9: Protozoa - Biology

9: Protozoa - Biology

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Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. Under certain conditions, some protozoa produce a protective form called a cyst that enables them to survive harsh environments. Cysts allow some pathogens to survive outside their host.

Thumbnail: A "Giant Amoeba", Chaos carolinense. (CC BY-SA 2.5; Dr.Tsukii Yuuji).


Bland J. Finlay , Genoveva F. Esteban , in Encyclopedia of Biodiversity (Second Edition) , 2013

Protozoa and Ecosystem Function

Protozoa are abundant. One gram of soil typically contains 103–107 naked amebae, 105 planktonic foraminiferans can often exist beneath 1 m 2 of oceanic water, and almost every milliliter of fresh water or sea-water on the planet supports at least 100 heterotrophic flagellates. When expressed in global terms, these numbers are very large, and an inevitable consequence of the persistence of such a large number of very small organisms is that migration rates will be relatively high. It follows that rates of speciation and extinction must be low, as will the consequent global number of species. It also follows that protozoa are unlikely to have biogeographies, and endemic species probably do not exist. The authors might expect that the local diversity of protozoa would account for a significant proportion of global diversity, even if at any moment in time much of this diversity is represented by rare or inactive individuals (e.g., cysts awaiting the arrival of suitable conditions). There is indeed good evidence for the global distribution of protozoan species, including the morphologically distinctive flagellate Rhynchomonas nasuta that has been found in most aquatic and terrestrial environments worldwide marine foraminiferans and ciliates found living in slightly salty water of desert oases, hundreds of kilometers from marine coasts the same radiolarian species living in high northern and southern oceanic latitudes the same pond-dwelling ciliates living in Australia and northern Europe and the cosmopolitan distribution of the same species of agglutinated foraminiferans in the deep-sea benthos. In general, protozoan morphospecies are ubiquitous and apparently cosmopolitan if the habitats to which they are adapted are distributed in different parts of the world ( Finlay, 1998, 2002 ). In accordance with this, the global number of protozoan species is indeed relatively modest ( Table 1 ), and the number of species that can be retrieved from a local area (e.g., a pond), in both active form and from a passive state is a significant proportion (usually at least 10% for various morphological-functional groups) of the global total. This fact may not be obvious from short-term ecological sampling programs because only a limited number of microbial niches are available at any moment in time.

Table 1 . Estimates of global species richness of extant free-living protozoa a

Ameboid protozoaSlime moldsDictyostelids6060
Rhizopod amebaeNaked180220400
Benthic, inshore4000 b 4000
Benthic, deep sea250 b 250
Actinopod amebaeAcantharians150150
Radiolarians, solitary750750
Radiolarians, colonial5050
Flagellated protozoaExcluding heterotrophic dinoflagellates c Marine plankton420420
Marine benthos330330
Freshwater and soil350350
Heterotrophic dinoflagellates 9001101010 d
Other mixotrophic flagellates 150 e
Ciliated protozoa 140016603060
Total 11890

Protozoa and other microorganisms have other special properties. Microbial activities interact strongly with physical and chemical factors in the natural aquatic environment (e.g., light transmission or the concentrations of essential nutrients) to create a continuous turnover of microbial niches. These niches are quickly filled from the locally available diversity of rare and dormant microbes, and the activities of the latter create further reciprocal interactions. Therefore, the diversity of active protozoan species in a pond, at any moment in time, is the result of preceding reciprocal interactions involving many biological and nonbiological factors, and the biodiversity of protozoa and other microbes is an integral part of ecosystem functions such as carbon fixation and nutrient cycling ( Finlay et al., 1997 ).

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

II. Method of locomotion

The above describe organ beat in a different way causing different types of movement in protozoans, so protozoans have several types of movement such as amoeboid, flagellar, ciliary, and metabolic movement. Some of the protozoans movements are described here –

1- Amoeboid movement

Sarcodina, certain Mastigophora, and Sporozoa have characteristic amoeboid movement. The process of amoeboid movement is done by pseudopodia formations, pseudopodia are formed by streaming flow of cytoplasm in the direction of movement.

2- Flagellar movement

Flagellar movement is present in Mastigophora, which bears one or more flagellum. There are three types of flagellar movement that are recognized.

A- Paddle stroke

This type of flagellar movement is first described by Ulehla and Krijsman in 1925. They describe that in this flagellar movement of the flagellum is sideway consist of effective stroke or down-stroke in the opposite direction of movement and relaxed recovery stroke, during recovery stroke flagellum brought forward again and ready for next effective stroke. As flagella give effective stroke in water in backward direction then water propels organism in the forward direction.

B- Undulating motion

In this type of movement wave-like undulation takes place from base to tip or from tip to base. If wave-like undulation takes place from tip to base, the animal is pulled in the forward direction, and if wave-like undulation takes place from base to tip animal is pulled in the backward direction. And when undulation is spiral animals rotate.

