All single-celled eukaryotes come under this kingdom.

Currently, we include Chrysophytes, Dinoflagellates, Euglenoids, Slime molds, and Protozoans under Protista.

Characteristics of members of Kingdom Protista:-

• They are primarily aquatic****.

• Being eukaryotes, the protistan cell body contains a well-defined nucleus and other membrane-bound organelles.

  • Some have motile organelles like cilia and flagella.

• Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

  • This kingdom forms a link with the others dealing with plants, animals, and fungi.

Chrysophytes:

Characteristics:

• They are found in freshwater as well as in marine environments.

  • They are microscopic and float passively in water currents (plankton)

• Most of them are photosynthetic.

  • Examples: Diatoms and Golden Algae (Desmids).

Diatoms:

• The cell walls form two thin overlapping shells, which fit together as in a soap box.

  • The walls are embedded with silica and thus the walls are indestructible.

    • They leave behind large amounts of cell wall deposits in their habitat and this accumulation over billions of years is referred to as diatomaceous earth.

      • This soil is used in polishing, and filtration of oils and syrups since it is gritty.

• They are the chief producers of the ocean.

Dinoflagellates:

• These organisms are mostly marine and photosynthetic and appear yellow, green, brown, blue, or red depending on the main pigments present in their cells.

• The cell wall has stiff cellulose plates on the outer surface.

• Most of them have two flagella - one lies longitudinally and the other transversely in a furrow between the wall plates.

  • Very often, red dinoflagellates undergo such rapid multiplication that they make the sea appear red (red tides).

    • Toxins released in such large numbers may even kill other marine animals such as fishes.

      • Example: Gonyaulax (red dinoflagellate).

Euglenoids:

• The majority of them are freshwater organisms found in stagnant water.

  • Instead of a cell wall, they have a protein-rich layer called a pellicle which makes their body flexible.

  • They have two flagella****, a short and a long one.

    • Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.

  • The pigments of euglenoids are identical to those present in higher plants.

    • Example: Euglena.

Slime Moulds:

• They are saprophytic protists.

• The body moves along decaying twigs and leaves engulfing organic material.

  • Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet.

  • During unfavorable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.

    • These spores possess true walls and are dispersed by air currents.

  • 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 protozoans:

Amoeboid Protozoans:

• These organisms live in freshwater, seawater, or moist soil.

  • They move and capture their prey by putting out pseudopodia(false feet).

    • Marine forms have silica shells on their surface.

• Some of them are parasites.

  • Examples: Amoeba, Entamoeba.

Flagellated protozoans:

• The members of this group are either free-living or parasitic.

  • They have flagella.

  • The parasitic forms cause diseases such as sleeping sickness.

    • Examples: Trypanosoma.

Ciliated protozoans:

• These are aquatic, actively moving organisms because of the presence of thousands of cilia.

  • They have a cavity (gullet) that opens to the outside of the cell surface.

• The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet.

  • Examples: Paramecium.

Sporozoans:

• It includes diverse organisms that have an infectious spore-like stage in their life cycle.

  • The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease that has a staggering effect on the human population.