Protozoa: Habitat, Architecture and Organelles

In this article we will discuss about Protozoa:- 1. Habitat of Protozoa 2. Architecture of Protozoa 3. Organelles.

Habitat of Protozoa:

Adaptation of Protozoa is ex­tended to all environments open to micro­organism. Free-living forms occur in fresh water lakes, ponds, open ocean, river and even in temporary pools. Soil and sands also form natural environment for many Protozoa.

Such Protozoa live in the film of mois­ture surrounding soil particles or sand grains. Parasite protozoa occur in many different species of animals, in certain plants and even in a few protozoans themselves. The microsporidian Nosema notabilis is a hyperparasite on myxosporidian Spherospora (Fig. 10.52).

In the body of the animal host they occur in the coelom, digestive tract, individual tissue cells and body fluids. Trypanosomes live in the blood plasma and malarial para­sites invade individual red blood cells and liver parenchyma cells and ultimately destroy them.

Certain parasitic forms stick to a single kind of host while there are others which can live successfully in a few closely related species of animals, or two or more different and unrelated species of hosts. For such Protozoa that can invade unrelated host spe­cies, a change of hosts is essential for the completion of its life cycle.

Of all the ani­mals, human beings are most hospitable to parasitic protozoa for as many as twenty five different species of parasitic Protozoa have been encountered in them.

Number:

The specific number of different kinds of Protozoa living today is a matter of speculation. It has been estimated that there are about 30,000 species. But this estimation is subject to inaccuracy as many current names of species represent duplication.

Protozoa or Protista:

All the types of nutrition, e.g., holozoic, saprozoic and holophytic are seen in the protozoan group. Flagellates like Euglena, contain chlo­rophyll and carry on photosynthesis like plants. Mycetozoa of protozoologists is the other name of slime molds of botanists (Fig. 10.53).

That means they are more plant-like than animal-like and as a result the ordinary criteria for separating plants from animals break down at the level of Protozoa. To avoid unwanted arguments in case of those animals stamped with such mixture of characters some inspired Biolo­gists have proposed a group, Protista, to include Protozoa, unicellular algae and other micro-organisms.

Theoretically the scheme has many ad­vantages but in practice the term protista has not been adopted because of proprietary interests of Botanists in Algae, Bacteriologists in Bacteria and Zoologists in Protozoa.

Architecture of Protozoa:

(i) Shape:

Excepting a few Rhizopods most Protozoa are with fixed shape and size char­acteristic for each species. The forms of Pro­tozoa may be flattened, spherical, oval or elongated and often bizzare shapes are also encountered. Though majority of Protozoa exhibit an important bilaterial symmetry, all types of animal-symmetry are witnessed in them.

Rhizopods and Foraminifera are asym­metrical. Heliozoa and Radiolarians exhibit spherical symmetry while radial symmetry is noted in sessile choanoflagellates, bilateral symmetry is apparent in Giardia or Octomitus.

(ii) Size:

Most of the Protozoa are micro­scopic in size and range from 2-4µ to several millimeter in length or diameter. Plasmodium residing within the RBC is considered as the smallest of all Protozoa. Radiolaria and Foraminifera are largest in size amongst the Protozoans. The shells of Foraminifera may attain a diameter of 2-15 mm. Spirostomum ambiguum is the largest among freshwater ciliates and is about 4.5 mm in length.

(iii) The Cell Body:

The basic and the fundamental component of the Protozoan body is protoplasm which is differentiated into nucleus, cytoplasm and cell membrane.

A. Nucleus:

The form, structure and size of Protozoan nucleus are extremely vari­able (Fig. 10.54). Most Protozoa contain a single nucleus and in many there are two or more. Giardia and Protoopalina contain two similar and identical nuclei while Euciliates and Suctoria bear dissimilar nuclei, i.e., micronucleus and macronucleus. This nuclear dimorphism is also present in certain foraminifera.

The macronucleus is consid­ered as ‘somatic or vegetative’ nucleus per­forming general metabolic activities while the small one is considered as ‘generative’ which looks after the reproductive part. In Pelomyxa, Opalina, Myxosporidia and Microsporidia there are numerous similar nuclei.

