Protozoa: Characteristics, Reproduction and Classification

In this article we will discuss about:- 1. Introduction to Protozoa 2. Cellular Characteristics of Protozoa 3. Classification 4. Reproduction.

Introduction to Protozoa:

Protozoa represent the most primitive group of animal organisms. They are unicellular eukaryotic cell wall-less motile organisms and form a very large highly diverse group originating from several phylogenetic lines. There are about 20,000 living species and many fossils which are now extinct. The majority of protozoa are free-living organisms in aquatic habitats and soil.

Some are parasitic and pathogenic. Others are symbiotic or commensal organisms, living in association with other organisms. Protozoa form an important link in the food chain of aquatic environments, both fresh water and marine. Many of them feed on other microorganisms, and they themselves are devoured by larger organisms.

A small group possess chromatophores and carry out photosynthesis e.g. Euglena. These photosynthetic protozoa are generally considered as algae. Majority of protozoa are, however, heterotrophic or holozoic i.e. they live by phagocytosis of other organisms.

As symbionts, protozoa may have photosynthetic algae as endosymbionts, e.g. Chlorella grows as an endosymbiont of Paramecium bursaria. Alternatively, protozoa can live symbiotically in animal organisms, e.g. the protozoa living in the gut of termites and rumen of herbivorous animals. Most protozoa are aerobic, mesophilic organisms, having an optimum temperature of 16°C to 25°C. Some are anaerobic e.g. Entamoeba.

Protozoa can also be parasitic causing various diseases in animals including humans. Among the common human infections caused by protozoa are diarrhoeal diseases due to Entamoeba histolytica and Sacrocystis sp., malaria due to several species of Plasmodium, giardiasis due to Giardia lamblia, African sleeping sickness due to Trypanosoma gambiense, kala-azar due to Leishmania donovani, toxoplasmosis caused by Toxoplasma gondia, vaginal infection due to Trichomonas vaginalis etc.

Some of these infections are caused by the specific protozoa living in the environment, while others are carried by specific vectors e.g. malarial parasites are borne by female mosquitoes of Anopheles and trypanosomes by tsetse fly. Malarial protozoa require both humans and mosquitoes to complete their life cycle. Such protozoal infections which are transmitted from animals to human are called zoonotic.

Cellular Characteristics of Protozoa:

On the basis of gross cell morphology and motility, protozoa are distinguished into four major types. These are flagellates, amoebae, ciliates and spore-forming protozoa. Although all of these types are unicellular, they exhibit a considerable diversity in size and form.

The parasitic zoo flagellate, Leishmania donovani, measures only 1 μm to 4 μm in length which is comparable with the size of average bacteria. On the other extreme, some amoebae can be as large as 600 μm in diameter. Some fossil protozoa are even larger in size.

The general cellular features of protozoa are typically those of eukaryotic organisms, though there are also some unique characteristics. The protozoal cells are not enclosed by a cell-wall, rather their protoplasts are protected by a special thin and pliable layer which is commonly known as a pellicle or periplast.

It is a part of the cytoplasmic membrane which is modified to a thickened outer layer. A speciality of many protozoal cells is that the cytoplasm is differentiated into an outer layer, called ectoplasm, and an inner layer, called endoplasm.

The ectoplasm is generally denser in consistency than the endoplasm. The protoplast in general consists of the ingredients typically present in eukaryotic cells, like membrane-bound nucleus, Golgi bodies endoplasmic reticulum, ribosomes and mitochondria.

In some protozoa mitochondria are totally absent. They are considered as primitive and placed in the group Archaezoa which include protozoa like Giardia, Nosema, Entamoeba etc. Some other protozoa have a primitive type of mitochondria with discoid cristae e.g. Trypanosoma, Naegelaria and Leishmania. Still others, like Paramecium, Plasmodium etc., have tubular cristae in their mitochondria.

In some protozoa belonging to Kinetoplastida, each cell has a single large tubular branched mitochondrion. In some protozoa, mitochondria are without any cristae. Thus, protozoa show a very broad range of variation in respect of mitochondrial characteristics.

Among other specialties of protozoal cells is the presence of several types of vacuoles. These are contractile vacuoles, food vacuoles and secretary vacuoles. The contractile vacuoles are found mostly in fresh-water free-living protozoa and usually absent in marine forms. They function as osmoregulatory organelles and expel excess water from the cell.

The contractile vacuoles collect fluid from the cytoplasm with the help of some feeder canals and discharge the collected fluid through a pore in the outer surface outside the cell. The food vacuoles serve for storing and digesting solid food particles acquired by the protozoa. The secretory vacuoles contain enzymes.

