9.1: Characteristics of Protozoa - Biology

9.1: Characteristics of Protozoa - Biology

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Learning Objectives

After completing this section you should be able to perform the following objectives.

  1. Briefly describe protozoa.
  2. Briefly describe 3 ways protozoans may reproduce asexually.
  3. Define the following:
    1. trophozoite
    2. protozoan cyst.

Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. Although there are nearly 20,000 species of protozoa, relatively few cause disease; most inhabit soil and water. Protozoa reproduce asexually by the following means:

  1. fission: One cell splits into two.
  2. schizogony: A form of asexual reproduction characteristic of certain protozoa, including sporozoa, in which daughter cells are produced by multiple fission of the nucleus of the parasite followed by segmentation of the cytoplasm to form separate masses around each smaller nucleus.
  3. budding: Buds form around a nucleus and pinch off of the parent cell.

Some protozoa also reproduce sexually by fusion of gametes (Figure (PageIndex{1})).

Exercise: Think-Pair-Share Questions

  1. Protozoa that cause gastrointestinal infections are capable of producing cyst forms as well as trophozoites. State why this is essential to these pathogens.

The Role of Protozoan Cytoplasmic Membrane Components in Initiating Body Defense

Initiation of Innate Immunity

In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns or PAMPs. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)

Components of protozoa that function as PAMPs include GPI-anchored proteins (GPI = Glycosylphosphatidylinositol) and mannose-rich glycans (short carbohydrate chains with the sugar mannose or fructose as the terminal sugar) that function as PAMPs. These mannose-rich glycans are common in microbial glycoproteins and glycolipids but rare in those of humans. These PAMPs bind to pattern-recognition receptors or PRRs on a variety of defense cells of the body and triggers innate immune defenses such as inflammation, fever, and phagocytosis.

Initiation of Adaptive Immunity

Proteins associated with protozoa function as antigens and initiate adaptive immunity. An antigen is defined as a substance that reacts with antibody molecules and antigen receptors on lymphocytes. An immunogen is an antigen that is recognized by the body as non-self and stimulates an adaptive immune response. The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes and T-lymphocytes by means of epitope-specific receptor molecules having a shape complementary to that of the epitope. The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor and is actually an antibody molecule. The receptor on a T-lymphocyte is called a T-cell receptor (TCR). This will be discussed in greater detail in Unit 6.

We will now briefly look at some medically important protozoa classified into phyla based on their motility. Illustrations can be found in your Lab Manual in Lab 20.


Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. Some protozoa can also reproduce sexually. Relatively few protozoa cause disease. The vegetative, reproducing, feeding form of a protozoan is called a trophozoite. Under certain conditions, some protozoa produce a protective form called a cyst. Components of protozoa that function as PAMPs include GPI-anchored proteins and mannose-rich glycans. These PAMPS bind to PRRs on various defense cells and trigger innate immunity. Protozoan molecules can also trigger adaptive immunity such as the production of antibody molecules against protozoan antigens.


Organisms known as protozoa include a wide range of organisms, most of which are free-living single-celled eukaryotes. Therefore, protozoa fit into the Domain Eukarya. Although the different phyla of the kingdom Protista are not closely related, they are nonetheless classified together because of their large differences from the other kingdoms of plants, animals and fungi. The name “protozoa” has a dynamic history, at one time including only the “animal-like” unicellular forms of life. Today, these heterotrophic protozoa are lumped together with the autotrophic algae and other simple forms of life into the Kingdom Protista. While all protozoa are eukaryotes, not all reproduce with the standard model of mitosis that is seen in higher animal cells. Many have complex cellular division that resembles binary fission in bacteria, on a larger scale. Some phyla in the Kingdom Protista are autotrophic cells, containing chloroplasts which can produce sugars from sunlight. Although only heterotrophic organisms were considered protozoa historically, this article will present many of the phyla within Protista that can photosynthesize sugars. Below is an image of a ciliate protozoa.

Characteristics of Protozoa

Protozoa do not have a cell wall and therefore can have a variety of shapes. Nevertheless, some of the protozoans have a pliant layer, a pellicle, or a stiff shell outside the cell membrane.

Protozoa vary in size and shape. Their sizes range from 10 to 55 micrometers, but they can be as large as 1 mm. The largest protozoa are called xenophyophores, which can measure up to 20 centimeters in diameter.

