Protozoa: Nutrition, Respiration and Excretion

In this article we will discuss about Protozoa:- 1. Nutrition in Protozoa 2. Respiration in Protozoa 3. Excretion 4. Reproduction.

Nutrition in Protozoa:

Nutrition is a process by which the individuals obtain nourishment. It includes ingestion, digestion, absorption and digestion.

The nutrition of protozoa is manifested by following ways (Fig. 10.68):

A. Holozoic or Zootrophic or Hetero­trophic nutrition,

B. Holotrophic or Autotrophic or Phytotrophic nutrition,

C. Saprozoic or Saprophytic nutrition,

D. Parasitic nutrition and

E. Myxotrophic nutrition.

(A) Holozoic or Zootrophic or Heterotrophic:

In this method animals and plants smaller than the body of the pro­tozoa are used as food sources (Fig. 10.68). The method is ultimately associated with in­gestion, digestion, assimilation and egestion.

I. Ingestion:

Most Sarcodina capture their food and take them inside the body through any part of the body. In Mastigophora and Ciliates food enters into the body through the cytostome. The lashing of flagella or cilia aids in bringing about the food particles to the cytostome. Food-getting is no problem to parasitic forms when they are inside the body of the host.

In Sarcodina the following methods have been observed for the ingestion of food par­ticles:

(a) Import:

The food is taken inside the body upon contact with little or no move­ment of body parts.

(b) Circumfluence:

The food is sur­rounded on all sides by the cytoplasm and is engulfed.

(c) Circumvallation:

The amoeba forms pseudopodia round the food particle and ingests it.

(d) Invagination:

The ectoplasm of the amoeba, when comes in contact with the food particle, is invaginated or in pushed into the endoplasm as a tube. The cell membrane at the point of contact dissolves. Certain para­sitic amoebae are capable of ingesting by in­vagination.

In certain ciliates like Coleps and Didinium the cytostome remains closed ordinarily but it expands to an enormous size during feeding and the whole food is ingested in toto.

In suctorians the tentacles are prehensile, and solid or liquid food matter is sucked in through the tubular tentacles.

II. Digestion:

The ingested food particles surrounded by a film of fluid remain in the endoplasm in ‘food vacuoles’. The food vacuoles always remain in motion in the en­doplasm and show ‘cyclosis’. The vacuole contains food particles in killed, paralysed or alive conditions. However, all activities on the part of the prey stop a few minutes after its entry into the food vacuole. Digestion is done within the food vacuoles.

Observations of the changes within the food vacuole of amoeba are:

(a) the fluid inside the vacuole first becomes acidic and then alkaline,

(b) cytoplasmic secretions do not bring about the increased acidity inside the vacuole. Probably respiration of the ingested organism and chemical changes associated with the death of the organism are respon­sible for the change in pH.

(c) Oxygen inside the vacuole decreases as it is being used up in the respiration of the organism inside the vacuole,

(d) Decrease in oxygen results in the death of the captured prey. The existence of some ‘lethal agent’ inside the vacuole is also advocated.

Specific information about the processes involved in digesting the food particles is wanting and nothing is known about the lo­calisation or distribution of enzymes within protozoan body.

However, the findings made so far have indicated that the digestion in Protozoa is carried on by enzymes. The ex­istence of enzymes like peptidase, protein­ase, amylase, lipase, succinic dehydrogenase and others in different protozoa has been demonstrated.

Recent findings have established the ex­istence of enzymes like acid phosphatase and esterase in Amoeba proteus. In recent years it has been advocated that protozoan cells con­tain elaborate set of hydrolytic enzymes and these enzymes participate in digestion. Some of these enzymes have been demonstrated as components of food vacuoles while others are secreted by the cell body into the sur­rounding medium.

These hydrolytic enzymes have been divided into two functional groups—one group remains engaged in the digestive processes occurring within the cell either in food vacuoles or in the cytoplasm, while the other group releases enzymes in the surrounding medium.

III. Egestion:

Non-digested residue is thrown out of the body through the plasma membrane or through cytopyge or tempo­rary cell-anus.

(B) Holotrophic or Auto­trophic or Phytotrophic:

This type of nutrition is equivalent to the photosynthe­sis of plants. The process involves the photolytic decomposition of H₂O ultimately lib­erating O₂ and reduction of CO₂ to form carbohydrates. Holophytic nutrition is pre­dominant in Phytomastigina and few chlorophyll-bearing ciliates.

