0The Kingdom Protista consists of eukaryotic protists. Members of this very diverse kingdom are typically unicelluar and less complex in structure than other eukaryotes. In a superficial sense, these organisms are often described based on their similarities to the other groups of eukaryotes: animals, plants, and fungi. Protists do not share many similarities, but are grouped together because they do not fit into any of the other kingdoms. Some protists are capable of photosynthesis, some live in mutualistic relationships with other protists, some are single celled, some are multicellular or form colonies, some are microscopic, some are enormous (giant kelp), some are bioluminescent, and some are responsible for a number of diseases that occur in plants and animals. Protists live in aquatic environments, moist land habitats, and even inside other eukaryotes.
It may be photosynthetic, holotrophic, saprotrophic, parasitic and symbionts. Some have mixotrophic nutrition (holotrophic + saprobic).
- The photosynthetic, floating protists are collectively called phytoplankton.
- The free-floating, holozoic protozoans are collectively termed zooplankton.
- Unicellular protists have been broadly divided in to three major groups:
(a) Photosynthetic Protists: Example: Dinoflagellates, Diatoms, Euglenoids
(b) Consumer Protists: Example: Slime moulds or Myxomycetes
(c) Protozoan Protists: Example: Zooflagellata, Sarcodina, Sporozoa, Ciliata
Major Groups of Protists
Chrysophytes
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This group includes diatoms and golden algae (desmids).
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They are found in fresh water as well as in marine environments.
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They are microscopic and float passively in water currents (plankton).
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In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box.
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The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as 'diatomaceous earth'.
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Being gritty this soil is used in polishing, filtration of oils and syrups.
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Diatoms are the chief 'producers' in the oceans
Dianoflagellates
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These organisms are mostly marine and photosynthetic.
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They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
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The cell wall has stiff cellulose plates on the outer surface.
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Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
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Very often, red dianoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides).
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Toxins released by such large numbers may even kill other marine animals such as fishes.
Euglenoids
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Majority of them are fresh water organisms found in stagnant water.
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Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
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They have two flagella, a short and a long one. The two flagella join with each other at a swelling called paraflagellar body. An orange red coloured eye-spot or stigma is located at the base of flagellum attached to the membrane of reservoir at the level of paraflagellar body.
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Both paraflagellar body; and eye spot act as photoreceptors and direct the organism towards the optimum light.
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Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in higher plants. Example: Euglena. They contain red pigment astaxanthin.
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Nutrition is holophytic (photoautotrophic), saprobic (e.g., Rhabdomonas) or holozoic (e.g., Peranema). Even holophytic forms can pick up organic compounds from the outside medium. Such a mode of nutrition is called mixotrophic.
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Euglena is a connecting link between animals and plants. Nutrition in Euglena is mixotrophic, when light is available it is photosynthetic, in darkness it is saprophytic absorbing food from surrounding water.
Slime Moulds-
Slime moulds are saprophytic protists.
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The body moves along decaying twigs and leaves engulfing organic material.
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Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet.
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During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. The spores are dispersed by air currents.
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They are extremely resistant and survive for many years, even under adverse conditions.
Protozoans
All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals.
There are four major groups of protozoan
Group 1. Flagellated ProtozoansCharacters:
(i) They possess flagella for locomotion.(ii) They may be free living aquatics, parasites, commensals or symbionts.
(iii) Zooflagellates are generally uninucleate, occasionally multinucleate.
(iv) The body is covered by a firm pellicle.
(v) Nutrition is holozoic, saproboic and parasitic
Examples:
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Trypanosome gambiense – The parasite of sleeping sickness. It was first observed by Forde in 1901. Fruce discovered that the parasite of sleeping sickness is transmitted by tse-tse fly. It causes Gambian sleeping sickness. The disease, also called Gambain trypanosomiasis, is found in western and central parts of Africa.
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Trypansoma rhodesiense - It causes Rhodesian sickness. The disease is also called Rhodesian trypanosomiasis. The parasite is transmitted by the bites of tse-tse fly (glossina palpalis and glossina morsitans). Initially parasite is present in the blood of man but later on it enters the cerebrospinal fluid.
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Trypanosome cruzi - It causes South American trypanosomiasis (also called Chagas disease). The symptoms of the disease are fever, diarrhea, anaemia and enlargement of lymphoid glands.
Group 2. Amoebid ProtozoansCharacters:
(i) They develop pseudopodia which are temporary protoplasmic outgrowths. They are of four types- lobopodia (broad and blunt), filppodia (slender, unsupported, independent), axpodia (slender with axial support) and reticulopodia (slender, reticulate).
(ii) pseudopodia are used for locomotion and engulfing food articles.
(iii) Sarcodines are mostly free living, found in fresh water, sea water and on damp soil. Only a few are parasitic.
(iv) The body may be covered with plasmalemma or a shell.
Examples: Amoebe, pelomyxa, entamoeba, radiolarians, foraminiferans, heliozoans.
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Amoeba proteus- The Proteus Animalcule. Amoeba was discovered by Russel Von Rosenhoff in 1755. H.I. Hirschfied(1962) has given a detailed account of the biology of amoeba. It is found in fresh water. Types of pseudopodia are lobopodia.
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Pelomyxa - It is also known as giant amoeba. The size is about 2.5 mm long. Pelomyxa occurs in fresh water. Nutrition is holozoic. The chief food article is diatoms.
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Entamoeba histolytica – Lamble (1859) discovered Entamoeba histolytica. Losch (1875) discovered its pathogenic nature. The life cycle of Entamoeba histolytica is monogenetic (single host life cycle). It resides in the upper part of the human large intestine and causes the disease known as amoebic dysentery or amoebiasis.
Group 3. SporozoansCharacters:
(i) All sporozoans are endoparsites.
(ii) Some sporozoans such as Eimeria cause severl diseases like coccidiosis in the birds.
(iii) Locomotory organelles (cilia, flagella,pseudopodia,etc.)are absent.
(iv) Nutrition is parasitic (absorptive). Phagotrophy is rare.
(v) The body is covered with an elastic pellicle or cuticle.
(vi) Contractile vacuoles are absent.
Group 4. Ciliated Protozoans
Characters:
(i) Ciliates are protozoan protists which develop a number of cilia during a part or whole of the life cycle.
(ii) Cilia are used for locomotion and driving food.
(iii) There is a high degree of morphological and physiological specialization.
(iv) Most ciliates are free living individuals in fresh and marine waters. A few are parasitic.
(v) The body is covered by a pellicle.
(vi) Nutrition is holozoic except in the parasitic forms.
(vii) there are definite regions for ingestion and egestion. The region of ingestion consists of , cytostome (mouth) and gullet.
Examples:
Paramecium, vorticella, opalina, balantidium.
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Paramecium- The slipper organism or slipper Animalcule. Paramecium is a free living ciliate which is found in fresh water. Most widely distributed species are paramecium caudatum and paramecium Aurelia. Nutrition is microphageal. Bacteria are its chief feed. Paramecium is a surface feeder. Pellicle maintains the shape. The cilia of the extreme posterior end longer and form a bunch called caudal tuft.
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