All unicellular eukaryotes, irrespective of their mode of nutrition, are included in the kingdom Protista in Whittaker's system. The term protista was coined by Ernst Haeckel. This kingdom forms a link between kingdom Monera on one hand and other three kingdoms i.e. , Plantae, Fungi and Animalia on the other hand. Protistans are ancestors of all multicellular eukaryotes (plants, fungi and animals).

Kingdom Protista includes

General Characteristics of Protista

1. Unicellular, eukaryotic organisms. Some are colonial without much cellular differentiation. Organisation at tissue level is absent.

2. Mostly aquatic organisms.

3. Cell structure is eukaryotic type having all kinds of membrane bound organelles and 80 S cytoplasmic ribosomes and cells may possess cellulosic cell wall.

4. Flagella and cilia have (9+2) pattern of microtubule organization consisting of tubulin protein.

5. Movement by pseudopodia, flagella or cilia where ciliary mode is fastest.

6. Mode of nutrition may be photosynthetic (holophytic), holozoic (ingestive), saprobic or parasitic (absorptive). Some have mixotrophic nutrition (photosynthetic and saprobic) as in Euglena.

7. Reproduction occurs by asexual and sexual means.

8. Life cycle is of 2 types-(i) Showing zygotic meiosis (ii) Showing gametic meiosis.

9. These are decomposers, photosynthetic or parasites. Parasitic protists may cause diseases like dysentery, malaria, sleeping sickness etc.

 Photosynthetic protists and Slime moulds are described below:

(1) Photosynthetic Protists 

These are popularly called protistan algae. Protistan algae constitute the major portion of the phytoplanktons. 

A. Diatoms

Diatoms are golden brown photosynthetic protists and are called Chrysophytes (including both diatoms and desmids). They are both aquatic and terrestrial. Some are marine. They support much of marine life. Their important characters are

These are microscopic organisms possessing varying colours.

They are basically unicellular, but may form pseudofilament and colonies, lacking flagella except in the reproductive stage. They may be free floating (phytoplanktonic) due to presence of light weight lipids.

The cellulosic cell wall is impregnated with silica to form transparent siliceous shell, known as frustule. Depending upon the symmetry, diatoms may be pennate type, having bilateral symmetry (e.g., Navicula) and centric type, having radial symmetry (e.g., Melosira).

The cell wall is characteristic, made up of two halves; one half covering the other (epitheca over  hypotheca) resembling a soap box.

The cell wall encloses the peripheral layer of cytoplasm (primordial utricle) surrounding a large central vacuole.

Nucleus lies in the central vacuole, suspended with the help of cytoplasmic strands.

Mode of nutrition is holophytic (photoautotrophic), photosynthetic pigments are chlorophyll a, chlorophyll c, b-carotene and special carotenoids containing fucoxanthin; xanthophylls like diatoxanthin, diadinoxanthin.

The reserve food is oil and a polysaccharide called leucosin (chrysolaminarin), volutin granules are also present.

They are responsible for almost 50% of the total organic matter synthesized in the biosphere.

Movement occurs by mucilage propulsion.

They mainly undergo asexual reproduction. The common mode of asexual reproduction is binary fission.

During binary fission, one half of the cell wall is retained by each of the daughter cells formed. The other half of the cell wall is secreted afresh.

Resting spores are called statospores (centric diatoms).

They reproduce sexually as well. Sexual reproduction varies from isogamy to oogamy. It involves gametic meiosis as diatoms are generally diploid (diplontic life cycle).

  Concept Builder

1. Silica shells of dead diatoms are nearly indestructible and thus, get accumulated at the sea bed. Such huge rock-like deposits of hard shells of diatoms constitute diatomaceous earth, which is mined to obtain a whitish powder called diatomite or kieselguhr or diatomaceous earth. Diatomite is rough and gritty. Because of these features, it is used in filters in brewing industry, sugarcane refineries, in polishes for metals, tooth pastes, for making insulating bricks, in insulation of refrigerators, houses and for making the latter sound proof, in the manufacture of dynamite, water glass or sodium silicate and strong acids. This is added to paint to increase their night visibility.

2. They are very good indicators of water pollution. Common examples of diatoms are Triceratium, Melosira, Navicula, Cymbella.

   B. Dinoflagellates

Dinoflagellates are golden brown photosynthetic protists, belonging to class Dinophyceae (Pyrrophyta). They are mainly marine, though few are fresh water forms.

They may appear red, yellow, green, brown or blue depending upon the main pigment present in cell.

     General characters : 

Unicellular, motile, biflagellate, golden brown photosynthetic protists (some are non-motile, amoeboid, palmelloid or filamentous).

They are mostly marine, some are found in fresh water.