Recent studies have shown that some species of photosynthetic Euglenae when kept in the dark place for some days or are grown in media rich in certain organic nutrients lose their chloro­phyll and tend to resemble the member of the genus Astasia.

Moreover, treatment of Euglena with Streptomycin at a higher than standard temperature produces irreversible loss of chlorophyll. Such experiments indicate that some colourless genera or Euglenoids like Astasia might have been derived secondarily from photosynthetic species.

(C) Saprozoic or Saprophytic:

In this process the nourishing substances enter into the body by diffusion through body surface and no organelle is involved. The nourishing substances are simpler com­pounds formed by the activities of bacteria on dead or decomposed bodies of animals or plants. Many free-living protozoans, specially the flagellates, nourish themselves by this process.

(D) Parasitic:

Many protozoa live in­side the body of other living organism and nourish themselves from the food of the host. In some cases the digested or decomposed substances of the hosts enter into the body of the parasites by diffusion as in Monocystis. However, many parasitic protozoa like Enta­moeba practice holozoic nutrition.

(E) Myxotrophic:

Flagellates like Eu­glena can nourish themselves in more than one method. On the demand of the external condition (in the absence of light) they can change their mode of nutrition from holophytic to saprophytic type.

Respiration in Protozoa:

Protozoa do not have any organellae for the process of respiration. The limiting permeable membrane acts as a respiratory surface. The free molecular oxy­gen from the surrounding media enters into the body by diffusion. Presence of a cyto­chrome system has been demonstrated in protozoa.

Protozoa which live as parasites in the digestive tube of higher animals do not get molecular oxygen in free state but get it by decomposing complex oxygen bearing substances present in the body of the host.

Anaerobic protozoa include Trypanosoma gambiense of vertebrate blood. While Histomonas meleagridis, a flagellate in the intestine of chicken can grow in presence of air as well as without it and is a ‘facultative aerobe’.

Excretion in Protozoa:

Waste products are water, carbon dioxide and nitrogenous compounds and remain in soluble forms. Waste materials are passed out of the body by diffusion or by the contractile vacuoles. Surrounding water is hypertonic to freshwater amoeba.

So, water constantly enters inside the body of amoeba through the cell surface. This excess water interferes with the body functions and is eliminated by the discharge of contractile vacuole. Marine or parasitic protozoa live in isotonic media and do not have contractile vacuoles.

Some amount of carbon dioxide is dif­fused out through the cell surface. Rest of the carbon dioxide and ammonia which remain in soluble state are thrown out of the body by the contractile vacuoles.

Insoluble substances in the form of crys­tals of calcium phosphate (recorded in Amoeba proteus) ureate, carbonate, oxalate and grains (haemozomin in haemosporidians) are often encountered. These substances are consid­ered as catabolic products. The way they are extruded is still in observation stage. In most protozoa, excretion of nitrogen occurs in the form of Ammonia and free amnio acid.

Response to Stimuli:

The reaction to stimulus in protozoa is expressed by movements. These movements may be clas­sified in two broad groups; Taxis in which the reaction is directed towards the stimulus, i.e., positive reaction and Kinesis in which the stimulus increases the random movement of the animals and as a result the animals tend to move away from the source of the stimulus, i.e., negative reaction.

Well-organ­ised structures for the reception and conduc­tion of a stimulus are lacking in protozoa.

In amoeba, a stimulus is first received by the body surface and then by the whole proto­plasmic body. The flagella or cilia, in flagel­lates and ciliates respectively, are in part sensory. In ciliates there are certain cilia that are non-vibratile and appear to be sensory in function. Sensory organellae such as stigma, ocellus, statocysts and concretion vacuoles occur in many forms.

Response to mechanical stimuli:

(1) An amoeba turns away, i.e., shows negative reaction when stimulated mechanically by the tip of a glass rod,

(2) A suspended amoeba shows positive reaction when it comes in contact with a solid surface. The tip of the pseudopodium in such cases touches the surface and then adheres to it by spreading out, and

(3) Positive reaction is shown by an amoeba during ingestion of solid food par­ticle.

Response to chemical stimuli:

An amoeba reacts negatively when it is brought in contact with salt solution, methyl green or methylene blue. Ciliates show positive reac­tion to acid solution up to certain concentra­tion. In higher concentrations the reaction becomes negative.

Chemotaxis is of great importance for the existence of protozoa since it helps them to find out the proper food. With the help of chemotaxis the parasitic protozoa find the specific site of their resi­dence in the body of the host. Chemotaxis is believed to have a role in the process of sexual reproduction in protozoa.

