Introduction of Protista

In biology, we come to know that even the tiniest and the most insignificant organisms have substantial roles to play when it comes to sustaining living organisms. Protista is one such group of organisms that form a core aspect of cell organization in spite of being minuscule in structure and capacity. The kingdom Protista was proposed by Ernst Haeckel in 1866, who was a zoologist of German origin.

One of the most unique characteristics of protists is that their cell distribution is an organized one and they also consist of nucleus and organelles. The Protista kingdom can be deemed fit to be called "aquatic organisms".Since protists are members of the eukaryotic family, they have organelles that are membrane-bound and also a well-defined nucleus.

Background Information on Protista

The classification of five kingdoms was given by Robert Whittaker in 1969, and the Protista kingdom was a part of those five kingdoms. During that time, protists were regarded as unicellular eukaryotic organisms. But with time, the kingdom Protista expanded and even multicellular organisms found their place in that kingdom. 

A special characteristic of the organisms grouped under this kingdom is that all of those organisms have a simplified cellular structure devoid of extraneous tissues and organs. The number of protists that have been discovered ranges from approximately more than 200,000. In fact, a huge number of protists are yet to be discovered beside the already-discovered humongous number of protists. 

Since protists are aquatic organisms, they form their habitat mostly in watery areas. They form the foundational structure of the ecosystem by being the dominant producers. In addition to that, protists come in multifarious sizes ranging from minuscule organisms to humongous seaweeds that are capable of growing approximately 200 feet in height.

Characteristics of Protista 

Since the kingdom of Protista is a diversified one, it is difficult to coherently characterize them in a simplified manner. The organisms that form a part of this kingdom are diverse and multifarious in terms of bodily structure, reproductive abilities, and nutrition.

However, the characteristics of Protista can be summarised in the following manner:

  • While some organisms may be unicellular, some others may be colonial or multicellular.

  • Since protists are a part of the eukaryotic kingdoms, their organisational structure can be varied. For instance, protists have multiple organelles that also include a nucleus, mitochondria, plastids, food vacuoles, and flagella.

  • One of the defining characteristics of protista is that they are capable of both sexual and asexual reproduction.

  • Protists are free-living organisms, but they are also capable of living interdependently with another organism.

  • The relationship between a protist and another organism is either a collaborative one or a parasitic one. In a collaborative relationship, both of them benefit from each other, whereas in a parasitic relationship, the protist is deriving the benefit by feeding off the host.

  • Usually, there are two different sides to protists - one side is beneficial to humans, while the other side harms them in the form of dangerous, life-threatening diseases. Protists are the bedrock of the food chain where they form a vital part of the plankton aquatic system.

  • Protists have the dual characteristics of both mobility and immobility. While some protists are ambulatory as they use cilia, flagella, and pseudopodia as fake feet to commute, some of them are stationary as well.

  • Another characteristic of protists is that they can be both autotrophic, that is, preparing their own food, and heterotrophic, that is, acquiring outside sources of nutrition. For example, Euglena performs mixotrophic nutrition as both holotropic (where the organism captures and ingests the food) and saprotrophic (where the organism releases enzymes that convert organic matter into simpler products) modes of nutrition can be observed in them. 

Classification of Protista

The categorization of protists is done on the basis of their nutritional habits, mobility and modes of reproduction. They can be broadly classified into three categories: 

  1. Protozoan protists

  2. Photosynthetic protists or Protist algae

  3. Slime moulds

Protozoan Protists

The origin of the word 'protozoa' lies in the Latin word for first animals. Protozoa come under the group of parasitic protists that feed on other organisms that are mostly dead, for nutrients. Moreover, they are heterotrophic unicellular organisms. Protozoa can be further classified on the basis of their motility. For example, protozoa that use cilia to move are called ciliates and those that use flagella are called flagellates. Amoebas are those protozoa that use pseudopodia to move and protozoa such as sporozoa are not capable of moving at all. Sporozoa are mostly parasitic as they make dead organisms their host. They are likely to inhabit areas that have continuous food supply as they are unable to collect food by themselves. 

The classification of protozoan protists is given below:

  • Flagellated Protozoans - Trypanosoma, Giardia, Leishmania, Trichomonas

  • Ciliated Protozoans - Paramecium, Vorticella, Opalina, Balantidium

  • Amoeboid Protozoans - Amoeba, Pelomyxa, Entamoeba

  • Sporozoans - Plasmodium, Monocystis, Eimeria


Photosynthetic Protists or Protist Algae

They are the protists that form the foundation of the food chain. Most organisms are dependent on organisms like algae for food and oxygen either directly or indirectly. They mainly include dinoflagellates, chrysophytes and euglenoids. A special characteristic of algae is that it is a major photosynthesizer of organisms, which makes it the dominant supplier of oxygen. Even though algae are grouped under the category of plant-like protists, they lack those parts that would define them as a plant (e.g. roots, stems, and leaves). Moreover, they are incapable of preventing water loss, so they have to inhabit areas that have an abundance of water. However, the one dominant characteristic that portrays them as plant-like is the presence of chlorophyll. The presence of chlorophyll in them makes them distinguishable and classifiable on the basis of different colours. 

Dinoflagellates are a crucial component of phytoplankton. Some of the examples of dinoflagellates are Glenodinium, Peridinium, Gymnodinium, Ceratium, etc.

Chrysophytes include diatoms and desmids. Some of the examples are Navicula, Cymbella, etc.

Euglenoids can be defined as Euglena-like organisms and they possess pellicle instead of cell walls. Some examples are Euglena, Phocus, etc.

Slime Moulds 

They resemble fungal organisms and come in dazzling colours of multiple variants. Moreover, they are likely to inhabit places that are murky and damp. Such organisms derive their nutrients from decaying and decomposing organic material. Some examples of slime moulds are Fuligo, Dictydium, etc.

In Summary

  • Protists come under the category of eukaryotes that are single-celled.

  • Protists form the foundational structure of the ecosystem by being the dominant producers.

  • The characteristics of protista are dual in nature - capable of sexual and asexual reproduction, cooperative and parasitic relationship, and they can be both mobile and immobile.

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FAQs on Protista

1. What are Protists?

Protists are organisms that belong to the Protista kingdom that are eukaryotic and generally unicellular, but sometimes multicellular and include most algae, protozoans, and some fungus-like organisms. They do not fit into any groups and are labelled as protists. Despite that, they have to reproduce to survive. The reproduction of protists are done either through asexual reproduction or sexual reproduction. They are found in wet and moist surfaces and thrive on nutrients from other organisms. They are sporadic in nature and are different from other organisms.  

2. Name 4 diseases caused by flagellated protozoans and their causative agents.

The diseases caused by ciliated protozoans are-

  1. Sleeping Sickness - Trypanosoma gambiense.

  2. Chagas Disease - Trypanosoma cruzi.

  3. Kala-Azar - Leishmania donovani.

  4. GiarDiasis - Giardia intestinalis.

3. State two features of dinoflagellates.

The features are mentioned below-

  • Nutrition is generally photosynthetic.

  • Bioluminescence is observed in some of the marine dinoflagellates such as Noctiluca.

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