C- Simple conical gyration

It is described in Butschli’s screw theory, this theory postulates spiral turning like a screw. This screw-like motion causes the pulling of the animal in the forward direction with spiral rotation as well as gyration of the animal. Although the exact mechanism for this type of flagellar beat is unknown, it is believed that axonemal fibers are involved in this process. Sliding tubules theory describe, doublet slide past each other, which is the cause of movement in flagella, and energy for this process is mitochondrial ATP.

3- Ciliary movement

In the case of ciliary movement, the cilia oscillate in a pendulum-like manner. In each oscillation, there is a fast effective stroke followed by the recovery stroke, like flagellar movement. During effective stroke cilia expel the water in the backward direction like an oar of the boat, and in response if this effective stroke water propels the animal in the forward direction. During recovery stroke, cilia come in forward direction ready for next effective stroke. Cilia neither beat simultaneously nor independently, cilia beat progressively in a characterized wave-like manner.

Mode of swimming by cilia

By ciliary movement animal directly does not follow the straight movement, they rotate spirally like a bullet of rifle in left-handed helix manner. It might be because cilia do not beat directly straight, beating is somehow obliquely toward the right and might be cilia at oral groove beat more obliquely and vigorously away from the mouth. This combined effect causes swimming movement in the animal.

4- Metabolic movement

This is due to the pellicular contractile structure. In this type of movement, organisms show gliding or wriggling, or peristalsis. Microtubules present in their pellicle is responsible for this type of movement.

Lab 3: Protozoans

The organisms referred to as protozoans (&ldquofirst animals&rdquo) constitute of diverse group of eukaryotic (mostly) unicellular organisms. In protozoans all life functions are carried out within the confines of a single cell. Although there are obviously no organs or tissues in protozoans, they are far from &ldquosimple&rdquo organisms as they are sometimes described. In fact, the cells of some species show the greatest complexity and internal organization of any organisms on Earth!

General protozoan characteristics include: small size, unicellular (but some species are colonial or have multicellular stages), body naked or covered by an exoskeleton (test) formed of silica or calcium carbonate. With over 64,000 living species, protozoans show a fantastic diversity of forms. Although they are found wherever life exists, protozoans always require moisture, which restricts them to a narrow range of environmental conditions in fresh water or marine habitats, the soil, decaying organic matter or inside the bodies of plants and animals. Many forms are ecologically important, forming essential links in food chains and decomposer systems.

About 10,000 species have close (symbiotic) relationships with animals or plants. These relationships may be mutualistic (both partners benefit), commensalistic (one benefits, while the other is neither helped nor harmed) or parasitic (the parasite benefits the host is harmed). In fact, some of the most important diseases of humans and domestic animals are caused by parasitic protozoans!

Although the protozoans used to be lumped into four groups based on their type of locomotion (i.e., whether they are propelled by flagella, cilia, pseudopodia or those forms that lack locomotor organelles), evidence from an analysis of the genes coding for the small subunit of ribosomal RNA as well as for several proteins has significantly changed (and continues to change) our concepts of the phylogenetic affinities and relationships not only of protozoan groups but of all eukaryotes and has forced a revision in protozoan classification. What follows, then, is an introduction to some of the currently recognized protozoan phyla as well as some of the more important clades and informal groupings of these organisms.

All members of this phylum move by flagella, whip-like projections composed of microtubules sheathed in an extension of the plasma membrane. Although some members of this phylum such as Euglena are autotrophic, a number of heterotrophic species cause serious diseases in humans and domestic animals. For example, Trypanosoma brucei causes African sleeping sickness in humans and a related disease in domestic animals. This disease, which is transmitted by the bite of a tsetse fly (Glossina spp.), causes death in about half of the infected individuals and permanent brain damage in many of those that survive.

Another dangerous euglenozoan parasite is Trypanosoma cruzi that causes Chagas&rsquo disease, which affects some two to three million people in Central and South America, 45,000 of which die each year. Finally, several species of Leishmania that are transmitted by the bites of sand flies cause serious diseases in humans that may affect the liver or spleen or cause disfiguring lesions of the mucous membranes of the nose and throat and skin ulcers.

This large and diverse group includes some of the most complex protozoans known such as Paramecium, Stentor, Spirostomum and Vorticella. Locomotion is always by cilia, and all forms are multinucleate, having at least one macronucleus (responsible for metabolic and developmental functions of the cell) and one or more micronuclei that are involved in sexual reproduction). Most are holozoic but a few forms are parasitic and cause damage to their hosts, including humans. Several parasitic species can cause serious problems for aquarium fish and fish in farm enclosures.

In addition to a number of complex organelles, many ciliates have a sculptured, rigid outer covering called a pellicle. Embedded in the pellicle are the cilia plus a number of thread-like structures called trichocysts. Upon mechanical or chemical stimulation, these trichocysts can be discharged to produce long, sticky protein threads that remain attached to the organism. Although the function of these structures is probably defensive, it has been hard to demonstrate this.

