Protozoa: Nutrition | Zoology Optional Notes for UPSC PDF Download

Nutrition: Mode # 1. Holozoic or Zoo-Trophic Nutrition

The majority of Protozoa primarily obtain their nutrition through holozoic means, akin to animals consuming solid food. Protozoa's diet typically comprises microorganisms such as bacteria, diatoms, rotifers, crustacean larvae, other protozoans, algae, small fragments from larger animals and plants, and so forth. This method of nutrition fundamentally encompasses the stages of food intake (ingestion), digestion, absorption, and the elimination of undigested remnants (egestion).

Ingestion

• Protozoa employ distinct methods of food intake, a process referred to as phagocytosis or phagotrophy. Flagellates, particularly those lacking pigmentation or chromatophores, capture food using their flagella.
• Food capture in flagellates occurs at specific locations on their unadorned bodies, like Budo, or via characteristic oral structures like in Euglena, where the cytostome and cytopharynx facilitate ingestion. Some flagellates, such as Peranema, use specialized rod-like structures called trichites for food capture.
• Sarcodina, on the other hand, employ pseudopodia, which form food cups to capture food. Amoeba's food ingestion process involves circumvallation, circumfluence, import, and invagination, as reported by Rhumbler (1930). Various types of pseudopodia, like axopodia in heliozoans and radiolarians, and reticulopodia in foraminifera, aid in capturing prey.
• Ciliates like Paramecium possess a well-developed feeding apparatus with a distinct cytostome, typically located at the base of the oral groove leading to the cytopharynx. Specialized cilia within this apparatus create a whirlpool of water current, directing food particles into the cytopharynx through the cytostome.
• Suctorians have a unique feeding mechanism. They use tentacles, often knobbed at their tips, to capture prey. Each tentacle contains a central tubular canal surrounded by a contractile sheath. Upon contact with the tips of the tentacles, the prey becomes immobilized and paralyzed by a secreted toxin. The suctorian then gradually sucks the prey's cytoplasm into its body through the central tubular canal of the tentacles.

mode of feeding in suctoria

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 possessing chlorophyll or related pigments are capable of synthesizing complex organic nutrients, similar to those produced by green plants, from simple inorganic substances. For instance, organisms like Euglena and Noctiluca fall into this category. These protozoa often contain protein structures known as pyrenoids, which serve as the focal points for the process of photosynthesis.
In contrast, some protozoa lack chromatophores but host chlorophyll-bearing algae like Zooxanthellae or Zoochlorellae, which engage in photosynthesis to produce organic food for the host. This phenomenon can be observed in organisms such as Stentor, Thalassicola, and Paramecium bursaria. Additionally, autotrophic forms of protozoa can obtain their nitrogen requirements from nitrates or ammonium compounds.

Nutrition: Mode # 4. Saprozoic Nutrition

Certain Protozoa obtain complex organic substances dissolved in solution by a process known as osmosis, a nutritional method referred to as osmotrophy. These Protozoa are categorized as saprozoic, and they rely on ammonium salts, amino acids, or peptones to meet their nutritional needs. The breakdown of decaying animals and plants in water results in the formation of proteins and carbohydrates.
Saprozoic Protozoa are typically parasitic in nature, such as Monocystis. However, some parasites like Entamoeba histolytica and Balantidium coli, which primarily feed holozoically, also have the ability to absorb dissolved organic substances through their general body surface. In contrast, certain colorless flagellates like Chilomonas, Polytoma, and various species of Euglena acquire nutrients from their surrounding environment through their overall 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:

• 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. 
• 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:

• Macrophagous feeders are those which feed on large pieces of food (Amoeba),
• 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).
• Fluid feeders are saprozoic and parasitic Protozoa which absorb liquid food through their surface (Monocystis).