Response to light stimuli:

All protozoa are indifferent to an ordinary light source. Amoeba shows a negative reaction to strong light source while positive reaction to light is shown by stigma bearing mastigophora.

Mastigophores in a jar concentrate at a place where the light intensity is maximum, if the jar is placed in a dark place the organisms remain scattered throughout the container, become inactive and encysted, and myxotropic forms start practising saprozoic method. Protozoa are sensitive to ultraviolet rays.

Response to electrical stimuli:

Protozoa in water have been subjected to electric cur­rent and it has been observed that amoeba shows negative reaction to anode and moves towards cathode, free-swimming ciliates move to the cathode excepting Paramoecium and Stentor which move to the anode.

Response to temperature stimuli:

There exists an optimum temperature range for each protozoan and it can withstand a little fluc­tuation of this range. In temperatures higher than the optimum, the metabolic activities of protozoa increase and reproduction, in quick succession, is followed. It is believed that temperature changes in the environment result in bringing forth different phases (trophic and cystic) in the life cycle of differ­ent amoebae.

Response to gravity:

Reaction to the forces of gravitation is dependent upon body or­ganisation and locomotor organelle. Bottom dwellers like Testacea show positive reaction to gravity while Paramoecium shows nega­tive reaction.

Response to water current:

Free-swim­ming forms orient themselves against the current. Paramoecium places itself in the line of current with anterior end upstream.

Reproduction in Protozoa:

Protozoans reproduce in a variety of ways and the process of reproduction is variable amongst different groups. But in all essence and purpose protozoan reproduction is nothing more than the division of the cell. It reproduces both asexually and sexually.

I. Asexual Reproduction:

When the continuity of species is main­tained without the participation of the gam­etes and the asexual reproduction takes place by the division of the body of individual into two or more parts, these parts give rise to the new individuals.

The asexual reproduction is of the follow­ing types:

A. Binary fission,

B. Multiple fission or Sporulation,

1. Schizogony or Agamogony

2. Gamogony

3. Sporogony

C. Plasmotomy

D. Budding and

E. Repeated fission.

A. Binary fission:

It is the usual method in which the body of the individual divides into two equal halves and the furrow ex­tends along the long and the extended axis of the body.

Depending upon the plane of division, the binary fissions are of the following cat­egories:

(i) Longitudinal binary fission:

The plane of constriction is along the long axis of the animal, e.g., Euglena, Vorticella, Trypanosoma etc.

(ii) Transverse binary fission:

The plane of division of the body constricts transversely, e.g., Paramoecium.

(iii) Oblique binary fission:

The plane of division is somewhat oblique, e.g., Ceratium.

The different organelles present in the body may divide or they may be retained by one of the daughter cells; while in the other cell regenerates the lost organelles. In extreme cases organelles disappear altogether and are regenerated by both the offspring.

(iv) Encysted condition:

In Colpoda, Tellina and in testaceans, binary fission takes place in encysted condition. One of the daughter individuals remains within the old test while the other moves away to form a new one.

Remarks:

In Stentor coeruleus, a process called physiological regenera­tion takes place occasionally and its biological significance is not known.

B. Multiple fission or Sporulation:

In multiple fission the body divides and a number of daughter individuals are formed. The nucleus divides a number of times and a multinucleate state results. The nuclei come to the periphery and gather some amount of cytoplasm round them. The cell-membrane breaks and daughter individuals correspond­ing to the number of nuclei are produced.

The number of individuals produced by multiple fission varies and sometimes as many as 1000 individuals are formed. Mul­tiple fission occurs in Foraminifera, Radiolarians and Sporozoans.

Multiple fission is also known by the following names:

1. Schizogony or Agamogony:

When the products of the fission directly develop into individuals as in Plasmodium in the red blood cells or hepatic cells of man.

2. Gamogony:

When the products are sex cells as the microgametocytes of Plasmodium.

3. Sporogony:

When it occurs following sexual fusion as in Monocystis and Plasmo­dium.

C. Plasmotomy:

It is the division of the cell-body without nuclear division and oc­curs in many multinucleate ciliates like Opalina.

D. Budding:

It is a process in which one or more individuals are produced on the body of the parent and are budded off. The indi­viduals generally do not resemble the mother and undergo further development before or after being free. Budding occurs only in Suctoria. The site of bud formation may be inside or outer side of the body.

1. Exogenous bud:

When the buds are constricted off to the exterior as in Noctiluca and some Myxosporidia.