Included in this group are many species that form a large component of the marine phytoplankton, making them some of the most important producers in marine environments. Toxins produced by excessive profusions (blooms) of some of these marine species can lead to the so called red tides that poison fish or lodge in shellfish, making them poisonous to eat! Others such as Noctiluca produce light (bioluminescence). Other important dinoflagellates are the zooxanthellae that are mutualistic in reef-building orals and giant clams. Without their photosynthetic activity, coral reefs (and all that depend on them) would cease to exist!

This group contains endoparasitic protozoans, all of which possess (at least at certain developmental stages) a specialized combination of organelles called an apical complex, which contains structures that aid in penetrating the host. Although there are a number of apicomplexans that cause disease in humans and their animals, the most serious of these is malaria, which is caused in humans by four species of Plasmodium that are transmitted by the bite of the female Anopheles mosquito. There are over 600 million people in the world with the disease, and each year about 2 million people (mostly children) die from its effects directly and many other die indirectly.

Parabasalids constitute another clade of flagellated protozoans that lack mitochondria. Although some parabasalids such as Trichonympha live as mutualists in the guts of termites and cockroaches where they (with the help of bacterial endosymbionts) produce enzymes that break down the wood (cellulose) in their host&rsquos diet, others are human pathogens.

Trichomonas vaginalis is a sexually transmitted parabasalid protozoan that causes urogenital tract infections. Infection with T. vaginalis is one of the most common and curable sexually transmitted diseases with five million new infections reported each year in the United States alone and over 200 million worldwide! The parasite reproduces asexually through longitudinal fission, but unlike many other protozoans, the organism does not have a cyst stage as part of reproduction.

This group contains amebas and other protozoans that move by using their mobile extensions of the cytoplasm called pseudopodia. These pseudopodia come in a variety of sizes and shapes, the most common of which are rather large and blunt. Some species have thin, needle-like pseudopodia, while others have ones that form a netlike mesh around the organism. Nutrition in most forms is holozoic by engulfing prey (phagocytosis).

Amebas are naked protozoans often found in shallow, clear water. Although most amebas are free-living, feeding on small organisms with their pseudopodia, some forms are parasitic and can cause problems for humans. For example, Entamoeba histolytica is an important intestinal parasite of humans living in parts of the world with poor sanitary facilities. The parasite (which causes amebic dysentery), is contracted by drinking water contaminated with human waste or by eating raw vegetables washed with such water. Under the right conditions, the feeding stage can explosively reproduce, erode the intestinal wall and generate ulcers. In addition to causing diarrhea, E. histolytica can create problems outside of the digestive tract by invading the blood stream. Once in the blood stream it can migrate to the brain, liver and lungs &ndash often with very serious outcomes.

Slime mold is a broad term describing fungus-like organisms that use spores to reproduce. Although slime molds were formerly classified as fungi, they are no longer considered part of this kingdom. Their common name refers to part of some of these organisms' life cycles where they can appear as gelatinous &ldquoslime&rdquo. Slime molds have been found all over the world and feed on microorganisms that live in any type of dead plant material. For this reason, these organisms are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs. However, in tropical areas they are also common on inflorescences, fruits and in aerial situations (e.g., in the canopy of trees). In urban areas, they are found on mulch or even in the leaf mold in gutters. Most slime molds are smaller than a few centimeters, but some species may reach sizes of up to several square meters and masses of up to 30 grams, and many have striking colors such as yellow, brown and white.

The true plasmodial slime molds exist in nature as a plasmodium, a multinucleate blob of protoplasm up to several centimeters in diameter, without cell walls and only a cell membrane to keep everything in. This &ldquosupercell&rdquo (a syncytium) is essentially a large ameba with thousands of individual nuclei that feeds by engulfing its food (mostly bacteria) with pseudopodia in a process called phagocytosis. Thus the slime mold ingests its food and then digests it.

When the plasmodium runs out of food, or environmental conditions become harsh, they often form elaborate (often beautiful) fruiting bodies made mostly from calcium carbonate and protein that produce spores that allow them to move to a new food source. These later germinate to form uninucleate amebas or flagellated swarm cells. These later fuse and then divide mitotically to form a plasmodium, completing the life cycle. One fascinating thing about plasmodial slime molds is that the millions of nuclei in a single plasmodium all divide at the same time. This makes slime molds ideal tools for scientists studying mitosis, the process of nuclear division.

Occasionally, during rainy periods, large plasmodia (up to a few meters in diameter) crawl out of the woods and into people's lawns and gardens. The plasmodium may be ugly to some, but it is not harmful. Slime molds cause very little damage. The plasmodium ingests bacteria, fungal spores, and maybe other smaller protozoa. Their ingestion of food is one reason slime molds are not considered to be fungi. Fungi produce enzymes exogenously (outside of their bodies) that break down organic matter into chemicals that are absorbed through their cell walls, not ingested.