Protozoa prefer living in moist and aquatic habitats. Their cysts can be found in the bleakest parts of the ecosphere.

Protozoa are found drifting in the oceans, seas, and freshwater. They are at the base of food chains.

The life cycle of protozoa changes between proliferative stages and dormant cysts.

When in the cystic stage, protozoa can live in utmost temperatures or harsh chemicals, or without nutrients, water, or oxygen for a long time. Being a cyst enables parasitic species to dwell on the host externally. This lets them transmit from one host to another. In the form of trophozoites, protozoa feed actively. The transition of a trophozoite to a cyst is called encystation and the transition back to a trophozoite is called excystation.

Would you like to write for us? Well, we're looking for good writers who want to spread the word. Get in touch with us and we'll talk.

The mode of nutrition of protozoa is heterotrophic, and most species obtain food by phagocytosis. Phagocytosis is the process where the cell changes shape by sending out pseudopodia to make contact with food particles.

Protozoa take food into the cell at a point called the cytostome. The food is ingested by them and lysosomal enzymes digest the food. There are also certain types of protozoa that take in food by their cell membranes. Some others such as the amoeba, surround food and absorb it. Others have mouth pores into which they pull in food.

Protozoans digest their food in spaces called vacuoles. Contractile vacuoles that are found in protozoa thriving in freshwater, excrete water that penetrates into the cells by osmosis. While chewing down the food, protozoans produce and release nitrogen.

Protozoa species move on their own by one of the three types of locomotor organelles such as flagella, cilia, or pseudopodia.

Protozoa reproduce by the method of binary fission or multiple fission. Some of the members reproduce by asexual mode, some by sexual means, and some by both.

Characteristics of Protista - Quiz on Protozoa

The organisms included in protista represents diverse ways of life. Many are photosynthetic autotrophs. They are collectively known as phytoplankton or microscopic, floating photosynthetic organisms. Protists (Protozoa) showing following characteristics:
a. Protists include solitary unicellular or colonial unicellular eukaryotic organisms which do not form tissues.
b. Simple multi nucleate organisms or stages of life cycles occur in a number of groups.
c. The organisms possess nuclear membranes and mitochondria.
d. In many forms plastids, (9+2 strand) flagella and other organelles are present.
e. The nutritive modes of these organisms include photosynthesis, absorption, ingestion and combination of these.
f. Their reproductive cycles typically include both asexual divisions of haploid forms and true sexual processes with karyogamy and meiosis.
g. The organisms move by flagella or by other means or are non motile.

Protozoa Structure

  • Protozoa are eukaryotic cells.
  • They are unicellular organisms.
  • Their size ranges from 1 micrometer to 200000 or may be up to 200000 micrometres in diameter.
  • The size of smaller protozoa is from 1 to 10 μm long.
  • They contain membrane bounded organelles in their cytoplasm such as ribosome, Golgi apparatus.
  • Protozoa contain a well organised nucleus which is covered with membrane.
  • The types of organelles present in protozoa vary from species to species. They contain some characteristic organelles such as the Trichocysts of Paramecium, certain skeletal structures, Contractile vacuoles.
  • The protozoa contain a vesicular nucleus. As such, the chromatic is scattered, the nucleus resulting a diffuse in look.
  • The vesicular nucleus of Phylum Apicomplexa contains one or more nucleoli with DNA whereas the DNA is absent in the endosome of trypanosomes.
  • They contain pseudopodia, flagella and cilia which help them in locomotion. These locomotory structures are covered by the plasma membrane.
  • Some protozoa also contain a rigid structure known as pellicle which gives them shape and also helps in twisting and bending during locomotion.

Subphylum III: Cnidospora

  • Spores have several cells having one or more polar filaments which are coiled threads and can be shot out, and one or more sarcoplasms or sporoplasms (analogous to sporozoites).
  • All are parasitic.
  • Zygote gives rise to one or more trophozoites without sporogony.

Class 1: Myxosporidea

  • Spores are of multicellular origin and large.
  • There are one or more sporoplasms with two or three valves.
  • They are parasites of fish.

Examples : Myxobolus, Myxidium, Ceratomyxa, etc.

