Phylum Protozoa: Nutrition and Parasitism
Posted on : 26-11-2017 Posted by : Admin

 

NUTRITION IN PROTOZOA

Protozoa obtain nutrition in many ways. Some synthesize their own food other get it synthesized by algae living in their cytoplasm and still others capture the food. Some Protozoa lead a parasitic life, usually doing no harm or very little harm to their hosts but occasionally cause serious diseases.

All types of nutrition are found in protozoa namely holophytic nutrition, holozoic nutrition, saprozoic nutrition, mixotrophic nutrition and parasitic nutrition. Their modes of nutrition are as follows:

Holophytic nutrition: The phytoflagellates possess chloroplasts and chromatophores to synthesize their food by photosynthesis. They utilize sunlight, carbon dioxide and water as raw materials. This method of self-feeding is referred to as Autotrophic phototrophy. The dextrose sugar paramylon synthesized is characteristic of euglenoid flagellates.

Holozoic nutrition: Most of the Protozoa derive nutrition by ingesting other organisms. This mode of nutrition is said to be holozoic. It involves development of organelles for food capture, ingestion, digestion, assimilation and egestion of undigested food materials. They capture their food by flagella, pseudopodia and trichites.

Some use axopodia, reticulopodia and tentacles to pull the prey that comes within their reach. In ciliates the ciliary oral apparatus is well developed for food capturing and driving it towards mouth or cytosome and then pushing it into the cytopharynx.

Pinocytosis: This method also called as cell drinking involves ingestion of liquid food by invagination through the surface of the body. The pinocytosis channels are formed at some parts of the body, which enclose the fluid from the surrounding medium. The lower ends of these channels are pinched as food vacuoles into the endoplasm. Pinocytosis is only induced by certain active substances in the medium surrounding the cell.  High molecular compounds from the external medium are absorbed by this method.

Saprozoic nutrition: This involves the absorption of food by osmosis, through the general body surface. So this method is referred as osmotrophy. The food mainly is the dead organic matter rendered so by the decomposing bacteria. This kind of nutrition is found in Mastigamoeba and also some of the colorless flagellates.

Suctorians feed on other ciliates with the help of their tentacles which have funnel ends. Each tentacle consists of a rounded rigid central tube. As soon as the prey is attached, the tentacles tips paralyses the prey with some hypnotoxin and gradually suck the body fluids with the central tubes.

Myxotrophic nutrition: This is a combination of more than one mode of nutrition. Many protozoa using photosynthesis as a means also take in some part of their diet in dissolved form by osmotrophy or solid form by phagocytosis. The best examples of this kind of nutrition are flagellates like Euglena and Peranema.

Nutrition of parasitic protozoa: The mechanisms used by parasitic protozoa are almost are similar to that of their non-parasitic protozoa.  Parasites inhabiting the intestine and blood have a distinct mouth through which food particles are ingested through the process of phagotrophy. The osmotrophic forms of protozoa are either coelozoic or histozoic. The coelozoic forms absorb their food by their cell surface. The histozoic forms feed on the substances by osmotrophy. Parasitic saprozoic forms may also directly use the serum of their host blood.


 

PARASITISM IN PROTOZOA

Parasitism is a Greek word which means eating beside one another (para=beside, sitos=food). Parasites are the species which live at the expense of certain other species. The other animals on which the parasites live are called as hosts. The parasites are biologically and economically closely connected with the hosts throughout their life time. Hence parasitism can be defined as an association between the parasites and their hosts. It is an association between two organisms whereby the parasite temporarily or permanently lives and feeds in or on the body of the host. Almost all the protozoan groups have parasitic species and the group Sporozoa is exclusively parasitic.

 

Types of parasites

Ectoparasites: These are the parasites which inhabit the external surface of their hosts.

Eg: Hydramoeba hydrozena feeding on the ectodermal cells of hydra,

       Icthyopthirius multifilis burying in the epidermis of freshwater fishes

Endoparasites: These are the parasites which inhabit the internal surfaces of their hosts. They may be further subdivided into the following types,

  • The parasites which dwell in the lumen of the alimentary canal of the host.