In contrast to the plasmodial slime molds, cellular slime molds, or social amebas, spend most of their lives as individual unicellular organisms, and as long as there is enough food (usually bacteria) the amebas thrive. However, when food runs out, they send out chemical signals to surrounding amebas, which then stream toward a central point, forming a slug like multicellular pseudoplasmodium (&ldquofalse&rdquo plasmodium), which can then migrate like a single organism. When conditions are right, the pseudoplasmodium stops migrating and forms a multicellular fruiting body. Some of the cells become spores that disseminate, while the rest form stalk cells whose only function is to raise the spores up into the air to be more easily caught in air currents.

Lab-3 01

This slide shows two exoskeletons, or tests, from a group of marine protozoans called foraminiferans. The shells of these ancient protozoans, which are composed of calcium carbonate, accumulate on sea bottoms and contribute over time to the formation of chalk and limestone. It is largely the bodies of these foraminiferans that have formed England's White Cliffs of Dover and the limestone used to build the Egyptian pyramids.

Lab-3 02

This slide contains a number of exoskeletons, or tests, of marine protozoans called radiolarians. These beautiful tests, which are abundant in marine sediments in many parts of the world, are composed of principally of silica.

Lab-3 03

This slide shows several stained specimens of Amoeba proteus (Proteus was a Greek god that could assume various forms). These relatively large protozoans use mobile extensions of the cytoplasm called pseudopodia for movement and food capture. Ingested food is surrounded by a food vacuole and digested by enzymes. Clear areas called contractile vacuoles collect excess water from the surrounding cytoplasm and discharge it to the outside of the body. Also note the darkly stained nuclei, which contain granular chromatin and control the activities of these unicellular organisms.

Photographs of living amoebas

Lab-3 04

This phase contrast microscope image shows a specimen of a living ameba. Note the large contractile vacuole on the left-hand side of the organism. This organelle is used to collect and expel excess water that enters the ameba by osmosis.

Lab-3 05

This phase contrast microscope image shows a living ameba. Note the many food vacuoles forming within this "well-fed" individual as well as the mobile extensions of the body called pseudopodia.

Lab-3 06

This phase contrast microscope image shows another living ameba using its pseudopodia (upper right hand corner) to surround a prey item. Once inside, the food will enter food vacuoles to be digested.

Lab-3 7

This is a slide of the large and complex ciliate Paramecium caudatum, which is often found in water containing bacteria and decaying organic matter. Note the large, kidney-shaped macronucleus that controls most of the metabolic functions of the organism. Located close to and often within a depression on the macronucleus is the much smaller micronucleus, which is involved in reproduction. As in other freshwater protozoans, contractile vacuoles are used to remove excess water that is constantly entering the organism by osmosis.

Photographs of Living Paramecia

Lab-3 08

This phase contrast microscope image shows two live specimens of Paramecium caudatum. Note the large contractile vacuole at the anterior end of the organism on the right (pointed to by the red arrow). This organelle is used to collect and expel excess water that enters by osmosis. Also note the oral groove on the surface of the organism. This depression leads to a permanent cell mouth called a cytostome through which food particles enter the protozoan.

Lab-3 09

  1. Food vacuole
  2. Oral groove
  3. Micronucleus
  4. Macronucleus
  5. Contractile vacuoles

This phase contrast microscope image shows a magnified view of a specimen of Paramecium caudatum. Note the large macronucleus and smaller micronucleus. The two fixed contractile vacuoles shown are filled with fluid soon to be expelled. Note the radial canals of this organelle that collect the fluid from cytoplasm. A food vacuole can also be seen in this specimen.

Lab-3 10

This phase contrast microscope image shows a highly magnified view of a another specimen of Paramecium caudatum. Note the large macronucleus, food vacuoles and two fixed contractile vacuoles. The radial canals that collect water from the cytoplasm and deliver it to the vacuole are easily seen in this specimen.

Lab-3 11

This slide shows a single Paramecium that is dividing in the process of asexual reproduction called binary fission. During this process, the micronuclei first divide mitotically and then redistribute themselves throughout the cytoplasm, after which the macronucleus elongates amitotically into two halves. On the specimen shown, this division of the macronucleus into two distinct halves has been completed.

Lab-3 12

The blue arrows point to a pair of conjugants

This slide shows a number of stained specimens of Paramecium engaged in various stages of a type of sexual reproduction called conjugation. During this process, two individuals of different mating types come together and form a cytoplasmic bridge between them. This is followed by a complex set of divisions and degenerations of the macronuclei and micronuclei that ultimately results in an exchange in genetic material between the conjugants analogous to the sexual reproduction seen in multicellular organisms.

Lab-3 13

The red arrows point to the macronuclei

This slide shows two stained specimens of the large, trumpet-shaped ciliate Stentor, a common inhabitant of freshwater lakes, ponds and streams. Although Stentor can use its cilia to actively move through the water column in search of food, it is often found attached by a long stalk to submerged sticks, stones and vegetation where it uses an array of complex ciliary organelles to draw food particles into its mouth (cytostome). Note the long, beaded macronuclei whose great size most likely reflect the special problems of controlling such a large cell.

Photograph of a Living Stentor

Lab-3 14

This phase contrast microscope image shows a living ciliate called Stentor. Note the long, trumpet-shaped body of this exceptionally large protozoan as well as the beaded macronucleus that carries control to all parts of this long and large cell.