Protozoan Taxonomy & Classification

Zoologists who specialize in the study of protozoa are called proto-zoologists. The protests base diversity of ultrastructure, life cycle, mitochondria, DNA sequence data, life styles and evolutionary lineages. Therefore, they cannot be put in a single kingdom. Thus classification scheme of protozoan have been changed. New evidences have been collected from electron microscopy, genetics. biochemistry and molecular biology – . These evidences shows that phylum protozoa has itself may phyla. Therefore, protozoa have been given the status of kingdom. Number of species of protozoan are 64,000. Most of these are fossils.

Recent Protozoan Classification

Kingdom Protozoa: Single-celled eukaryote. lacking collagen and cell walls. It has following phylums:

1. Phylum Chlorophyta: Unicellular and multicellular, photosynthetic pigments present: Reserve food material is starch: biflagellated stages present: free living autotrophs: some are heterotrophic. Examples: Chlamydomonas, volvox.

The Discicristates: This is an informal group. This group possess disc-shaped mitochondria, cristae.

2. Phylum Atostlata: They contain an axostyle which is made of microtubules. This phylum has single class.

Class Parabaselea: They contain large Golgi bodies associated with karymastigont: thousands of flagella present: mostly symbiotic: living in host ranging from human to termites to wood roaches. .

Order Trichomonadida: Some kinetosomes associated with rootlet filaments parabasal body present: no sexual reproduction: all parasites. Examples: Dientamoeba, Trichomonas

3. Phylum Euglenozoa: Cortical microtubules are present flagella are present mitochondria with discoid nuclei: nucleoli present during mitosis.

(a) Subphylum Euglenida: Contain pellicular microtubules that stiffens the pellicle. Mostly found in freshwater habitat and are photosynthetic.

Class Euglenoidea: Two flagella with different structures some species with light sensitive pigments and chloroplast. Example: Euglena

(b)Subphylum Kinetoplasta: Mitochondria contain a disc of DNA. Class : Trypansomatidea: One or two flagella present: single mitochondria: Golgi bodies present all parasites. Examples: Leishmania. Trypansoma

4. Phylum Retortamonada: Lack Golgi bodies and mitochondria three anterior and one posterior flagellum free living or parasitic. All members have a prominent body of massed DNA within the mitochondrion called kinetoplast.

Class Diplomonadea: One or two kinetosomes with a nucleus (a karyomastigont): individual karyomatigonts with one to four flagella cysts present: parasitic or free living.

Order Diplomonadida: Two karyomastigonts each with four flagella: a variety of microtubular bandsExamples: Giardia, Entermonas, Spironucleus, Trigonomas.

The Alveolata: All members in this informal heading possess flattened membranous sacs (alveoli) underneath the plasma membrane. The mitochondrial cristae are tubular.

5. Phylum Apicomplexa (formerly sporozoa): Contain an apical complex used to penetrate host cells cilia and flagella absent in adults but present in certain reproductive stagescysts often presentall parasitic.

(i) Class Gregarinea: Gametes are similar in size and shape: zygotes forming oocvsts with gametocysts they are parasites in body cavities or digestive tract in invertebrates. Examples: Gregarina, Monosystis

(ii) Class Coccidea: Mature gamonts intracellular: most species live inside the vertebrates. Examples: Babesia, Cyclospora, Cryptospordidium, Emeria, Toxoplasma, plasmodium.

6. Phylum Ciliophora: Cilia present two types of nuclei: binary fission and sexual reproduction present. Examples: Balantidium. Paramecium. Stentor, Tetrahyymena, Trichodina, Vorticella

7. Phylum Dinozoa (formerly dinoflagellata): Two flagella present: chlorophylls present free living or parasitic, planktonic. or mutualistic. Examples: Noticiluca, Zooxanthella, Peridinium,Ceratium, Gymodinium.

Ameobozoans: It is an informal heading because these members do not form monophyletic group. All members moves by pseudopodia asexual reproduction by fission most free living some species are obligate pathogens of human and mammals: all have branching tubular mitochondrial cristae. There is not flagellate stage in their life cycle.