The macronu­cleus offers variation in its form and struc­ture. It is compact, spherical or ellipsoidal in most cases. In Vorticella it is much elongated. In Spirostomum and Stentor, it is like a chain of nodes joined to one another by filaments. The chromatin of the meganucleus is called trophochromatin while that of micronucleus is made up of idiochromatin.

Protozoan nucleus may be vesicular or compact. Vesicular nucleus consists of a nu­clear membrane which is very thin and deli­cate but nucleoplasm is distinct and chroma­tin content is less. The compact nuclei are always massive as they contain large amount of chromatin substance and a comparatively small amount of nucleoplasm. The macronu­cleus of ciliates is of compact type.

Within the nucleus there are two general kinds of nucleolus-like bodies which contain no DNA and are called ‘endosomes’ or nucleoli. The endosome in Euglenoids is cen­trally located, made up of three concentric zones and divides during nuclear division.

In parasitic amoebae like Entamoeba the endosome is composed of a number of ir­regular masses, each containing numerous small particles and it disappears during mitosis.

Repeated mitosis in the nucleus with no dissolution of nuclear membrane result in some cases in the formation of a polyenergid nucleus. Each polyenergid nucleus bears many sets of chromosomes which are finally distributed to the daughter cells. Polyenergic states are believed to be antithesis of sporulation.

B. Cytoplasm:

It is the extra-nuclear part of the Protozoan body. The cytoplasm is colourless, homogeneous, and in optical observation presents granulated, vacuolated, reticulated or fibrillar texture. The cytoplasm is differentiated into ectoplasm and endoplasm. The ectoplasm is also called the ‘Cor­tex’.

The cortex is hyaline and gelatinous and may be defined as a part of cytoplasm bounded externally by plasma membrane and internally by endoplasm. In Ciliophora, the cortex houses a number of organelles. The endoplasm is voluminous and fluid (Plasmasol).

C. Cell Surface:

The cytoplasm bears a protective envelope which may be present in the following textures:

1. Plasmalemma:

An extremely thin membrane which regulates the entry and exit of materials between the organism and the surrounding medium in which it lives.

2. Pellicle:

The surface layer becomes gelated and forms a visible firm pellicle. The pellicle is living and is often sculptured as in Paramoecium.

3. Cuticle:

The outer limiting surface may be a cuticle as in Monocystis. The cuticle is dead and may be made up of nitrogenous elements, carbohydrate or cellulose.

4. Shell:

The whole body may be encased in a close fitting shell having an aperture through which protoplasm may be extruded. The shell is composed of nitrogenous ele­ments in Arcella, of silicious plates in Euglypha or calcium carbonate as in Foraminifera. Cysts are temporary shells with no opening.

D. Cytoplasmic Inclusions:

1. Stored food:

Polysaccharides, lipids and rarely nitrogenous materials remain embedded in the cytoplasm. Polysaccharides are present in the form of starch granules, leucosin and paramylum bodies as in Stentor and cysts of Entamoeba. The synthesis of polysaccharides in Phytomastigophores is in­dependent of the presence of chlorophyll.

Presence of glycogen has been reported in any Sarcodina and Ciliates. In some para­sitic protozoa glycogen synthesis is stepped up during cyst formation. These glycogen are never identical with the glycogens of other higher animals. A peculiar type of glycogen which is neither typical glycogen nor true starch is encountered in the endoskeletal plates of certain ciliates which live in the rumen of cattle.

Histochemical tests have shown that carbohydrate is stored by Amoeba in the form of glycogen. Washed amoeba, if treated with radioactive glucose, the isotope is recovered from the Amoebic glycogen.

Lipids, i.e., fat and other related sub­stances, remain stored and distributed throughout the cytoplasm as small globules or in some cases they remain localised in a particular area of the body. This stored oil in some Phytomastigophora presents a disagree­able taste and colour.

Nitrogenous reserves as chromatoid bod­ies of Entamoeba are also recognised. They are seen in the cysts, and the characteristic forms of chromatoid help in distinguishing one species from the other. The chromatoid bodies are made up of protein and RNA, and till date their function is unknown. ‘Volutin granules’ containing RNA have been reported in many Protozoa.