Lab-3 15

This slide shows numerous stained specimens of the ciliate Vorticella attached to a small piece of debris by long contractile stalks. Cilia around the mouth create water currents that draw small food particles into the organism.

Photograph of living Vorticella

Lab-3 16

This phase contrast microscope image shows a living Vorticella. Note the long stalk by which this ciliate is attached to the substrate (a piece of pond debris). Although this stalk can reach a length of 3,000 microns, it can be retracted in a fraction of a second when the organism is disturbed (see next photo in the series).

Lab-3 18

This slide shows three stained specimens of an exceptionally large ciliate called Spirostomum. This spiral-shaped protozoan can reach a length of 3 mm and has a highly contractile body. Like Stentor, it also has a long, beaded-macronucleus.

Lab-3 19

Living in the digestive tracts of most termites (and some cockroaches) are mutualistic parabasalids of the genus Trichonympha that help their hosts digest cellulose and other structural components of wood. Surprisingly, the protozoans themselves lack the ability to produce cellulases and must depend on a population of endosymbiotic bacteria to produce these enzymes. In exchange for this service, the protozoans and their endosymbionts benefit from a continuous supply of energy-rich cellulose and from the suitable environment of the host's gut.

Interestingly, although Trichonympha has a large number of typical eukaryotic flagella that surround most of the organism, it also harbors a population of motile spirochaete bacteria that cling to sites on the protozoan lacking flagella. At present, researchers are unsure as to the role these ectosymbiotes play in the protozoan&rsquos ecology.

Photographs of living Termite Gut Flagellates

Lab-3 20

This phase contrast microscope image shows the large protozoan Trichonympha that inhabits the gut of primitive termites. Other smaller zooflagellates as well as bacterial species can also be seen on the slide.

Lab-3 21

This phase contrast microscope image shows a more magnified view of the large zooflagellate Trichonympha that inhabits the gut of primitive termites.

Lab-3 22

This slide shows two specimens of Paramecium that have been treated with a special stain that highlights a structure called the pellicle, a semi-rigid outer covering that provides support for the cilia which project through it. On the slide, these structures appear to be composed of numerous ridges and grooves.

Lab-3 23

This slide shows a blood smear containing the flagellate Trypanosoma brucei that causes African sleeping sickness in humans. Although there are two subspecies of the parasite that cause slightly different forms of the disease, both are transmitted by the bite of the tsetse fly (Glossina). Numerous purple-stained trypanosomes (pointed to by the blue arrows) can be seen among the lightly stained, circular erythrocytes (red blood cells). A large, darkly stained lymphocyte (white blood cell) can also be seen on the slide.

Lab-3 26

Trypanosoma cruzi is a parasitic protozoan that causes the potentially fatal Chagas&rsquo disease. Transmission occurs through the bites of the assassin or &ldquokissing&rdquo bug (Triatoma) when feces containing an infective stage of the parasite are deposited on the skin surface. Because the bite can cause pain and itching, the feces often get scratched into the wound or may be picked up by the hand and transferred to the eye, where they enter through the mucus membrane. Transmission can also occur through contaminated blood transfusions.

Chagas&rsquo disease presents one of the highest disease burdens in Latin America. Approximately 16-18 million people are currently infected, 50,000 of which die each year. There are currently no good drugs available to treat disease, so elimination efforts primarily involve vector control and blood screening to prevent new infections.

Lab-3 27

Trichomonas vaginalis is a small anaerobic, parabasalid protozoan that moves with the aid of four whip-like flagella that protrude from its front end. It also has a fifth flagellum extending rearward from an undulating membrane that allows the parasite to attach to and tear the urethra or vaginal walls, causing inflammation that aids in speeding and intensifying infection. The adults (called trophozoites) then live in the urinary or reproductive tracts, until they are passed onto their next human host though unprotected sex.

Lab-3 25

Leishmania is another trypanosome that infects humans. Like Trypanosoma brucei, the parasite requires two hosts to complete its life cycle: a mammal and an insect. Leishmania causes two forms of disease: cutaneous leishmaniasis and visceral leishmaniasis. The former typically results in cutaneous lesions that are often self-limiting. The latter is much more serious, often resulting in the destruction of the phagocytic cells of the immune system that can lead to secondary infection and eventual death of the human host.

Lab-3 24

This slide shows a blood smear taken from an individual infected with malaria, which is caused by the apicomplexan parasite Plasmodium. Although most of the red blood cells in the smear appear normal, notice the cell infected with an intracellular feeding stage of the parasite called a trophozoite (1). After feeding on the red blood cell&rsquos hemoglobin, the parasite undergoes a form of asexual reproduction called schizogony (multiple fission), which results in the production of a number of nuclei seen in the red blood cell (2) above and to the left of the trophozoite. After cytokinesis is completed, the cell will rupture and release newly formed daughter cells called merozoites. It is the synchronous destruction of many erythrocytes and the release of their contents that produce the alternating bouts of fever and chills characteristic of this debilitating disease.