Rhizopodans: locomotion by lobopodia, filopodia or protoplasmic flow. Examples: Amoeba proteus, Entamoeba, Diffugia, Arcella

The Cercozoa: This is an informal heading. This is very diverse group. It is defined exclusively by Molecular characteristics. It includes nonphotosynthetic amoebae, amonoflagellates and very large number of zooflagellates in soil and freshwater. All have tubular mitochondria! cristae.

8. Phylum Granuloreticulosa: They move by reticulopodia secrete calcium carbonate tests. pseudopodia protrude through numerous pores.

Class Foraminifera: It includes foraminiferans some species form symbiotic association with algae. These have an extensive fossil record. Examples: Globogernia, Vetebranlia.

9. Phylum Radiozoa: All members possess radiating microtubular supports called axopodia. They move by these axopodia. It includes radiolarians. Examples: Actinophyrys. Clatrulina


The word "protozoa" (singular protozoon or protozoan) was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals" (ur- ‘proto-’ + Thier ‘animal’). [12] Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. [6] Originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoa and polychaete worms. [13] The term Protozoa is formed from the Greek words πρῶτος ( prôtos ), meaning "first", and ζῶα ( zôa ), plural of ζῶον ( zôon ), meaning "animal". [14] [15] The use of Protozoa as a formal taxon has been discouraged by some researchers, mainly because the term implies kinship with animals (Metazoa) [16] [17] and promotes an arbitrary separation of "animal-like" from "plant-like" organisms. [18]

In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa (animals). [19] At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates and flagellated algae) and Rhizopoda (amoeboid organisms). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology" [20] —and the term came into wide use.

As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany. [21]

Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, but can also feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms or living beings, are animals or plants." [16] As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (Rhodophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.

Six years later, Ernst Haeckel also proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.

Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. [21] A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula. [22] [23] [24]

While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and foraminifera (all members of the SAR supergroup). In its current form, his kingdom Protozoa is a paraphyletic group which includes a common ancestor and most of its descendants, but excludes two important clades that branch within it: the animals and fungi. [8]

Since the protozoa, as traditionally defined, can no longer be regarded as "primitive animals" the terms "protists", "Protista" or "Protoctista" are sometimes preferred. In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists. [25]

Size Edit

Protozoa, as traditionally defined, range in size from as little as 1 micrometre to several millimetres, or more. [26] Among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter. [27]

Species Cell type Size in micrometres
Plasmodium falciparum malaria parasite, trophozoite phase [28] 1–2
Massisteria voersi free-living cercozoan amoeboid [29] 2.3–3
Bodo saltans free-living kinetoplastid flagellate [30] 5–8
Plasmodium falciparum malaria parasite, gametocyte phase [31] 7–14
Trypanosoma cruzi parasitic kinetoplastid, Chagas disease [32] 14–24
Entamoeba histolytica parasitic amoebozoan [33] 15–60
Balantidium coli parasitic ciliate [34] 50–100
Paramecium caudatum free-living ciliate [35] 120–330
Amoeba proteus free-living amoebozoan [36] 220–760
Noctiluca scintillans free-living dinoflagellate [37] 700–2000
Syringammina fragilissima foraminiferan amoeboid [27] up to 200 000

Habitat Edit

Free-living protozoans are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. Some species thrive in extreme environments such as hot springs [38] and hypersaline lakes and lagoons. [39] All protozoa require a moist habitat however, some can survive for long periods of time in dry environments, by forming resting cysts that enable them to remain dormant until conditions improve.

Parasitic and symbiotic protozoa live on or within other organisms, including vertebrates and invertebrates, as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms others may be significant causes of diseases, such as babesia, malaria and toxoplasmosis.

Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoans such as Trichonympha and Pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules. [40] A wide range of protozoans live commensally in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as Trichomonas, and ciliated protozoa, such as Isotricha and Entodinium. [41] The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms. [42]

Feeding Edit

All protozoans are heterotrophic, deriving nutrients from other organisms, either by ingesting them whole or consuming their organic remains and waste-products. Some protozoans take in food by phagocytosis, engulfing organic particles with pseudopodia (as amoebae do), or taking in food through a specialized mouth-like aperture called a cytostome. Others take in food by osmotrophy, absorbing dissolved nutrients through their cell membranes. [ citation needed ]

Parasitic protozoans use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell. [43]

Protozoa may also live as mixotrophs, supplementing a heterotrophic diet with some form of autotrophy. Some protozoa form close associations with symbiotic photosynthetic algae, which live and grow within the membranes of the larger cell and provide nutrients to the host. Others practice kleptoplasty, stealing chloroplasts from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning plastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. These, in turn, may be passed along to dinoflagellates of the genus Dinophysis, which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months. [44]

Motility Edit

Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of whip-like structures called flagella), ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of foot-like structures called pseudopodia). Some protozoa are sessile, and do not move at all.

