The following points highlight the seven important modes of nutrition in Protozoa. The modes are: 1. Holozoic or Zoo-Trophic Nutrition 2. Pinocytosis 3. Autotrophic or Holophytic Nutrition 4. Saprozoic Nutrition 5. Parasitic Nutrition 6. Coprozoic Nutrition 7. Mixotrophic Nutrition.
Nutrition: Mode # 1. Holozoic or Zoo-Trophic Nutrition:
Majority of Protozoa nutrite holozoically, i.e., like animals on solid food. The food of Protozoa consists of microorganisms like bacteria, diatoms, rotifers, crustacean larvae, other protozoans, algae, small fragments of large animals and plants, etc. This mode of nutrition essentially involves the processes like intake of food, i.e., ingestion, digestion, absorption and egestion of undigested residues.
Ingestion:
The mode of food ingestion in Protozoa is characteristically referred to as phagocytosis or phagotrophy. In fact, in flagellates which are colourless or who have lost their chromatophores capture food with the help of their flagella.
The captured food is ingested either at definite sites on their naked bodies like Budo or through characteristic oral apparatus like Euglena where cytostome and cytopharynx help in ingestion. In some other flagellates like Peranema, special rod-like structures called trichites help in capturing the food.
In Sarcodina, pseudopodia help in food capturing by forming food cups. Rhumbler (1930) has reported that the ingestion of food in Amoeba occurs by circumvallation, circumfluence, import and invagination. Different types of pseudopodia like axopodia in heliozoans and radiolarians; reticulopodia in foraminifera’s also help in catching the prey.
In ciliates like Paramecium, the feeding apparatus is well developed with a definite cytostome. The cytostome is usually present at the base of the oral groove leading into the cytopharynx. The feeding apparatus is provided with some specialised cilia. The beating of the cilia of cytopharynx creates a whirl pool of water current. The food particles in the water current are directed into the cytopharynx through cytostome.
The mode of feeding in suctorians is very characteristic; they feed with the help of their tentacles which are mostly knobbed at their tips (Fig. 23.6A). Each tentacle consists of a central tubular canal surrounded by a contractile sheath. The prey, as and when comes in contact with the tips of tentacles, soon gets adhered and paralyzed by some toxin secreted by the suctorian.
The prey’s cytoplasm is then gradually sucked into the suctorian body through the central tubular canal of the tentacles (Fig. 23.6B).
Digestion:
Digestion in Protozoa is intracellular within food vacuoles. The food vacuoles undergo changes in pH and in their size during digestion. At first the contents of the food vacuole are acidic and the vacuoles decrease in size, during this phase living prey dies.
After the initial acid phase the cytoplasm of the protozoan produces enzymes in an alkaline medium, the enzymes pass into the food vacuoles and the vacuoles increase in size and become alkaline.
Then the contents of the vacuoles are digested. In fact, proteolytic and carbohydrate digesting enzymes are reported in Protozoa; the proteins are converted into dipeptides in acidic medium and the dipeptides into amino-acids in alkaline medium. The carbohydrates are hydrolysed in alkaline medium. The fat digesting enzymes have also been reported in some Protozoa.
Absorption and Assimilation:
The digested food from the food vacuole is diffused out into the endoplasm and finally assimilated in the body to manufacture the protoplasm. The excess of food is stored in form of glycogen paramylon, Para glycogen bodies in the endoplasm.
Egestion:
The un-digestible remains of the food are egested out from the body at anybody surface, e.g., in Amoeba. But ciliates possess a definite opening for the egestion of undigested remains called cytoproct or cytopyge.
Nutrition: Mode # 2. Pinocytosis:
Pinocytosis or cell-drinking has also been reported in some Protozoa like Amoeba proteus, and also in certain flagellates and ciliates. It is related to the ingestion of liquid food by invagination of the general body surface. It may occur at any part of the body; during pinocytosis, some pinocytic channels are formed from the outer body surface deep into the body.
The inner ends of these channels’ contain pinocytic vesicles or pinosomes which get separated after engulfing liquid food through the channels. The separated pinosomes become the food vacuoles. This process is induced in presence of certain salts and some proteins.
Nutrition: Mode # 3. Autotrophic or Holophytic Nutrition:
Protozoa with chlorophyll or some allied pigment can manufacture complex organic food, like those of green plants, from simple inorganic substances, e.g., Euglena, Noctiluca. Often there may be protein bodies called pyrenoids which are the centres of photosynthesis.
Some Protozoa have no chromatophores but they have chlorophyll-bearing algae Zooxantliellae or Zoochldrellae which manufacture organic food for the host by photosynthesis, e.g., Stentor, Thalassicola, Paramecium bursaria. Nitrates or ammonium compounds are sufficient as the source of nitrogen for autotrophic forms.
Nutrition: Mode # 4. Saprozoic Nutrition:
Some Protozoa absorb complex organic substances in solution through the body surface by the process of osmosis called osmotrophy. These Protozoa are called saprozoic. Saprozoic forms need ammonium salts, amino acids, or peptones for their nutritional requirements. Decaying of animals and plants in water forms proteins and carbohydrates.
The saprozoic Protozoa are usually parasites like Monocystis. But some parasites, like Entamoeba histolytica and Balantidium coli feeding holozoically also absorb dissolved organic substances through their general body surface. However, some colourless flagellates like Chilomonas, Polytoma and species of Euglena absorb nutrients from their surrounding environment through their general body surface.
Nutrition: Mode # 5. Parasitic Nutrition:
The parasitic forms feed either holozoically or saprozoically.
Thus, the parasites may be grouped into two categories on the nature of food and their mode of feeding:
(i) Food-robbers:
The parasites feeding upon the undigested or digested foodstuffs of their hosts are known as food-robbers, such as some ciliate parasites like Nyctotherus, Balantidium. These parasites feed holozoically on solid food particles, while few others like Opalina feed upon the liquid food by the process of osmosis through their general body surfaces. The food-robbers are generally non-pathogenic to their hosts.
(ii) Pathogenic:
The protozoan parasites causing harm to their hosts, usually feed upon the living tissues of the host. They absorb liquid food through their general body surface, e.g., Trypanosoma, Plasmodium, etc.
Nutrition: Mode # 6. Coprozoic Nutrition:
Certain free-living protozoans are in habit of feeding upon the faecal matters of the other organisms like Clamydophrys and Dimastigamoeba.
Nutrition: Mode # 7. Mixotrophic Nutrition:
Some Protozoa nourish themselves by more than one method at the same time or at different times due to change in environment. This is called mixotrophic nutrition, e.g., Euglena gracilis and Peranema are both saprozoic and autotrophic in their nutrition, and some flagellates are both autorophic and zootrophic.
However, Protozoa which feed on a large variety of food organisms are called euryphagous, and those which feed only on a few kinds of food are stenophagous.
On the basis of the nature of food and feeding mechanism in Protozoa, they are placed in the following groups:
(a) Macrophagous feeders are those which feed on large pieces of food (Amoeba),
(b) Microphagous feeders are those which feed on very small particles, they rarely stop feeding and their food is drawn in with a current of water (Paramecium).
(c) Fluid feeders are saprozoic and parasitic Protozoa which absorb liquid food through their surface (Monocystis).