This kingdom includes all single-celled eukaryotes and serves as an evolutionary link to the plant, animal, and fungi kingdoms.
General Characteristics:
Primarily aquatic.
Eukaryotic cell structure with a well-defined nucleus and other membrane-bound organelles.
Some possess cilia or flagella for movement.
Reproduce both asexually and sexually (via cell fusion and zygote formation).
Major Groups of Protista
Chrysophytes: Includes diatoms and desmids (golden algae). They are microscopic plankton found in freshwater and marine environments.
Diatoms: Are major photosynthetic producers (“chief producers”) in the oceans. Their cell walls are embedded with silica, making them indestructible. These walls fit together like a soapbox and accumulate over billions of years to form diatomaceous earth, which is used in polishing and filtration.
Dinoflagellates: Mostly marine and photosynthetic organisms. Their cell walls have stiff cellulose plates. They typically have two flagella. Rapid multiplication of some species, like Gonyaulax, can cause red tides and release toxins harmful to marine life.
Euglenoids: Mostly freshwater organisms found in stagnant water (e.g., Euglena).
Instead of a cell wall, they have a flexible, protein-rich layer called a pellicle.
They are photosynthetic in sunlight but behave as heterotrophic predators when deprived of light. Their pigments are identical to those of higher plants.
Slime Moulds: Saprophytic protists. Their body moves along decaying organic material.
Under suitable conditions, they form an aggregation called a plasmodium, which can grow to span several feet.
In unfavorable conditions, the plasmodium forms fruiting bodies that produce extremely resistant spores.
Protozoans: Heterotrophic organisms that live as predators or parasites, considered primitive relatives of animals.
Amoeboid protozoans: Move and capture prey using pseudopodia (false feet), like Amoeba. Some are parasitic (e.g., Entamoeba).
Flagellated protozoans: Have flagella for movement. Some are parasitic and cause diseases like sleeping sickness (e.g., Trypanosoma).
Ciliated protozoans: Are aquatic and use thousands of cilia for movement. They have a cavity (gullet) to ingest food (e.g., Paramoecium).
Sporozoans: Have an infectious, spore-like stage in their life cycle. The most notorious is Plasmodium, the parasite that causes malaria.