The body is enclosed by a rigid coat called theca or lorica consisting of 2 to many articulated or sculptured plates of cellulose and pectin , hence are also called armoured dinoflagellates.

Theca has generally two grooves i.e., longitudinal called sulcus and transverse called cingulum or annulus or girdle.

Flagella are heterokont (different). One is longitudinal and other is transverse. The flagella pass out through the pores in the lorica and lie in the grooves. The transverse flagellum lies in the circular groove and the longitudinal flagellum in the longitudinal groove. The longitudinal flagellum is narrow, smooth directed posteriorly and the transverse flagellum is ribbon like

Both are oriented at right angle to each other producing spinning movements. Therefore, these protists are also called 'whirling whips'.

Most of the species have brown, green or yellow chromatophores with chlorophyll a, c,        a-carotene, xanthophyll (e.g., Peridinin). Plastids are generally surrounded by 3-membrane envelope and contain 3-thylakoid lamellae. They are autotrophic or photosynthetic (Ceratium), a few are saprobic or parasitic.

Reserve food is carbohydrate and oils.

Nucleus is relatively larger in size, has condensed chromosomes even in interphase, chromosomes do not have histone. Nuclear envelope and nucleolus remain present even during cell division. This organisation is called Mesokaryon (Dodge, 1966).

A non-contractile vacuole called pusule is present near the flagellar base. It may have one or more vesicle and takes part in floatation and osmoregulation.

Some dinoflagellates possess trichocysts and cnidoblasts like those of coelentrates.

Reproduction is commonly asexual and occurs through cell division.

Isogamous and anisogamous sexual reproduction is reported from some dinoflagellates                                               

e.g., Ceratium.

Life cycle involves zygotic meiosis (Ceratium, Gymnodinium). Gametic meiosis occurs in Noctiluca.

Concept Builder

1. Some marine dinoflagellates show bioluminescence, i.e., emit light, e.g., Noctiluca, Pyrodinium, Pyrocystis. Due to phosphorescence the sea glows at night.

2. Some dinoflagellates like Gonyaulax catenella produce a toxin called saxitoxin into the sea water which is highly poisonous to vertebrates, e.g., fishes and other aquatic animals. Marine shell fish consume dinoflagellates and accumulate the poison which is not harmful to the shell fish (mussel) but upon being consumed causes severe illness in man called paralytic shell fish poisoning (PSP) and even prove fatal.

3. Some dinoflagellates proliferate in large number and cause red tide of the sea, e.g., Gonyaulax, Gymnodinium.

C. Euglenoid (Euglena-like)

   It is a group of chlorophyllous and non chlorophyllous flagellate protists. Largest genera being Euglena amongst them.

Euglenoids are unicellular, flagellate protists found in water or damp soil. Majority of them are fresh water organisms found in stagnant water.

Body is spindle shaped with blunt anterior end and pointed posterior end.

Cell wall is absent but a covering periplast or pellicle is present which is proteinaceous (elastic) in structure.

Locomotory organs are flagella.

The cell bears a single long tinsel type flagellum (stichonematic) arising at the anterior end. Actually, there are two flagella but one of these is reduced. The longer flagellum has two branches at the base each having its own basal granule. In the area of union of two flagella is present a photosensitive paraflagellar body.

Myonemes are oblique but parallely arranged strips in pellicle. Euglenoids perform creeping movement of contraction and expansion with the help of myonemes which is called metaboly or euglenoid movement.

The apical end of the cell bears an invagination with three distinct parts, i.e., mouth (cytostome), canal (gullet or cytopharynx) and reservoir. It helps in the ingestion of solid food particles.

Stigma or an eye spot is attached to the membrane of the reservoir at the level of paraflagellar body and along with it seems to be involved in perception of light stimulus. It contains photosensitive red-orange pigment called astaxanthin.

A contractile vacuole occurs in the anterior end of the cell just below the reservoir, meant for osmoregulation and excretion.

Single large nucleus lies near the centre of the protoplast.

Nutrition in Euglena viridis is photoautotrophic. However, it is capable of getting nourishment from dead and decaying organic matter in the substrate by secreting digestive enzymes (saprophytic nutrition) in the absence of light. This dual mode of nutrition is termed as mixotrophic. Holozoic nutrition is absent in Euglena. Some forms are holozoic (Paranema) or saprobic (Rhabdomonas).

Photosynthetic pigments are chlorophyll a, chlorophyll b, xanthophyll and b-carotene.

Reserve food material is paramylon, stored in cytoplasm in the form of paramylum granules. They are chemically b 1,3-glucans.