Pellicle Edit

Unlike plants, fungi and most types of algae, protozoans do not typically have a rigid cell wall, but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoans, such as the ciliates and euglenozoans, the cell is supported by a composite membranous envelope called the "pellicle". The pellicle gives some shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In ciliates and Apicomplexa, the pellicle is supported by closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the euglenoids and the ciliate Paramecium. In some protozoa, the pellicle hosts epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili). [45]

Life cycle Edit

Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

Protozoans reproduce asexually by binary fission or multiple fission. Many protozoan species also exchange genetic material by sexual means (typically, through conjugation), but this is generally decoupled from the process of reproduction, and does not immediately result in increased population. [46]

Although meiotic sex is widespread among present day eukaryotes, it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in gene detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution. [47] (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles Amoebozoa, Giardia lamblia, Leishmania, Plasmodium falciparum biology, Paramecium, Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.

Historically, the Protozoa were classified as "unicellular animals", as distinct from the Protophyta, single-celled photosynthetic organisms (algae), which were considered primitive plants. Both groups were commonly given the rank of phylum, under the kingdom Protista. [48] In older systems of classification, the phylum Protozoa was commonly divided into several sub-groups, reflecting the means of locomotion. [49] Classification schemes differed, but throughout much of the 20th century the major groups of Protozoa included:

    , or Mastigophora (motile cells equipped with whiplike organelles of locomotion, e.g., Giardia lamblia) (cells that move by extending pseudopodia or lamellipodia, e.g., Entamoeba histolytica) , or Sporozoa (parasitic, spore-producing cells, whose adult form lacks organs of motility, e.g., Plasmodium knowlesi)
      (now in Alveolata) (now in Fungi) (now in Rhizaria) (now in Cnidaria)

    With the emergence of molecular phylogenetics and tools enabling researchers to directly compare the DNA of different organisms, it became evident that, of the main sub-groups of Protozoa, only the ciliates (Ciliophora) formed a natural group, or monophyletic clade (that is, a distinct lineage of organisms sharing common ancestry). The other classes or subphyla of Protozoa were all polyphyletic groups composed of organisms that, despite similarities of appearance or way of life, were not necessarily closely related to one another. In the system of eukaryote classification currently endorsed by the International Society of Protistologists, members of the old phylum Protozoa have been distributed among a variety of supergroups. [50]

    As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoan species include both herbivores and consumers in the decomposer link of the food chain. They also control bacteria populations and biomass to some extent.

    Disease Edit

    The protozoan Ophryocystis elektroscirrha is a parasite of butterfly larvae, passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. Infection creates a culling effect, whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration. [51] This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected. [52]

    Identification of Free-living Protozoa | Experiment

    There are more than 20,000 known species of free-living protozoa. They are identified with the help of their structural characteristics observed under microscope.

    The structural characteristics of few of the free-living protozoa and meaning of these structures have been given in Figure 9.1.

    Materials Required:

    Pond water, sampling bottle, dropper, glass slide, methylcellulose, cover slip, compound microscope


    1. Pond water is collected in a bottle from the bottom layer of the pond.

    2. A drop of the pond water is placed at the center of a clean slide.

    3. A drop of methylcellulose is added to the water on the slide to slow down the movement of the protozoa.

    4. One edge of a cover slip is placed against the outer edge of the drop of water.

    5. After the drop of water spreads along the inner edge of the cover slip, the cover slip is gently lowered onto the slide.

    6. The slide preparation is observed under low power and high power objectives of a compound microscope with diminished light.


    Protozoa can be seen in the drop of water. Sketches of the protozoa are drawn as observed under the microscope. They are identified by comparing their structures with those of different protozoa available in the literature (Figure 9.1).