Majority of protozoa are motile in some stage or other. Locomotion may be due to pseudopodia, flagella or cilia. Protozoa can respond to different stimuli and, in doing so, they actively move towards or away from the stimulus. Such response is known as taxis. Membrane-bound proteins are involved in responding to specific environmental stimuli.

Pseudopodial locomotion is characteristic of amoeboid protozoa. Pseudopodia are protrusions of the cytoplasm. They are used for both locomotion and collection of solid food particles which are engulfed by phagocytosis (phagotropism). Several such pseudopodia may be projected from a single amoeba. The characteristic movement, known as amoeboid, is exhibited only when the cell is in contact with a solid surface. The movement is thought to be caused by streaming of the endoplasm.

Flagellar locomotion is characteristically found in the flagellated protozoa, like Trypanosoma, Giardia etc. Flagella are long (up to 200 μm), thin membrane-bound cylindrical locomotion organelles. Protozoal flagella are in general, of eukaryotic type having two central and nine pairs of outer microtubules. The outer pairs of microtubules are connected with each other and also with the inner pair.

Microtubules are made of a protein, called tubulin and the connections between microtubules are made of another protein, dyenin. The microtubules together form an elastic filament which is known as axoneme. It is covered by a contractile cytoplasmic outer membrane. The flagella are connected to the cytoplasmic basal bodies, called kinetosomes. They move in an undulating manner to drive the cells. Flagellar movement requires hydrolysis of ATP i.e. it is an energy-consuming phenomenon.

Ciliary movement is observed in the ciliate protozoa, like Tetrahymena, Paramecium etc. Cilia are shorter than flagella, generally not exceeding 20 μm in length, but they have the same basic structure as that of flagella and are connected to the cytoplasm by basal bodies.

They propel the cells by beating. All cilia beat the surrounding water simultaneously producing waves over the surface of the cell. Cilia are present in great numbers, arranged longitudinally or spirally on the protozoal cell.

In contrast, flagella occur singly or in a pair per cell. In trypanosomes, a special locomotion organelle, known as an undulating membrane present. It is a thin membranous structure bordered by a flagellum. Its vibration causes an undulating movement.

Another characteristic feature of some protozoa, like the ciliates, is that they possess two different types of nuclei, a macronucleus which is typically polyploid and a micronucleus which is diploid. The ciliates are also characterized by the presence of an opening in the anterior part of the cell. It is known as cytostome and is used for taking up food particles from the environment. There is also an opening in the posterior part of a ciliate protozoan. This opening, called cytoproct is used for release of undigested food particles outside the cell.

Protozoa belonging to the group Apicomplexa, commonly known as sporozoa possess a specialized complex of organelles at their apical part. The complex includes an apical ring and rhoptry, and also microneme organelles. These organelles are used by sporozoa for penetration of the host cells. All sporozoa are parasitic.

Besides the cellular features of protozoa, other membrane-bound organelles exist in specific groups. These include glycosomes which are unit-membrane bound organelles containing some early enzymes of the glycolytic pathway. These organelles are characteristically present in the group, known as kinetoplastida (Trypanosomes).

Some anaerobic protozoa lacking mitochondria possess another membrane-bound organelle, called hydrogenosomes. These organelles are involved in ATP generation and evolution of molecular hydrogen. Another type of membrane-bound organelles which are granular in appearance and present in the peripheral regions of amoebae are known as extrusomes. They are supposed to secrete mucous materials which help in amoeboid movement in these protozoa.

Except the photosynthetic flagellates, all other protozoa are heterotrophic. Some protozoa inhabiting in environments, rich in organic substances absorb the nutrients through the cell membrane. Others living in environments having large number of bacteria use phagocytosis for engulfing the microbes.

The engulfed food particles are transferred to digestive vacuoles where they are digested with the help of enzymes and soluble products are transported into the cytoplasm through the vacuolar membrane. Some representative types of protozoa and some cellular features are diagrammatically illustrated in Fig. 5.49.

Classification of Protozoa:

Protozoa are a highly diverse group of eukaryotic unicellular organisms having a wide range of size, morphological characteristics and physiological properties. They have been placed in the subkingdom Protozoa under the kingdom Protista by Whittaker in 1969.

The creation of the subkingdom of Protozoa is more a matter of convenience than classifying them into a phylogenetically coherent assemblage. From the evolutionary point of view, the protozoa without mitochondria are considered as the most primitive. Such protozoa are generally spindle-shaped with flagella projecting from the front.