Under favourable conditions, they mainly reproduce by longitudinal binary fission. During unfavourable conditions, palmella stage and cysts are formed for perennation. Sexual reproduction is not known to occur in euglenoids, e.g., Euglena and Paranema.

 Concept Builder

1. Euglena is producer-decomposer protist.

2. It is studied as plant as well as animal and is called as plant animal.

3. Plant characters of Euglena:

  • Presence of chloroplast with photosynthetic pigments.
  • Holophytic nutrition

4. Animal characters of Euglena:

  • Absence of cell wall and presence of proteinaceous pellicle.
  • Presence of stigma and paraflagellar body.
  • Presence of contractile vacuole.
  • Presence of longitudinal binary fission.

  (2) Slime moulds or consumer-decomposer protists 

They were included in class myxomycetes of fungi in two-kingdom classification. They were called mycetozoa by DeBary as they are closely related to animals. Mycologists include them in gymnomycota. Because of their nature they are called protistan fungi. 

    General characteristics of the slime moulds are :

They are usually free-living, creeping over debris like fallen leaves and rotting logs of wood.

They have naked protoplast, not covered by any cell wall in vegetative stage.

They lack chlorophyll and have saprobic or phagotrophic mode of nutrition.

During life cycle they are amoeboid and non-cellulosic, but spores have cellulosic wall so that their vegetative phase resembles with animals while reproductive phase resembles with plants.

Amoeboid plasmodial stage resembles protozoa and spore forming nature is like fungi.

Spores are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.

Reproduction is both asexual and sexual. This group is represented by two separate types of organisms i.e. acellular and cellular.

    A.    Acellular or Plasmodial slime moulds 
        General characters: 

Slimy masses found on decaying leaves and lumber.

Somatic body is free living, multinucleate, naked, diploid mass called Plasmodium. Movement occurs by means of pseudopodia.

During unfavourable conditions, entire plasmodium forms many fructifications/fruting bodies (polycentric). The fruiting body is called sporocarp which contains a stalk having a sporangium at its tip. The wall of sporangium is called peridium.

Sporangium has an intricate network of cytoplasmic threads called capillitium.

DiploId protoplast forms haploid spores by meiosis.

Spore wall is double, outer wall is spiny and sculptured.

On germination, spores produce biflagellate swarm cells or non-motile myxamoebae which act as gametes.

Sexual reproduction is isogamous.

Diploid zygote directly forms the plasmodium which becomes multinucleate by repeated mitotic divisions of the diploid nucleus.

Chief mode of nutrition is saprotrophic.

Vegetative reproduction is by fission .

e.g., Physarum, Physarella, Fuligo, Dictydium, Lycogala

B.    Cellular slime moulds or communal slime moulds 
General characters : 

Wall less, uninucleate myxamoebae present. Complete absence of flagellated cells during life cycle.

Formation of pseudoplasmodium stage as a result of chemotactic movement of myxamoebae due to release of cAMP and acrasin.

Sporangia are naked.

Spores have cellulosic wall.

Sexual reproduction is anisogamous.

Common cellular slime mould, Dictyostelium, is a colonial form in which hundreds of uninucleate, haploid amoeboid cells are aggregated without any fusion to form a colony.

The colony gives the appearance of single multinucleate mass of protoplasm and thus, called pseudoplasmodium.

Under exhausted food supply and stimulation by cAMP and chemical acrasin, many cells come close together by chemotactic movement during the formation of pseudoplasmodium.

Pseudoplasmodium exhibits primitive form of multicellularity and division of labour.

So these are also called as communal slime moulds.

On these basis cellular slime moulds are regarded as advanced protists and primitive fungi.

During unfavourable conditions, the myxamoebae may form a cyst called microcyst for perennation and dispersal.

Under dry conditions, the pseudoplasmodium produces stalked sporocarp, which may be branched or unbranched, each branch bearing single sporangium terminally (monocentric).

Sporangium is wall less. Within the sporangium, amoeboid cells become rounded to secrete a spore wall around.

On the approach of favourable conditions, spores are liberated.

Each spore germinates by rupturing cellulosic wall to form myxamoeba and the myxamoebae may live independently, multiply by repeated mitotic divisions or get aggregated to form pseudoplasmodium.

Sexual reproduction is anisogamous type. During sexual reproduction, a number of myxamoebae form a clump.

One of the myxamoeba becomes larger and engulfs the surrounding smaller myxamoebae.

The plasmogamy occurs and the fused protoplast secretes a thick wall around to form macrocyst.

In the macrocyst, karyogamy occurs and it thus, becomes zygote.

It is followed by meiosis and several mitotic divisions to form a large number of haploid myxamoebae, which are released by rupture of macrocyst wall.

  • e.g., Dictyostelium, Polysphondylium.