2. Mitochondria:

Mitochondria are present in all aerobic species. Number of mitochondria present in an organism are dependent on the volume of that particular organism. In Tetrahymena there are many mitochondria while in avian malarial para­site Plasmodium lophurae there are one or two mitochondria.

Structurally and functionally the Protozoan mitochondria differ very little from that of higher animals. The mitochon­dria occur as small spherical, oval, rod-shaped or filamentous bodies. They may be evenly distributed in the cytoplasm or may be local­ised in position as they are arranged between the kinetostomes of cillia in Opalina and Paramocium.

3. Golgi apparatus:

Presence of Golgi bodies in the form of compact, flattened and plate-like vesicles has been reported in Amoeba and Pelomyxa with certainty.

4. Cytoplasmic pigments:

Pigment gran­ules of various colours—violet, blue, green, yellow, pink, red occur in the cytoplasm of phytoflagellates and ciliates, Red pigments occur in Phytomonadina and Euglena. When exposed to bright light these red pigments increase in number and mask the usual green colour of the animals. The pink pigments of Blepherisma undulans is toxic to other ciliates.

Organelles in Protozoa:

A. Vacuoles:

Several kinds of vacuoles occur in differ­ent protozoa.

1. Contractile vacuoles:

Definition:

An intracellular, small incon­spicuous, membrane bound, spherical, fluid filled vesicle that maintains the osmoregulation in some protozoans.

Occurrence:

They are mainly found in freshwater protozoans such as sarcodines, flagellates and ciliates. It is also found in some marine forms but totally absent in parasitic forms such as Sporozoa.

Position:

The position, number and ac­cessory structures of the contractile vacuoles are different in different Protozoa (Fig. 10.56). In Amoeba, the position of the vacuole changes with the movement of the organism. In ciliates and flagellates the position is more or less fixed.

In many Heliozoans contractile vacuoles occur in the ectoplasm. In Balantidium and Nyctotherus the contractile vacuole is situated close to the cytopyge. Deep seated contractile vacuoles are often provided with a delicate duct which connects the vacuole with the pore on the pellicle as in paramoecium woodruffi.

Number:

The number of vacuoles varies in different groups of protozoa but remains constant in a particulaar species. It is single in Amoeba and Euglena but two in paramoecium.

Shape:

The contractile vacuoles are gen­erally sherical in shape, and simplest spheri­cal form is found in Amoeba but in many ciliates they have become star-shaped (e.g., Paramoecium) because of a number of collect­ing canals (5-12) which radiate from the main vacuole (Fig. 10.39).

Each canal consists of a narrow elongated terminal part, a swelling ampulla and a short injector canal which opens into the vacuole (Fig. 10.40). In flagellates like Euglena, the contractile vacuole is more complicated and a number of small acces­sory vacuoles are located around the main contractile vacuole.

Ultrastructure:

The contractile vacuole complex consists of a contractile vacuole proper and a system of vesicles and tubules called the spongiome (Fig, 10.57) which lies between the mitochondria and plasmalemma and helps in collection of fluid from the surrounding cytoplasm and transfers into the contractile vacuole.

A discharge pore is situ­ated in the plasmalemma through which water expels to the outside. The diameter of vesicles is about 1 µm. The breadth of the boundary membrane of the vacuole is about 0.5 µm. The volume of vesicles varies from 20 nm to 100 nm. Small fluid-filled vesicles contain polyribsomes. Rough walled vesicles are similar in structure to E.R. of mammals.

Types of contractile vacuoles:

The con­tractile vacuoles may be following type on the basis of complexity of the spongiome:

(i) In the first kind only collecting tubules are present in the spongiome and col­lecting fluid is expelled out through the pore of the plasmalemma (Fig. 10.57).

(ii) In the second kind, the tubules and vesicles are present in the formation of spongiome and the tubules collect fluid from the cytoplasm. The fluid is trans­ferred to the permanent collecting ca­nal which is dilated to form ampulla. The above mentioned two types are found in ciliates (Fig. 10.57).