2. Endogenous bud:

When the buds are formed in the brood chamber or internal spaces of the mother body and come out later as in Testaceans, Arcella, Suctorians and many Myxosporidia.

E. Repeated fission:

In which equal division of the nucleus occurs twice or thrice forming four or eight nuclei which do not separate till the process for which the nu­cleus divides is complete as in the micronucleus of Paramoecium and Volvox.

II. Sexual Reproduction:

Sexual reproduction is one when it takes place by the union of two entire individuals or it involves merely the nuclear exchange and their subsequent fusion.

In Protozoa the sexual reproduction occurs by the following processes:

A. Syngamy or Copulation:

In which union of two sexual cells, called gametes, occur.

On the basis of structure and behav­iour of the sexual units the following types of syngamy can be recognised:

(a) Hologamy:

In which no true gamete formation takes place but two mature trophic individuals unite with each other and fusion of both nucleus and cytoplasm takes place. It occurs in few flagellates and rhizopods.

(b) Isogamy:

The copulating sex units are similar in size and form and cannot be mor­phologically distinguished from each other though there exist physiological differences. The units are generally produced by multi­ple fission. Isogamy is common in Formaminifera, Gregarines and Phytomonadina like Copromonas.

(c) Anisogamy:

It is fusion of dissimilar gametes. The copulating sex units are dis­similar in size, form and behaviour. The large and non-motile unit is called female or macrogamete and the small mobile one is termed male or microgamete in such fusion. They widely occur in Phytomonadina and Sporozoa, e.g., Plasmodium.

(d) Oogamy:

In this case the gametes are quite dissimilar. The female gamete is non- motile egg and the male is a flagellate and motile sperm. It is found in Volvex.

(e) Paedogamy:

When the fusing pronu­clei are present in two different cells derived from a single parent cell, the process is called paedogamy. The process has been observed in Actinophrys sold by Blar (1922) and in some Myxosporidia.

Significance of syngamy:

(i) Syngamy brings about a nuclear re­organization, and physiologically it has distinct effects.

(ii) It brings two previously separated lines of heredity in close association.

(iii) It increases diversity among the off­spring.

B. Conjugation:

Conjugation may be defined as a temporary union of two indi­viduals belonging to same species for the purpose of exchange of nuclear material. Conjugation is a complex process in which several nuclear divisions occur both in the preparatory and post-conjugation phases and one of these divisions is meiotic in nature. Conjugation occurs in Euciliates and Suctorians.

Significance of conjugation:

(i) Conjugation helps in rejuvenescence to gain vigour and vitality.

(ii) It brings about the genetic recombi­nation, and the origin of genetic vari­ations takes place.

(iii) Reorganesation of nuclear apparatus takes place between the individuals.

Aberrant reproduction in Paramoecium:

Peculiar variation in behaviour of Paramoecium in nuclear division during conjugation is encountered.

These variations in behaviour have been classified in the fol­lowing ways:

(a) Autogamy:

Very similar to conjuga­tion but all the changes occur in a single individual. It is accompanied by fusion of pronuclei and meiosis and provides an op­portunity for the reshuffling of genes. Auto­gamy occurs in Paramoecium aurelia.

(b) Endomixis:

Woodruff and Erdmann (1914) first described the process in Paramoecium aurelia. This is very similar to conjugation but nuclear changes are restricted to a single individual. Fusion of pronuclei and meiosis does not occur, though a new meganucleus is formed out of the micro- nuclear material as in conjugation.

Remarks:

Diller (1936) and Sonneborn (1947) reported that they do not get any evidence in Paramoecium.

(c) Hemimixis:

The process was observed by Diller (1936) in Paramoecium aurelia and P. multimicronucleatum. In this case the meganucleus behaves in a strange fashion. It divides into two or a part of it may be pro­truded into the cell mass. The meganuclear activity is independent of cell division or syngamy.

Parthenogenesis:

In case the syn­gamy is missed, gametes develop parthenogenetically. The examples are Actinophrys, Polytoma and Chlamydomonas.

Regeneration:

Protozoa possess a remarkable power to regenerate lost parts, provided nuclear material is included. When an amoeba is cut into two parts and the parts are kept in proper environment—the part without nucleus degenerates while the nu­clear part regenerates. Shell of Foraminifera regenerates if broken.

Besides these restora­tive regenerations in protozoa there occur regenerations of lost parts like cilia, flagella, cytostome and vacuoles after asexual and sexual reproduction. The process of morpho­genesis in regeneration and reorganisation has been a subject of research.

