What are protozoa classification and types?

Definition

Protozoa are members of an informal grouping of simple,
usually unicellular, heterotrophic phyla that share similar characteristics. Some
protozoa are pathogenic.

Classification

Traditionally, the kingdom Protista (also known as Protoctista) is
made up of simple eukaryotic organisms that do not fit neatly into any of the
other kingdoms. Often its members are more closely related to members of other
kingdoms than to each other. For many years, this kingdom has been informally
divided into three subgroups: the algae, which are photoautotrophs; the protozoa,
which are ingestive heterotrophs; and the fungus-like protists, which are absorptive heterotrophs. These informal groupings
sometimes break down, as among the euglenids and dinoflagellates, in which there
are species that are photoautotrophs, species that are either absorptive or
ingestive heterotrophs, and species that can switch between autotrophy and
heterotrophy. Most taxonomists agree that the kingdom Protista is artificial (not
monophyletic) and should be split into multiple kingdoms.

Like the taxonomy of the protists in general, the taxonomy of the protozoa is continually changing, as more species are subjected to rigorous cytochemical and genomic analysis. All protozoa are nucleated and are considered to be eukaryotic, although some may show both eukaryotic and prokaryotic characteristics. The normal classification hierarchy that starts with kingdom is often not used with Protista. The species are simply placed in taxa according to possible evolutionary and biochemical similarities without labeling the taxa.

Characteristics and Habitat

Protists can live in fresh- or marine-aquatic habitats, in damp soil, or in other organisms such as parasitic or commensal organisms. All can reproduce asexually, while some, like the alveolates, can also reproduce sexually.

Most excavates reproduce asexually and have flagella, a cytoskeleton of
microtubules, modified mitochondria, and a large ventral groove used in feeding.
Diplomonads, like Giardia spp., have two haploid nuclei, multiple
flagella, and a modified mitochondrion, the mitosome, that is unable to perform
cellular respiration because it lacks electron transport. All are anaerobic with
bacteria-like anaerobic metabolism, and some are parasitic in the guts of animals.
The related parabasalids also have a modified mitochondria-like structure called a
hydrogenosome. This double membrane structure produces hydrogen as a by-product of
the oxidation of pyruvate, much like hydrogen-producing bacteria. Parabaslids also
have a prominent Golgi body (or Golgi apparatus) called the parabasal body that is
involved in protein synthesis. Parabasalids also have an undulating
membrane thought to be involved in locomotion. Many digest cellulose in the guts
of termites and ruminants, while others, such as Trichomonas
spp., are commensal or parasitic in animals.

The euglenoid kinetoplastids have a single large mitochondrion with discoid inner membranes. The mitochondrial deoxyribonucleic acid (DNA) is enclosed in a structure called the kinetoplast. Kinetoplastids also have Golgi bodies and a haploid nucleus. Some are parasitic, such as Trypanosoma spp., Leishmania spp., and Chrythidia spp. Among the nonparasitic kinetoplastids are some of the few colonial protozoa.

Alveolates are recognized by their alveoli, the membrane-bound vesicles just
inside the plasma membrane. Almost all have plastids of red algal origin. Among
the heterotrophic dinoflagellates are no important human pathogens, although many
photosynthetic dinoflagellates produce toxins that can affect humans and other
animals. All apicomplexans, formerly called sporozoa, are nonmotile parasites,
contain a modified plastid called the apicoplast, produce spores, and have sexual
reproduction. The most prevalent protistan disease of humans, malaria, is
caused by the apicomplexan Plasmodium spp. All ciliates, as the
name implies, have cilia, either on the entire surface or clustered in tufts or
rings. They also have two types of nuclei, macronuclei and micronuclei, and
reproduce both sexually and asexually. All, except for the occasionally pathogenic
Balantidium, are nonpathogenic.

Amebozoans all have lobe- or tube-like pseudopodia for locomotion and lack cilia or flagella at any stage of the life cycle. Most cannot reproduce sexually. The nucleus has a prominent nucleolar region called the endosome, and many can produce resting stages called cysts. Gymnamebas, like Amoeba proteus, are free-living, unicellular amoeba that produce multiple pseudopods. They usually are aquatic and feed on bacteria or detritus. Entamoeba spp. are also unicellular, produce a single pseudopod at a time, and are parasitic in animals.

Impact

Many protozoa are parasitic pathogens. They cause a number of
diseases in humans and exact a huge toll on the peoples of developing countries.
The various Plasmodium spp., for example, cause the different
forms of malaria, a disease that has had a devastating effect on the developing
world.

The protozoan Giardia intestinalis (also known as G.
lamblia) causes diarrhea in hikers and backpackers, especially, and
Trichomonas vaginalis causes vaginitis and
urethritis. Trypanosomes are responsible for sleeping
sickness (T. brucei) and Chagas’
disease (T. cruzi), and because they can
change their surface proteins, they often elude the host’s immune
system. Leishmania spp. most commonly cause
cutaneous leishmaniasis, an ulcerative disease of the skin.
Toxoplasma gondii can be transmitted to humans through cat
feces and can cause toxoplasmosis. Entamoeba
hystolytica is the only major amebic pathogen in humans and causes
amebic
dysentery.

Bibliography

Katz, Laura, and Debashish Bhattacharya, eds. Genomics and Evolution of Microbial Eukaryotes. New York: Oxford University Press, 2008. A collection of articles about the modern evolutionary taxonomy of the protists.

Margulis, Lynn, and Michael Chapman. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. 4th ed. New York: Academic Press/Elsevier, 2009. Describes many protozoa, including their morphology and natural history.

Parker, Steve. Protozoans, Algae, and other Protists. Mankato, Minn.: Compass Point Books, 2009. Although written for middle-school students, this book provides good coverage for all general readers studying the basics of protozoa.

Rogers, Kara. Fungi, Algae, and Protists. New York: Britannica Educational Press, 2011. A middle- and high-school-level book with broad coverage of protozoa.