(iii) In the third type a permanent pore is lacking and vacuoles are formed by the fusion of small fluid filled vesicles. The vacuoles are disappeared after the discharge of solutes (e.g., Amoeba).

(iv) In the fourth type the vacuole is filled by the conspicuous ampullae and through a permanent pore the fluid is expelled out. The ampullae, permanent pores and bundle of microtubules are absent in the third and fourth types.

Formation of the contractile vacuole:

All the contractile vacuoles pass through a cycle. The origin of a new vacuole involves the fusion of many small vacuoles in the cytoplasm.

The young vacuoles grow in vol­ume (diastole) by fusion of other small vacuoles or by receiving contribution of fluids from visible canals. When the volume reaches its maximum the contents are discharged to the outside (systole) through the pores in the pellicle or into the gullet (Fig. 10.56).

Functions of contractile vacuole:

(i) Osmoregulation:

All freshwater protozoa solve the con­stant osmotic problem with the help of the contractile vacuoles. The plasma­-lemma in these organisms is semiper­meable.

And as the concentration of water in the cytoplasm is lower than that of the surrounding medium a con­stant flow of water into the animal body occurs. Water passes more rap­idly into the body than it leaves. The organisms get rid of these excess wa­ter by pumping them out with the help of contractile vacuoles and prevent the body from being waterlogged.

(ii) Excretion:

(a) Some amount of nitrogenous wastes are voided along with the discharged water.

(b) Ludwing confirms that the con­tractile vacuole not only regulates the osmotic pressure but also helps in the excretion of CO₂.

(iii) It is considered by the differences in pulsation frequency that contractile vacuole is mainly excretory in marine protozoa, but excretory and osmotic pressure regulate in freshwater protozoans.

2. Food vacuoles:

They are temporary vacuoles and are universally present in holozoic Protozoan which take in whole or parts of other organ­ism. In forms which do not have a cytostome the food vacuoles assume the shape of the food. The food vacuoles are spherical in forms with cytostome.

A number of food vacuoles may remain present at a time and they con­tain food particles at different stages of di­gestion. Food vacuoles in ciliates show a circulatory movement or cyclosis within the endoplasm. Food vacuoles are absent in saprozoic protozoan.

3. Sensory vacuoles or Concretion vacuoles:

Certain parasitic ciliates of the families Butschliidae and Paraisotrichidae have a number of vacuoles located in the anterior region of the body under a pellicular cap. The vacuolar cavity contains a number of granules, called statoliths and a number of fibrils join the vacuole with the pellicle. These vacuoles are considered as statocysts and excretory vacuoles.

4. Superficial vacuoles:

Superficial vacuoles are found in passively floating Sarcodina which have a foamy outer cytoplasm. These thin-walled vacuoles pre­sumably containing a light weight fluid or gas maintain the organism at a particular depth. When the vacuoles collapse, the ani­mal sinks. When new vacuoles develop, the organism rises. Thus the superficial vacuoles help in floatation.

B. Mouth and Associated Or­ganelle:

Amoeboid organisms feed on bacteria or other small organisms and the ingestion of the food particle involves the formation of a food-cup to enclose the prey or the forma­tion of a gullet-like structure.

In few phagotrophic ciliates and flagellates a gullet is formed during ingestion and persists dur­ing the active life of the organisms. Certain euglenoid flagellates possess an accessory rod-like apparatus which helps in punctur­ing the body wall of the host too large to engulf whole. Both the food cup and gullet are extra-temporary structures.

Permanent and specialised feeding organelles called cytostome is encountered in ciliates like Paramoecium and Tetrahymena (Fig. 10.58). In Tetrahymena, there are three membranelles in the roof of the mouth and a membrane along the left margin. A membranella is composed of a double cili­ary lamella fused to form plate. The mem­brane is thin, transparent and bears one or two rows of cilia fused together.

The mem­brane is larger in Tetrahymena compared to other ciliates. In Hypotrichias the membranelles are very well developed but the mem­brane is ill developed. In ciliates the different stages of specialisation of the cytopharynx from an humble beginning to a complex end are readily recognisable.

In suctorians the tentacles play the role of the gullet. They feed on other ciliates which sometimes include an organism many times larger than itself (Fig. 10.58).

