to live and reproduce in the presence of oxygen and facultative
anaerobes like Staphylococci bacteria that can survive in the presence or
absence of oxygen.
Classification of Bacteria Based on Cell Wall Content
Gram-stain technique is also used to classify
bacteria. Whereas the cell wall of certain bacteria contains a thin layer of a
polymer known as peptidoglycan (Gram-negative bacteria) in between the inner
and outer lipid membrane, others have a thicker layer of the polymer in addition
to lipoteichoic acid (Gram-positive bacteria).
Because of the think polymer in
their cell wall, the Gram-positive retain the crystal violet dye of Gram stain
and appear purple under the microscope. However, Gram-negative bacteria have a
thinner layer and are therefore unable to retain the color. As a result, they
stain pink/red by taking Safranin stain.
- Lactobacillus etc
Unlike bacteria, protozoa are eukaryotic unicellular
organisms. A majority of protozoa are free living (they can exist on their own)
while others infect higher animals and can cause diseases.
cells, protozoa portray traits that are common among other animal cells. This
is due to the fact that they possess a nucleus and a number of other important
organelles within the cell membrane. Given that there are different types of
protozoa, classification is largely based on means of locomotion.
The phylum Sarcodina is the largest phylum of protozoa
and contains an estimated 11,500 species of protozoa. Amoeba proteus and
Entamoeba histolytica that belong to this phylum move through the streaming of
In addition, unicellular organisms that belong to phylum
Sarcodina also use temporary pseudopodia, which are projections of the protoplasm
commonly, referred to as false feet.
Mastigophora is also a phylum of protozoa and
includes the likes of Giardia lamblia and Trichomonas vaginalis. While some
have temporary pseudopodia, a majority of Mastigophora are flagellates, which
means that they are propelled by a whip-like structure during motion.
flagellates may have a one or more flagella that make movement possible. Some in
the phylum Mastigophora are free- living organisms (Cercomonas longicauda etc)
while others exist as parasites (e.g Trypanosoma gambiense).
Unlike the Phylum Mastigophora, unicellular organisms in the Phylum Ciliophora possess cilia, which are short hair-like projections that propel them and ensure their movement.
Some of the more common examples include Paramecium caudatum and Vorticella campanula, which are free living. Known as ciliates, these unicellular organisms may also have these projections around the mouth for feeding. The parasitic form of Ciliophora includes Balantidium coli.
Essentially, Sporozoa are parasitic forms that lack locomotor structures. Common examples of Sporozoa include the Plasmodium species that cause malaria in human hosts.
According to the Center for Disease Control and Prevention, Earth is home to an estimated 1.5 million species of fungi. Of these, over 800 species of the single-celled fungi (yeast) have been identified.
Like protozoa, yeast are also eukaryotic cells, which means that they possess a cell nucleus and other cell organelles.
They are typically found in environments that are moist and include:
The Species Saccharomyces Cerevisiae
This is the budding yeast that is known to convert carbohydrates to carbon dioxide and alcohols. This specie of yeast is particularly used in the food industry and particularly for making bread. The carbon-dioxide produced during the breakdown of carbohydrates makes the dough rise. Given that it also produces alcohol in the process, the yeast is also used in the alcohol industry.
While some yeast are beneficial and are used in such
industries as the food industry, others are pathogenic and tend to cause
diseases. These are mostly opportunistic pathogens, which means that they cause
infections among people with poor immune systems. An example of pathogenic
yeast includes Cryptococcus neoformans which has been shown to cause systemic
Algae includes a diverse group of photosynthetic
organisms that can be found in a wide range of habitats (aquatic to land). The
unicellular algae are mostly plant-like autotrophs that can make their own
There are five major divisions of unicellular algae including:
Chlorophyta (Green Algae) - Chlorophyta are green in
color because of the presence of chlorophyll. They may also contain a
carotenoid pigments and includes the likes of Chlamydomonas, which use
flagellum for movement.
Charophyta - The division Charophyta
includes some members of class Zygnemophyceae such as Zygnematales. Motile
cells in this division also possess flagella and are mostly biflagelated.
Euglenophyta - A good example of this
division is the Euglena cell. These cells possess a large nucleus as well as a nucleolus. They also have chlorophyll and the
caroteid pigments that make photosynthesis possible. These cells also use
flagella for movement.
Chrysophyta - Diatoms are some of the
most common Chrysophyta. They are enclosed
in a cell wall that is made up of silica and may exist as single cells or in
Pyrrophyta - Some of the most common
Pyrrophyta includes dinoflagellates like marine planktons. They also include
amoeboid cells and possess chlorophyll and such pigments as the carotenoid and
Like bacteria, Archaea are prokaryotes, which
means that they lack a well defined nucleus and membrane-bound organelles.
Although they are prokaryotes like bacteria, the Archaea are distinct with
regards to their biochemistry, which distinguishes them from both bacteria and
other unicellular eukaryotes. For instance, unlike bacteria, the cell wall of Archaea
In addition, the lipids of Archaea lack fatty acids (fatty
acids are replaced by isoprene units), which can be found in both bacteria and
eukaryotes. Although the identification and classification of Archaea has been
shown to be rather difficult, they can be grouped in the following phyla.
Crenarchaeota - The phyla Crenarchaeota
is largely composed of hyperthermophiles and thermoacidophiles both of which
may be described as extremophiles.
Extremophiles can be found in marine
environments as well as other extreme environments such as hot and acidic
springs. A good example of extremophiles is the Sulfolobus acidocaldarius that can
be found in terrestrial solfataric springs.
Euryarchaeota - this phylum is largely
composed of halophiles (e.g Halobacterium) and methanogens (e.g Methanococcus).
The halophiles are mostly found in such salty environments as the Dead Sea while
methanogens can be found in animal intestines (cows and human beings) as well
as in wetlands.
Korachaeota - Unlike Crenarchaeota and
Euryarchaeota, the phylum Korachaeota has been shown to consist of more
primitive members that are thermophilic in nature. They are also only found in
hydrothermal environments and include the Species Candidatus.
More on Cells:
Eukaryotes - Cell Structure and Differences
Prokaryotes - Cell Structure and Differences
Diatoms - Classification and Characteristics
Protists - Discovering the Kingdon Protista in Microscopy
Fungi - Mold Under the Microscope , Aspergillus type
Algae - Reproduction, Identification and Classification
Protozoa - Anatomy, Classification, Life Cycle and Microscopy
Bacteria - Morphology, Types, Habitat, looking at anaerobes
Archaea - Definition, Examples, Characteristics and Classification
Bacteria under the Microscope
Here, learn more about Cell Culture, Cell Division, Cell Differentiation and Cell Staining as well as Gram Stain. And check out information on Cell Theory.
Check out Multicellular Organisms - Development, Processes and Interactions
Return from Unicellular Organisms to MicroscopeMaster Research Home
Hawksworth, D.L. 2001. The magnitude of fungal diversity: the
1.5 million species estimate revisited. Mycological Research 105:1422-1432.
Kallmeyer, J., Pockalny, R., Adhikari, R., Smith, D. &
D'Hondt, S. Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1203849109
Kenneth J. Loceya and Jay T. Lennona (2015)
Scaling laws predict global microbial diversity.
M.J. Benton Biodiversity on land and in the sea.
Geol. J., 36 (2001), pp. 211-230
Protozoa: Structure, Classification, Growth, and
Development. Robert G Yaeger 1996.The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press.
All rights reserved.