Protists are simple eukaryotic organisms that are neither plants nor animals or fungi. Protists are unicellular in nature but can also be found as a colony of cells. Most protists live in water, damp terrestrial environments or even as parasites.
The term ‘Protista’ is derived from the Greek word “protistos”, meaning “the very first“. These organisms are usually unicellular and the cell of these organisms contains a nucleus which is bound to the organelles. Some of them even possess structures that aid locomotion like flagella or cilia.
Scientists speculate that protists form a link between plants, animals and fungi as these three kingdoms diverged from a common protist-like ancestor, billions of years ago. Though this “protists-like” ancestor is a hypothetical organism, we can trace some genes found in modern animals and plants to these ancient organisms.
Therefore, these organisms are traditionally considered the first eukaryotic forms of life and a predecessor to plants, animals and fungi.
Pritish Kumar gives an illustration about Protists characteristics and classifications below:
Characteristics of Kingdom Protista
The primary feature of all protists is that they are eukaryotic organisms. This means that they have a membrane-enclosed nucleus. Other characteristic features of Kingdom Protista are as follows:
These are usually aquatic, present in the soil or in areas with moisture.
Most protist species are unicellular organisms, however, there are a few multicellular protists such as kelp. Some species of kelp grow so large that they exceed over 100 feet in height. (Giant Kelp).
Just like any other eukaryotes, the cells of these species have a nucleus and membrane-bound organelles.
They may be autotrophic or heterotrophic in nature. An autotrophic organism can create their own food and survive. A heterotrophic organism, on the other hand, has to derive nutrition from other organisms such as plants or animals to survive.
Symbiosis is observed in the members of this class. For instance, kelp (seaweed) is a multicellular protist that provides otters, protection from predators amidst its thick kelp. In turn, the otters eat sea urchins that tend to feed on kelp.
Parasitism is also observed in protists. Species such as Trypanosoma protozoa can cause sleeping sickness in humans.
Protists exhibit locomotion through cilia and flagella. A few organisms belonging to kingdom Protista have pseudopodia that help them to move.
Protista reproduces by asexual means. The sexual method of reproduction is extremely rare and occurs only during times of stress.
Classification of Protista
Kingdom Protista is classified into the following:
Protozoans are unicellular organisms. Historically, protozoans were called “animal” protists as they are heterotrophic and showed animal-like behaviours.
There are also parasitic protozoans which live in the cells of larger organisms. Most of the members do not have a predefined shape. For instance, an amoeba can change its shape indefinitely but a paramecium has a definite slipper-like shape. The most well-known examples of protozoans are amoeba, paramecium, euglena. Unlike other members of this group, euglena is a free-living protozoan that has chlorophyll, which means it can make its own food.
The protozoans can be divided into four major groups:
- Amoeboid protozoans – Mostly found in water bodies, either fresh or saline. They have pseudopodia (false feet) which help to change their shape and in capturing and engulfing food. E.g. Amoeba
- Flagellated protozoans – As the name suggests, the members of this group have flagella. They can be free-living as well as parasitic. E.g. Euglena
- Ciliated protozoans – They have cilia all over their body which help in locomotion as well as nutrition. They are always aquatic. E.g. Paramecium
- Sporozoans – These organisms are so-called because their life cycle has a spore-like stage. For example, the malarial parasite, Plasmodium.
Slime moulds are saprophytic organisms (they feed on the dead and decaying matter). These are tiny organisms that have many nuclei.
Usually, Slime moulds are characterized by the presence of aggregates called plasmodium and are even visible to the naked eye.
Chrysophytes, Dinoflagellates and Euglenoids
These form another category under kingdom Protista. These are generally single-celled or multicellular organisms. These are photosynthetic, found mostly in freshwater sources or marine lakes. They are characterized by a stiff cell wall.
Example of chrysophytes include diatoms and golden algae. They are characterized by the presence of a hard siliceous cell wall. Diatomaceous earth is formed due to the accumulation of cell wall deposits. They are photosynthetic organisms.
Dinoflagellates are photosynthetic and found in various different colors, according to the pigment present in them. They show bioluminescence and known to cause red tide.
Euglenoids are the link between plants and animals. They lack a cell wall but perform photosynthesis. In the absence of sunlight, they act as a heterotroph and feed on small organisms. The outer body covering is a protein-rich layer known as a pellicle. E.g. Euglena, Trachelomonas, etc.
Economic Importance of Protists
- Protists serve as the foundation of the food chain.
- Protists are symbionts – having a close relationship between two species in which, one is benefited.
- Some protists also produce oxygen and may be used to produce biofuel.
- Protists are the primary sources of food for many animals.
- In some rare cases, Protists are harvested by humans for food and other industrial applications.
- Phytoplankton is one of the sole food sources for whales
- Seaweed is an alga, which is considered a plant-like protist.
- Zooplankton is fed on by various sea creatures including shrimp and larval crabs.