The tentacles adhere round the prey and it can hold an organism many times larger than it. Soon after adhesion the protoplasm of the captured ciliate starts flowing round the tentacle to the base where a food vacuole develops. The lining of the Suctorian tentacle is con­tractile and there is some sort of peristaltic action.

C. Cytopyge or Cell Anus:

The indigestible residue of food in case of the ciliates is thrown out through a particu­lar spot, called cytopyge or cell anus (also called cytoproct). It lies at the posterior ven­tral side of the body. The spot is recognisable when excrement is actually being cast away. This is again a temporary structure. In Nyctotherus a permanent cytopyge is seen.

D. Chromatophores, Pyrenoids and Stigma:

Chromatophores, i.e. chloroplastid and some non-green organelles are restricted to plant-like flagellates. The chromatophores occur in discoid, ovoid, band-like, rod-like or cup-like forms. In Chlamydomonas a single cup-shaped chromatophore is found and it is considered as a primitive form. This cup may be subdivided into pairs of lateral lobes or even to separate lobes.

Some of the Euglenidae contain many flattened chromatophores arranged near the surface of the body. In Peridinium chromatophores are arranged near the surface of the body and form anastomo­sing network. Electron micrograph studies have revealed that chromatophores are dou­ble membranes and have a lamellar structure in which electron opaque layers alternate with electron transparent layers.

Electron opaque are believed to be laden with photosynthetic pigments. Chlorophyll is the most predomi­nant pigment in the chromatophores but there are other pigments present in significant amounts.

These pigments are greenish yel­low, yellow red, brown and even blue and when present in superabundance they mask the green chlorophyll. The cytoplasmic pink pigment of Blepherisma is toxic to several other ciliates and to small metazoons. Even the annelid worm, Dero is susceptible to it. When exposed to very bright light, Blepherisma falls a victim of its own pigment or a toxic product of the pigment.

Pyrenoids are structures which usually remain associated with the chromatophores though all chromatophores bearing flagellates do not possess them. The structure of the pyrenoids varies from solid bodies to aggre­gate of granules. In Euglena the pyrenoid is encased in a layer of paramylum while in Chlaydomonas it is often surrounded by starch granules.

From this close structural relationship it is suggested that pyrenoids are functionally involved in the synthesis of starch and other polysaccharides. However, there must be other machinery for the syn­thesis of these substances as there are certain flagellates without pyrenoid, which can synthesise such polysaccharides.

Stigma or eye-spot occurs in many chlo­rophyll bearing and a few colourless flagel­lates. The stigma contains reddish pigments presumed to be carotenoid. The stigma of Euglena shows a mass of reddish granules embedded in a matrix. It is a discoid body, placed close to the gullet.

The flagellum which arises from the base of the reservoir through the gullet bears a small granule or a paraflagellar body at the level of the stigma. In Volvox and related colonial types the stigma is made up of a concave mass of pigments and a hyaline lens.

The role of the stigma is to help in the orientation of the flagellates towards a suitable light source. From the work on Euglena it is assumed that the parabasal body of the flagellum is a light sensitive structure and it becomes stimulated by the light energy which the stigma absorbs.

E. Neuromotor Organelle:

A well-defined system of nerves is lack­ing in the protozoan. But it has been seen that the cilia of the ciliates are capable of making a well co-ordinated movement. It is known that the ciliary co-ordination is due to the presence of certain fibrillar system in Epidinium. The presence of a neuromotor apparatus in the system of Epidinium is ad­vocated.

This apparatus consists of a central motor mass, called the Motorium, located in the ectoplasm, and from it definite strands radiate to the roots of the membranellae, cytopharynx and other structures.

Similar apparatus has been observed in Balantidium, Paramoecium and many other ciliates. Klein (1926) by silver-impregnation method has demonstrated the presence of such radiat­ing fibrils and has designated the fibres as silver lines and the whole complex as silver line system.

F. Protective or Supporting Organelles:

1. Pellicle:

Outside the plasma membrane many protozoa have a differentiated pelli­cle, i.e., a continuous covering which may be more or less flexible. The thick pellicles often show surface decorations in the form of ridges, papillae or pits.