Nuclear Division:

A. Mitosis:

The modes of nuclear divi­sion during reproduction are worthy of con­sideration. Earlier the existence of mitotic phenomenon in protozoa used to be disre­garded and it was advocated that in proto­zoa there occurs ‘amitosis’ or an unusual type of mitosis.

Now it has been made evident that the nuclear division in protozoa passes through all the steps of mitosis and is iden­tical with those of metazoan cells in most cases and in the rest they are abbreviated.

The mitotic phenomenon in protozoa is de­scribed in the following ways:

(1) Eumitosis:

When there is distinct chromosome formation and chromosomes on the whole behave like those of the metazoan. Eumitosis is a common feature of free-living forms.

(2) Paramitosis:

The chromosomes dur­ing paramitotic division do not shorten at metaphase and remain asymmetrically arranged on the equator of the spindle. The sister chromatids do not lie side by side but hang together at one end. As a result during separation they present a picture of false transverse division of chromosomes.

Paramitosis occurs in Coccidians, Dinoflagellates, etc.

(3) Cryptomitosis:

In cryptomitosis trans­lation of the chromatin material into chro­mosomes is lacking and the whole chroma­tin material is lodged as a mass on the equa­tor of the spindle. The chromatin mass be­comes divided into two halve which go to the two poles.

Cryptomitosis occurs in parasitic and coprozoic forms like Hoylosyoridium and Naegleria.

B. Meiosis:

The Protozoan nuclei undergo divisions prior to sexual reproduction. And it is expected that one of these divisions should be meiotic in nature so that the con­stancy of the number of chromosomes could be maintained.

Information about meiosis in protozoa is scanty or fragmentary. Meiotic division in protozoa may occur before the formation of gametes (pre-gametic) or after the fusion of gametes (post-zygotic). Pregametic meiosis occurs in Paramoecium and post-zygotic in Telosporidia.

Cytoplasmic Division:

The division of nucleus is followed by division of cytoplasm and extra-nuclear organelles, such as chromatophores and pyrenoid, blepharoplast and kinetosome. But nuclear division in encysted condition results accumulation of cytoplasm round each nu­cleus and there is no cytoplasmic division in true sense.

Encystment:

Many protozoa exhibit a phase reversal. At one phase of life cycle they remain active and carry on vital life processes and in an­other phase they become inactive and dis­card most of the life processes. The active phase is called trophic or trophozoite stage and the inactive phase is called cyst and cystic stage. That means many protozoa are capa­ble of existing alternately as trophic and cystic forms.

During the transformation from trophic to cystic the trophozoites cease to ingest, extrude remains of food particle and become round in appearance. This phase is called the pre-cystic phase. De-differentiation of the whole organism now occurs and cell organellae like cilia, peristome, axostyle, contractile vacuole, etc., are absorbed. Finally, they secrete substances which solidify and form resistant walls round the organism.

Thus a cyst is formed. The number of walls in a cyst varies from 1-3. The cysts are capable of remaining viable for a long time. The wall of the cysts contains silliceous plates in Euglypha, cellulose in Phytomonadina and chitinous elements in most cases.

Low and high temperature, evaporation, change in pH, accumulation of metabolic products and even overpopulation are the conditions which lead a protozoa to encyst.

Cysts may be protective when it is formed in unfavourable conditions, it may be repro­ductive when fission occurs inside the cyst and it may be digestive when it is formed immediately after food intake and overfeeding. On encountering a suitable and proper environment excystment occurs.

During excystment various organellae char­acteristics of the organism are re-differentiated and reformed. The trophozoite emerges through a pore on the cyst wall. Little is known about the specific mechanism by which an aperture is formed on the cyst wall.

The rupture of the cyst wall may be caused by the following processes:

(i) Increase in the internal pressure by accumulation of water inside the cyst causes the rupture of the cyst wall which loses is rigidity and resistance.

(ii) Pseudopodial activity inside the cyst wall leads to the formation of aper­ture on the cyst wall.

(iii) Dobell has proposed that in Enta­moeba secretion of an enzyme is re­sponsible for the dissolution of the cyst wall.

Experimentally encystment can be in­duced by the addition of fresh culture me­dium, distilled water, organic infusions and by lowering the temperature or changing the pH.

The success and wide distribution of pro­tozoa are probably due to its ability to en­cyst. The cysts are minute and are easily transported from one place to another by various agents, such as wind, water current, soil particles, insects, birds and other ani­mals.

Encystment is an essential and important phase of life in most protozoa though the presence of this phase has not been encoun­tered in Paramoecium and Amoeba proteus.