Protista

The Kingdom Protista consists of simple eukaryotic organisms. The members of this kingdom are less complex in structure than the other eukaryotic organisms. Although Protists are unicellular, they can be found in colonies of cells. Even though Protists do not share many similarities, they are classified under the same kingdom as they do not fit under the other kingdoms of the Five Kingdom Classification System. 

Some members of this kingdom are capable of photosynthesis. At the same time, the others may live in mutualistic relationships with other species of Kingdom Protista. Few Protists are also responsible for causing diseases in plants and animals. Generally, all Protists live in aquatic environments. Their habitats include- freshwater, marine environments, damp soil, and snow. 

Characteristic features of Protists

The single-celled eukaryotes are known to be Protists. But, numerous other characteristics put Protists under the Protista Kingdom. 

1. Usually, the species under this kingdom are aquatic and live in soil or other moist areas.
2. Usually, Protists are unicellular. But, the Giant kelp is an exception as it is multicellular. 
3. As Protists are eukaryotes, they have a nucleus and membrane-bound cell organelles. 
4. The mode of nutrition for the members of Kingdom Protista can be either autotrophic or heterotrophic. 
5. Some Protists show symbiosis. For example, the Kelps protect from predators the sea otters. In return, the sea otters eat sea urchins that feed on kelp.
6. These organisms also show parasitism. 
7. Locomotion in Protists is done using cilia and flagella. Few Protists also have pseudopodia for movement. 
8.  The members of Kingdom Protista usually use asexual means for reproduction. Protists rarely reproduce through sexual means of reproduction, mostly in times of stress. 

Classification of Protista Kingdom

The Kingdom Protista is further divided into the following groups:

• Chrysophytes

Diatoms and golden algae or desmids come under the Group Chrysophytes of Protista. Both freshwater and marine environments are known to be the habitats of the species of Chrysophytes. These passively float on water and are microscopic organisms. Chrysophytes are usually photosynthetic. The cell walls in diatoms form two thin overlapping shells, fitting together as in a soapbox. The cell walls of these organisms are embedded with silica; this makes the walls indestructible. These walls form diatomaceous earth by leaving large cell deposits that accumulate for billions of years in their habitats. 

• Euglenoids

This Group of the Kingdom Protista is considered the link between plants and animals. Mostly, euglenoids live in freshwater. Euglenoids have a layer rich in proteins called a pellicle in place of cell walls. This layer provides flexibility to the body of Euglenoids. These organisms possess two flagella. One of these flagella is short, and the other is long. Although Euglenoids are autotrophs, they become heterotrophs in the absence of sunlight and feed on smaller organisms. They have pigments similar to higher plants. 

• Dinoflagellates

This Group consists of marine and photosynthetic Protists. Based on the primary pigment present in their body, they can be green, brown, blue, red, or yellow in appearance. They have stiff cellulose plates on the outer surface of their cell walls. Most Dinoflagellates have one longitudinally located flagella, and the other is located transversely in a furrow between the wall plates. 

• Slime Moulds

The saprophytic Protists are classified under this Group. Under suitable conditions, Slime Moulds form plasmodium, an aggregation that grows and spreads over several feet. But, during unfavourable conditions, this Plasmodium develops fruiting bodies with spores containing true walls at their tips by differentiating. This allows them to survive for numerous years, even under adverse conditions. 

• Protozoans

The heterotrophic Protists come under this Group. Protozoans live as predators or parasites. Protozoans are further classified into:

1. Amoeboid Protozoans: This classification of Protozoans live in freshwater, seawater, or moist soil. As in Amoeba, these Protozoans have pseudopodia which they use to capture their prey. Some of these are also known to be parasitic. 
2.  Flagellated Protozoans: The members of flagellated Protozoans are either free-living or parasitic. As the name suggests, they have flagella. Trypanosoma that causes sleeping sickness in humans is one of the Protista examples of parasitic flagellated Protozoans. 
3. Ciliated Protozoans: These are aquatic organisms that move freely due to the presence of cilia. The gullet is the cavity in these organisms that open outside the cell surface. The cilia move coordinately to move the water-laden food into the gullet. 
4. Sporozoans: These are the organisms that have an infectious spore-like stage in their life cycle. The malarial parasite Plasmodium is one of the Protista examples of Sporozoans. 

Conclusion

The Kingdom Protista is an extensive collection of single-celled eukaryotes divided into several groups. The members of this kingdom also have ecological significance. For example, they serve as the foundation of the food chain. The importance of this kingdom makes it a diverse study topic, and its groups make this easily possible. Protists have fewer similarities among them. Still, their unique characteristic features make its members stay under this kingdom.