The pellicle in ciliates is perforated through which cilia and trichocysts emerge. The chief component of pellicle in case of Amoeba is polysaccharide and in Euglena the principal component is protein.

2. Exoskeleton:

Instead of a pellicle or in addition to a pellicle there occurs a covering which is exoskeletal in nature. These cover­ings are made up of inorganic materials in many flagellates and Sarcodina. In testate Sarcodina the covering is made up of sili­ceous plates. The theca of Dinoflagellates is a close fitting one while the test and the lorica are loose fitting ones.

3. Central capsule:

The central capsule of the Radiolarians contains the nucleus or the nuclei and is a specialised chamber where reproductive processes go on. This capsule is considered as a protective organelle.

4. Endoskeletal plates:

In many ciliates belonging to the family of Ophryoscolecidae which reside as commensals in the stomach of ruminants, the presence of a conspicuous endoskeletal plate is encountered. These plates arise from the oral region and run to the posterior region and are made up of hemicellulose or prismatic blocks of paraglycogen.

5. Axostyle and parabasal apparatus:

It is a flexible rod-like structure running through the whole length of the body. Many Polymastigina and Hypermastigina bear axostyle. It is believed to perform a support­ive function. Parabasal apparatus is a small or a long body coiled around the axostyle and its function is unknown.

6. Costa and Cresta:

In Trichomonad flagellates a delicate filamentous structure extending from the blepharoplast to the base of the undulating membrane is seen. It acts as a support to the undulating membrane. Cresta is a triangular membrane that extends posteriorly from its anchorage near the nu­cleus and is of unknown function.

7. Oral basket:

In many Gymnostomalous ciliates the cytopharyngeal wall is lined by a number of rod-like structures. These rods form a sort of basket and are considered as protective structures.

8. Trichocysts:

These unique organelle are seen only in Holotricha. The trichocysts are pyriform, fusiform or cylindrical in ap­pearance and are embedded in the ectoplasm. They remain arranged at right angles to the body surface or in an oblique fashion. A trichocyst is made up of two parts—the tip and shank and the main body (shaft). The tip is straight, curved or bent and is connected with the pellicle.

The tip is provided with a cap. The main body is a straight rod which gradually tapers into a sharp point at the end opposite to the tip. The size of the trichocysts ranges from 20-40 µ in length. Electron micrography has revealed cross-striations in the shank and a highly retractile granule at the base of the tip. Speculations run wide about the specific role of the trichocysts.

They are considered as defensive organelle in Paramoecium. They serve as offensive organelle and help Didinium in capturing food. Some consider them as organelle for attachment while oth­ers believe that trichocysts secret salts of sodium, potassium and calcium and are os­moregulatory in nature. Even they are con­sidered as secretory organelles while produce materials for the development of lorica.

Ciliary structures:

In some species or groups the cilia are fused variously to form various organelles such as undulating mem­branes, membranelles and cirri.

1. Undulating membrane:

It is a flattend, thin sheet of cilia found in the buccal cavity of Holotricha and Heterotricha.

2. Membranelles:

There are a smaller number of cilia of two or three adjacent rows, adhere to form a triangular plate-like or­ganelle, called membranelle, found in the peristomial region of Spirotricha arranged in spiral rows. These structures are used for food capturing rather than locomotion.

3. Cirrus (PI. Cirri):

A cirrus is a large, stiff bristle-like organ composed of a number of cilia arranged in two or three rows which fuse together and form a cirrus. The cirri are found in Hypotricha.

Stiff and immobile cilia in between vibratile cilia occur in Stentor. The tentacles of Suctorians are considered as modified cilia.

Physiology:

There are many minute parts or organelle within the body of protozoa. These parts maintain the physiological activities in the cell body of the protozoa.

It is difficult to get a thorough insight into the physiological problems of protozoa, and/Calkins (1933) has very aptly stated, “Physiological problems of protozoa begin where similar problems of the Metazoa leave off, namely the ultimate processes of the single cell”. However, infor­mation on various physiological activities of protozoa are under an accelerated progress through experimental researches.