The amitochondriate protozoa (Archaezoa) include genera like Trichomonas, Giardia, Nosema, Chilomastix etc. Among the mitochondriate protozoa, those with discoid cristae (discocristate) are considered as more primitive than the tubulocristate forms. Examples of discocristate protozoa are Trypanosoma, Leishmania, Naegeleria etc. The ciliates and apicomplexan protozoa, like Tetrahymena, Paramecium, Balantidium, Plasmodium, Toxoplasma etc. represent the most advanced types.

An expert committee of the society of Proto-zoologists proposed a scheme of classification of the protozoa in 1980. The proposed classification recognized Protozoa as a sub-kingdom of the kingdom Protista and classified protozoa into six phyla.

These are Sarcomastigophora, Labryinthomorpha, Apicomplexa, Microspora, Myxozoa and Ciliophora. However, this classification is not phylogenetic. A phylogenetic classification of protozoa based on r-RNA homology is in the process of making.

On the basis of studies made till now, a classification system based on the evolutionary history has divided the subkingdom of Protozoa also into six phyla.

These have been designated as:

Archaezoa,

Microsporidia,

Rhizopoda,

Apicomplexa,

Ciliophora and

Euglenozoa.

The two classification schemes are shown in Table 5.4 and Table 5.5:

Similarities in r-RNA sequences as well as the presence of membrane-bound alveoli (cavities) beneath the cell surface have prompted some biologists to bring the ciliated protozoa, the apocomplexans and the dinoflagellates into a common phylum having the designation Alveolate.

Reproduction in Protozoa:

Protozoa reproduce by both asexual and sexual means, though sexual reproduction is less common and occurs in certain groups. Most protozoa reproduce asexually by cell division producing two equal or sometimes unequal cells. The cell division in certain protozoa is longitudinal, while in others transverse. In some protozoa multiple fission or schizogamy is known to occur.

In this process, the nucleus divides mitotically to produce a large number of nuclei before the cell divides. Each nucleus, with the surrounding cytoplasm, forms a daughter cell. The daughter cells then separate. Multiple fission is best known in the malarial parasite, Plasmodium, though it has also been reported in certain amobae, like foraminiferans and radiolaria.

Many protozoa also reproduce asexually by budding. In this process, daughter nuclei produced by mitotic division migrate into a cytoplasmic protrusion (bud) which is ultimately separated from the mother cell by fission.

Asexual reproduction in amoebae takes place by binary fission. Before division, the pseudopodia are withdrawn into the protoplast. Then the nucleus divides and the protoplast is divided by a constriction in the middle resulting in production of two daughter amoebae.

In the flagellated protozoa also binary fission occurs, but the fission is generally along the long axis of the cell (longitudinal binary fission). The flagella are regenerated from the basal bodies which divide before the nuclear division starts. In the ciliates, fission takes place in a transverse plane at right angles to the long axis.

The fission develops across the narrow part of the protozoan cell. This type is sometimes referred to as homothetogenic fission. Ciliates generally possess two types of nuclei, a macronucleus and a micronucleus. The nuclei also divide prior to cell division, but in different manners.

While the micronucleus divides by regular mitosis, the division of the macronucleus is amitotic. The macronucleus elongates and pinches off in two fragments. Each daughter cell receives a copy of the micronucleus and the macronucleus.

Sexual reproduction has been observed in some protozoa. As in other eukaryotic organism, sexual reproduction in protozoa also involves meiotic division. Sexual union between two haploid nuclei results in diploid zygotic nucleus in which meiosis takes place.

In sporozoa (Apicomplexa), like Plasmodium (malarial parasite), a motile microgamete fuses with a non-motile macrogamete (anisogamy) to form a diploid zygote. Another mode of sexual reproduction is conjugation which generally occurs in the ciliate protozoa, like Tetrahymena and Paramecium.

Conjugation of ciliates is an elaborate process in which two individuals unite with each other by fusion of their pellicles and nuclei are exchanged. Apparently, the nuclei act as gametes. The ciliates possess two different types of nuclei, — the micronucleus and the macronucleus. Only micronuclei take part in conjugation. The organisms are normally diploid in respect of the micronuclei.

A diploid micronucleus divides meiotically to form four haploid nuclei of which three degenerate. The remaining nucleus divides mitotically to produce two haploid nuclei which are potential gametes.

One of these two is exchanged between the two partners of conjugation and fuse with the one remaining in each partner to produce the diploid zygotic nucleus in both conjugates. The zygotic nucleus then divides mitotically to form eight daughter nuclei from which a micronucleus and a macronucleus are formed.