Lab-3 28

This image shows a model of a relatively large protozoan called Amoeba. Amebas use mobile extensions of the cytoplasm called pseudopodia (4) for movement and food capture. Protozoans that form pseudopodia have two type of cytoplasm, an outer, more viscous portion called the ectoplasm and an inner, more fluid portion called the endoplasm. When a pseudopodium begins to form, a clear space at the leading edge of the pseudopodium called the hyaline cap (5) appears. After this occurs, endoplasm begins to flow into this space, causing the pseudopodium to be pushed forward through the medium. In addition to their locomotor role, pseudopodia can be used to engulf prey in a process known as phagocytosis. Once ingested, food enters food vacuoles (3) where it is digested by enzymes released from lysosomes. Clear areas called contractile vacuoles (2) collect excess water that enters by osmosis from the surrounding cytoplasm and discharge it to the outside of the body. Also note the darkly stained nucleus (1), which controls the activities of this unicellular organism.

Lab-3 29

This image shows a model of the large, complex ciliate protozoan known as Paramecium. These unicellular organisms are often found in water containing bacteria and decaying organic matter. Note the large, kidney-shaped macronucleus (1) that controls most of the metabolic functions of the organism. Located close to (and often within a depression on the macronucleus) is the much smaller micronucleus (2), which is involved in reproduction. As in other freshwater protozoans, contractile vacuoles (4) are used to remove excess water that enters the organism by osmosis. In addition to these organelles, note the ciliated oral groove (5) that directs food to a permanent opening called the cytostome, or cell mouth (6). Once inside the cell, the food is surrounded by food vacuoles (3) and is digested by enzymes released by lysosomes. Some species also maintain a permanent opening to outside called a cytoproct ("cell anus"). Located beneath the plasma membrane is a stiff but flexible structure called the pellicle that provides support for the protozoan, enabling it to maintain its shape. Embedded within this pellicle are the cilia that project through it as well as numerous thread-like structures called trichocysts (7). Upon mechanical or chemical stimulation, these trichocysts can be discharged (as shown on the model) to produce long, sticky protein threads that remain attached to the organism. It is believed that these structures can be used for defense.

Classification of Protozoa

Classification of Phylum Protozoa

There are present different phyla of protozoa such as

(i). Phylum Euglenida

  • They contain pellicle and flagella which help them in locomotion.
  • The pellicle is located beneath the cell membrane and made of protein strips. It is the characteristic feature of Euglenida.
  • Some Euglenidas are autotrophs, they contain chloroplasts which help them in photosynthesis to make their foods.
  • Others use dissolved nutrients to get their foods. While few of them are parasitic.

(ii). Phylum Kinetoplastida

  • They are protected by a pellicle which is made up of microtubules.
  • They contain a single, much enlarged and elongated, mitochondrion.
  • Some of them are parasitic such as leishmaniasis that cause disease in humans

(iii). Phylum Ciliophora

  • They contain cilia for locomotion, which are much smaller structures than flagella.
  • They act as parasites in the digestive tracts of larger organisms.
  • The entire cell is covered with cilia, which propel the cell forward. Each cilia gives a forward-moving power stroke, then whips back to the starting position in the recovery stroke.
  • Some ciliates are found at the bottom of marine environments, which are known as the benthic zone.
  • The free-swimming organisms and sessile use cilia to filter food material from the water.

(iv). Phylum Apicomplexa

  • These are parasitic organisms, which enter into their host cells by using apical complexes. They are much more resistant inside the cell and get better access to nutrients.
  • They can hide from the immune system by changing the proteins exposed on their cell surface, that is why it is difficult to treat them by medicine.

(v). Phylum Dinoflagellata

  • They contain flagella for locomotion and pellicle. The pellicle of Dinoflagellata is made up of a series of vesicles beneath the cell membrane which makes them rigid.
  • Some of them protect their cells by filling their vesicles with polysaccharides and forming armor.

(vi). Phylum Stramenopila

  • This phylum contains different varieties of organisms, from the shelled diatoms to brown and golden algae.
  • They contain shells, scales, or tests that support the cell.
  • The tests of diatoms are made of silicate, others use calcium carbonate or protein to make their shells.

(vii). Phylum Rhizopoda

  • This phylum includes the amoebas. These are small, unicellular protozoa and don’t contain any hard covering.
  • They extend their cytoplasm in the environment for locomotion. These extensions of amoeba are known as the pseudopodia.

(viii). Phylum Actinopoda

  • They contain characteristic axopodia. These are sharp spines extend from the cell used for locomotion and feeding.

(ix). Phylum Granuloreticulosa

  • Granuloreticulosa has an immense industrial value.
  • They produce tests at the bottom of the ocean, where they fossilize together and form chalk, limestone, and marble.

The pyramids of Egypt were built from stones which originated from the shells of these protozoans.

(x). Phylum Diplomonodida

  • They contain flagella (around 8)for their locomotion.
  • An example of Diplomonodida is genus Giardia which causes flu-like symptoms and diarrhea in humans.