    Protozoa are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are several times larger than bacteria - ranging from 1/5000 to 1/50 of an inch (5 to 500 µm) in diameter. As they eat bacteria, protozoa release excess nitrogen that can then be used by plants and other members of the food web.

    Protozoa are classified into three groups based on their shape: Ciliates are the largest and move by means of hair-like cilia. They eat the other two types of protozoa, as well as bacteria. Amoebae also can be quite large and move by means of a temporary foot or "pseudopod." Amoebae are further divided into testate amoebae (which make a shell-like covering) and naked amoebae (without a covering). Flagellates are the smallest of the protozoa and use a few whip-like flagella to move.

    Protozoa play an important role in nutrient cycling by feeding intensively on bacteria. Notice the size of the speck-like bacteria next to the oval protozoa and large, angular sand particle.

    Credit: Elaine R. Ingham. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

    Bacteria ingested by an amoeba.

    Credit: No. 35 from Soil Microbiology and Biochemistry Slide Set. 1976. J.P. Martin, et al., eds. SSSA, Madison, WI. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

    Flagellates have one or two flagella which they use to propel or pull their way through soil. A flagellum can be seen extending from the protozoan on the left. The tiny specks are bacteria.

    Credit: Elaine R. Ingham. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

    Ciliates are the largest of the protozoa and the least numerous. They consume up to ten thousand bacteria per day, and release plant available nitrogen. Ciliates use the fine cilia along their bodies like oars to move rapidly through soil.

    Credit: Elaine R. Ingham. Please contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

    What Do Protozoa Do?

    Protozoa play an important role in mineralizing nutrients, making them available for use by plants and other soil organisms. Protozoa (and nematodes) have a lower concentration of nitrogen in their cells than the bacteria they eat. (The ratio of carbon to nitrogen for protozoa is 10:1 or much more and 3:1 to 10:1 for bacteria.) Bacteria eaten by protozoa contain too much nitrogen for the amount of carbon protozoa need. They release the excess nitrogen in the form of ammonium (NH4+). This usually occurs near the root system of a plant. Bacteria and other organisms rapidly take up most of the ammonium, but some is used by the plant. (See figure below for explanation of mineralization and immobilzation.)

    Another role that protozoa play is in regulating bacteria populations. When they graze on bacteria, protozoa stimulate growth of the bacterial population (and, in turn, decomposition rates and soil aggregation.) Exactly why this happens is under some debate, but grazing can be thought of like pruning a tree - a small amount enhances growth, too much reduces growth or will modify the mix of species in the bacterial community.

    Protozoa are also an important food source for other soil organisms and help to suppress disease by competing with or feeding on pathogens.

    Where Are Protozoa?

    Protozoa need bacteria to eat and water in which to move, so moisture plays a big role in determining which types of protozoa will be present and active. Like bacteria, protozoa are particularly active in the rhizosphere next to roots.

    Typical numbers of protozoa in soil vary widely - from a thousand per teaspoon in low fertility soils to a million per teaspoon in some highly fertile soils. Fungal-dominated soils (e.g. forests) tend to have more testate amoebae and ciliates than other types. In bacterial-dominated soils, flagellates and naked amoebae predominate. In general, high clay-content soils contain a higher number of smaller protozoa (flagellates and naked amoebae), while coarser textured soils contain more large flagellates, amoebae of both varieties, and ciliates.

    Nematodes and Protozoa

    Protozoa and bacterial-feeding nematodes compete for their common food resource: bacteria. Some soils have high numbers of either nematodes or protozoa, but not both. The significance of this difference to plants is not known. Both groups consume bacteria and release NH4+.

    Bug Biography: Soil Dwelling Vampires

    Most protozoa eat bacteria, but one group of amoebae, the vampyrellids, eat fungi. The perfectly round holes drilled through the fungal cell wall, much like the purported puncture marks on the neck of a vampire's victim, are evidence of the presence of vampyrellid amoebae. The amoebae attach to the surface of fungal hyphae and generate enzymes that eat through the fungal cell wall. The amoeba then sucks dry or engulfs the cytoplasm inside the fungal cell before moving on to its next victim.

    Vampyrellids attack many fungi including root pathogens, such as Gaeumannomyces graminis, shown in the photo. This fungus attacks wheat roots and causes take-all disease.

    Watch the video: General characteristics and classification of Protozoa lecture (February 2023).