Eg: Entamoeba histolytica, Giardia lamblia, Isospora hominis, Balantidium coli

  • The parasites which reside in the mouth cavity of hosts.

Eg: Trichomonas tenax, Entamoeba gingivalis

  • The parasites which inhabit the genital tract of the hosts.

Eg: Trichomonas vaginalis

  • The parasites which live within the tissues of hosts. These may enter the host tissues through the skin or even from the digestive tract.

Eg: Trypanosoma, Plasmodium, Leishmania, Babesia

Hyperparasites: These are the protozoans which parasitize other parasitic protozoans. In other words these are parasites on parasites.

Eg: Zelleriella, Nosema notabilis, Sphaerospora polymorpha

Pathogenic parasites: Generally the parasitic protozoans are not always pathogenic but sometimes these parasites are pathogenic and can cause grave diseases in humans and other animals. These parasites which are causing disease are called as pathogenic parasites.

Eg: Leishmania donovani, Plasmodium vivax, Trypanosoma gambiense

 

Host specificity

The protozoan parasites are generally host specific and in this regard two trends are seen.

  • Firstly, some of the parasites like Trypanosome, Entamoeba and Eimeria successfully parasitize wide range of hosts.
  • Secondly, some parasites like Plasmodium restrict themselves only to few specific hosts.

 

Transmission

Protozoan parasites are transmitted by various ways to their hosts. The following are the few examples of various ways used by the protozoan parasites to reach their hosts,

Protozoan parasite
Transmssion type
Transmission method
Entamoeba gingivalis Direct transfer By mechanical contact like kissing.
Entamoeba histolytica Contaminative transfer By cysts in contaminated food or water
Trypanosoma sps. Inoculative transfer By invertebrate vectors
Plasmodium sps. Inoculative transfer By invertebrate vectors
Babesia sps. Congenital transfer By invasion of ovary or eggs
Eimeria tenella Contaminative transfer By cysts in contaminated food or water

 

Life cycle of the protozoan parasites

Many of the protozoan parasites have single host throughout their life cycle and only a part of the life is spent outside the host. These parasites with only one host in their life cycle are called as monogenetic parasites. For example, Eimeria and Monocystis

Many other protozoan parasites have two or more hosts through their life cycle. These two hosts included in the life cycle of the protozoan parasite belong to separate animal groups. The two hosts are designated as primary host or definitive host, in which the ancestors of the parasite have evolved. The other host is called as secondary host or vector or intermediate host. This vector acts as the transmitting agent for the parasite. These parasites with more than two hosts in their life cycle are called as digenetic parasites. For example, Trypanosoma and Plasmodium

Sometimes a reservoir host can harbor a pathogen indefinitely with no ill effects. A single reservoir host may be reinfected several times.

If the parasite undergoes part of its life cycle in vector, then its transmission is called as cyclical. If the parasite does not undergo part of its life cycle in vector, then its transmission is called as mechanical transmission.

 

Effects of protozoan parasites on their hosts

The parasites can bring about several changes within their hosts. Some parasites can also prove to be injurious to the host, while others may produce no effect on the host. The following are the few examples,

  • Entamoeba histolytica destroys the large intestine of the host and this in turn causes ulcerations.
  • Eimeria stiedae causes hyperplasia of the liver cells of rabbits.
  • Polymnia nebulosa which is a parasite of earth worm brings about hypertrophy of the sperm mother cells.
  • Plasmodium gallinaceum which is malarial parasite of bird clogs fine blood capillaries


  1. Diffrentiate between ectoparasites and endoparasites. Also give examples for each.
  2. What are pathogenic parasites? Give few examples from Phylum Protozoa.
  3. Explain general life cycle of protozoan parasites.
  4. What are the effects of protozoan parasites on their hosts.
  5. Name the types of nutrition found in Protozoa.
  6. Describe the process of pinocytic nutrition.
  7. Explain the mechanism of feeding in suctorians.
  8. What is myxotrophic nutrition?

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