(xi). Phylum Parabasilida

  • They contain thousands of flagella, and contain a fiber that attaches the Golgi apparatus to the base of the flagella.
  • Some of them show symbiotic relationships with insects, mainly those insects eat wood.
  • They release enzymes to break the cellulose.

Classification of Protozoa Based on the Mode of Existence

There are about 21,000 species of free-living protozoa and 11,000 species of parasitic microbes which are lives in both vertebrates and invertebrates.

The free-living protozoa live everywhere, they can be found in water, soil, etc. They cause disease in humans. Hence, based on the habitat of free-living protozoa they are classified into these following groups such as

(a). Acanthamoeba

  • They can be found in soil and water.
  • They are responsible for chronic granulomatous amebic encephalitis, amebic keratitis, granulomatous skin as well as lung lesions.

(b). Naegleria fowleri

  • They are mainly found in moist soil.
  • They are responsible for the acute primary amebic meningoencephalitis.

(c). Balamuthia mandrillaris

  • They are responsible for sub-acute to chronic granulomatous amebic encephalitis and also cause granulomatous skin and lung lesions.

(d). Sappinia diploidea

  • They can be found in Elk and buffalo feces, Soil, water.
  • The following symptoms can be observed in a Sappinia infected patient Headache, Sensitivity to light, Nausea or upset stomach, Vomiting, Blurry vision, Loss of consciousness.

Classification of Protozoa based on the Mode of Nutrition

Protozoa are classified into three main category based on their mode of nutrition such as

(a). Autotrophs

  • They produce carbohydrates or foods from carbon dioxide and water through photosynthesis.They contain chlorophyll.
  • They use acetates, simple fatty acids and alcohols as a main source of carbon.
  • In presence of light they act as autotrophs, while in dark they switch to heterotrophs.
  • Some examples of Photo-autotrophic protozoa are Euglenida, Cryptomonadida, Volvocida (both autotrophy and heterotrophy).

(b). Heterotrophs

  • Most of the protozoa comes under this category. They feed on bacteria (microbivores) or algae(herbivores) or may be in both bacteria and algae (carnivorous ).
  • They are divided into two distinct groups based on their entry point of food such as those that have a mouth/cytostome and those that lack a mouth or a definite point of entry for food.

(c). Chemoheterotrophic


Leishmaniasis is a vectorborne disease that is transmitted by sand flies and caused by obligate intracellular protozoa of the genus Leishmania. Human infection is caused by more than 20 species. These include the L. donovani complex with 2 species (L. donovani, L. infantum [also known as L. chagasi in the New World]) the L. mexicana complex with 3 main species (L. mexicana, L. amazonensis, and L. venezuelensis) L. tropica L. major L. aethiopica and the subgenus Viannia with 4 main species (L. [V.] braziliensis, L. [V.] guyanensis, L. [V.] panamensis, and L. [V.] peruviana). The different species are morphologically indistinguishable, but they can be differentiated by isoenzyme analysis, molecular methods, or monoclonal antibodies.


Organisms that are single celled, swim in water and consume food are generally called protozoa. They belong to the Kingdom Protista and are classified into different phyla based on how they move. If you take a drop of pond water and observe it under the microscope, you can often see tiny little organisms swimming around. In fact, Anton van Leeuwenhoek, one of the first scientists to observe these creatures under the microscope gave them the name “animalcules”, as if they were a combination of animals and molecules. Though protozoa may be tiny and unicellular, they have fascinating complexity.

Take for instance, the amoeba, which belongs to the Phylum Sarcodina. This single-celled protist can be any shape it wants because its membrane is flexible and it can push its cytoplasm around to change its shape. The word “amoeba” means “to change”. When you first look into the microscope for an amoeba, you may miss it because it does move slowly. It seems to take a lazy approach to life by casually stretching out its cytoplasm into extensions called pseudopodia . These extensions can also trap smaller protists within them, which create a food vacuole where the amoeba can digest them. In this case, slow doesn’t mean harmless – the smaller protists really don’t even sense the danger.

Another interesting protozoan is the paramecium. It moves using tiny hair-like structures on its surface called cilia . In fact, the paramecium belongs to a whole group of protists that move using cilia, the Phylum Ciliophora. Compared to the amoeba, the paramecium is fast swimmer. It is so fast that when looking for it under the microscope it may zoom right over your viewing field before you have a chance to really even see it. For this reason, biologists add a thickening agent to the water to slow the paramecium down so it can be seen more clearly. You can also place obstacles on the slide to get in its way, such as cotton fibers. Once you have the paramecium slowed or trapped, you can see many amazing features within it.

The paramecium has two nuclei. One nucleus controls the cells activities, and the other functions in sexual reproduction. As the paramecium swims forward, it will roll its body so you can see both sides. On one side is an indentation called the oral groove . The paramecium sweeps food into this opening, which then forms a food vacuole within the cell where digestion occurs. Like the amoeba, paramecium generally eat protists that are smaller than they are. The oral groove is also used in sexual reproduction, where two paramecia join together and exchange DNA. Once they separate and divide by mitosis , the new paramecia are different from the original parent.

Both the amoeba and the paramecium live in fresh water, and due to osmosis, water will tend to enter their cells. These two protists must have a strategy for removing the excess water (or they might explode!). The organelle called the contractile vacuole does the job. It serves as a water pump to remove the extra water that builds up in the cell. Under the microscope, the contractile vacuole will often look like a clear air bubble within the cell.

The amoeba and paramecium are just two of the many protozoa you can find living in pond water. There are other groups like the Zoomastigina phylum which include protists that move using a tail like structure called a flagella . The euglena has a flagella, but it is sometimes classified as an algae because it can photosynthesize – use light to create food like a plant.

There is even a group of protists that are parasitic and live within a host. Malaria is an illness caused by a protist that infects the blood through the bite of a mosquito. If a person is infected by malaria, they will suffer from chills and fever and overall weakness, and could even die. Generally, most protists are harmless and can be studied safely in a biology laboratory.

1. What organelle is used to remove excess water in protozoa?
a . food vacuole b . contractile vacuole c . nucleus

2. Which of these protists moves the fastest? a. amoeba b . paramecium

3. Where do the amoeba and the paramecium live? a. pond water b. sea water c. within the blood

4. The word “amoeba” means: a. to change b . unicellular c. slow mover

5. To what Kindgom and Phylum does the paramecium belong?
a. Protista, Sarcodina b . Protista, Ciliophora c. Ciliophora, Sarcodina

6. A pseudopodia is a(n ): a. row of cilia b . type of protist c . extension of the cytoplasm

7. Which is an illness caused by a protist: a. swine flu b . small pox c . malaria

8. Food is digested within: a. the oral groove b. food vacuoles c. the nucleus

9. Tiny hairlike structures located on the surface of the cell are called: a . cilia b . flagella c . pseudopodia

10. Which of the following is unicellular? a. amoeba b . paramecium c . both

11. Mitosis is a type of: a. movement b . cell division c . amoeba

12. Protozoa are grouped into different phyla based on:
a. their color and size b . how they move c. where they live

13. If you are studying a paramecium, what should you do to the slide?
a. heat it b . add cotton fibers c. turn it upside down

14. How does an amoeba catch its food?
a. by trapping it within its pseudopodia b . by sweeping food into its oral groove

15. The Blepharisma is protist related to the paramecium. It has cilia to help it move. How would you classify the blepharisma?
a. Phylum Sarcodina b. Phylum Zoomastigina c. Phylum Ciliophora

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Protozoan Diseases

Common infectious diseases caused by protozoans include:

These infections are found in very different parts of the body. Malaria infections start in the blood, giardia starts in the gut, and toxoplasmosis can be found in lymph nodes, the eye, and also (worrisomely) the brain.

Other protozoan disease include:

  • African trypanosomiasis ("sleeping sickness"): Caused by Trypanosoma brucei gambiense (98% of cases) and Trypanosoma brucei rhodesiense (2%). Both are spread by tsetse fly bites.  
  • Amoebic dysentery: Due to Entamoeba histolytica),   which causes diarrhea and GI upset. It can also travel through the walls of the intestines and go into the bloodstream and on to other organs, like the liver, where it can create liver abscesses.

Can Sleeping Sickness Be Eradicated?

The flies that spread sleeping sickness live in at least 36 countries. The disease causes serious neurologic effects, and the treatment is difficult. In poorer, resource-limited areas, it's hard to identify and treat.

Most cases occur in the Democratic Republic of the Congo, where plans are in the works to greatly reduce the spread of the disease and its burden—and possibly even drive this protozoa into extinction.

Ecology of Protozoa

This book is written for ecologists and protozoologists. Ecologists who study environments and biotic communities in which protozoa are im­ portant should find this book especially useful. During the last decade it has become clear that protozoa play important roles in natural eco­ systems, but few ecologists have a feeling for the functional properties and the diversity of these organisms. Protozoa pose or exemplify many general problems of population and community ecology, and of evo­ lutionary biology. In most respects the general ecological properties of protozoa are not fundamentally different from those of larger organisms yet, due to their small size, short generation times, and ubiquitous oc­ currence they often present ecological phenomena in a new and dif­ ferent light. To this should be added that protozoa are well-suited for experimental work. Despite these advantages, the study of protozoa has played a relatively modest role in the development of ecology and ev­ olutionary biology, primarily, I believe, because most ecologists are unfamiliar with these organisms. I hope this book will attract more attention to these favorable characteristics of protozoa. I also hope that this book may make protozoologists aware of new aspects of their pet organisms. For a long time (that is, until the fun­ damental distinction between prokaryotic and eukaryotic cells was rec­ ognized) protozoa were believed to represent the simplest form of life. They were therefore extensively used for the experimental study of basic questions of cell biology.

Watch the video: Protozoa - Ring Around My Rosie 992021 (January 2023).