Protists: Definition, Types, and Examples

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Protists Definition

Protists are the eukaryotic organisms having a well-defined nucleus in their cell. They are classified as a separate group of organisms in the five-kingdom classification system. Most of the protists are unicellular organisms; however few of them are multicellular.

What are Protists?

The organisms that belong to the kingdom Protista are called protists that include protozoa, algae, and slime molds. Protists consist of a highly organized cell nucleus and cellular organelles. Locomotory organs such as cilia and flagella are also present in some organisms. They are generally found in water or damp terrestrial environments or some organisms are also found as parasites in higher plants and animals. According to evolution theory, protists are considered the common ancestor of plants, animals, and fungi. Protists are assumed to be the predecessor of all higher organisms including plants, animals, and fungi. Examples of protest include Protozoa, algae, and slime molds. All of them have microbial structures and show very low similarity among themselves.

History of Classification

The earliest classification classified all the organisms into three kingdoms- animal, plant, and mineral. In the 1860s, John Hogg included Protoctista with unicellular organisms. They are defined as a primitive form of higher organisms. Later, Ernst Haeckel replaced the term “protoctists” with “protist”. He also classifies the organisms into the “Protista” and also included anucleated microorganisms.

Later, based on the 3-scheme classification system Whittaker proposed 5 kingdom classifications. Whittaker classified all organisms into four kingdoms:

(1) Fungi

(2) Plantae

(3) Animalia

(4) Protista.

Later he separated all prokaryotes from kingdom Protista and place them into a new kingdom “monera”.

Characteristics of Protists

The characteristic features of Protists are as follows:

• They are microscopic, eukaryotic organisms.

• They consist of the mitochondrion.

• Some of the protists are parasitic such as Trypanosoma protozoa. They are usually found in an aquatic, moist environment.

• Most of the protists are unicellular organisms however some of them are multicellular such as kelps that can grow up to 100 ft. in height.

• They can be autotrophic or heterotrophic, some organisms also live in symbiotic association.

• They consist of locomotory organs such as cilia and flagella or sometimes pseudopodium.

• They reproduce mainly by asexual and vegetative methods however they also exhibit sexual reproduction.

Types of Protists

They are classified into three types:

1. Animal-like protists- motile and heterotrophic

2. Plant-like protists- autotrophic protists that carry out photosynthesis

3. Fungi- like protists- reproduce via spores and have a cell wall. They are heterotrophic.

The kingdom is highly diverse thus the classification and taxonomy of the protists is ever-changing. They are further classified into various groups based upon the shape, size, nuclear structure, and cytoplasmic organelles.

Autotrophs

Autotrophic protists can synthesize their food by photosynthesis. They do not consist of locomotory organs thus are non-motile. They consist of chlorophyll and other plant-like pigments to carry out photosynthesis. The pigments can vary among different autotrophic protists. For example- green algae have chlorophyll, brown algae consist of fucoxanthin, phycoerythrin found in red algae. Autotrophic protists carry out 40% of the world,s total photosynthesis.

Heterotrophs

Some heterotrophic protists are motile and the movement takes place with the help of cilia, flagella. They do not perform photosynthesis thus cannot synthesize their food. They depend upon other sources for their energy needs.

Mixotrophs

The mixotrophs fall between heterotrophs and autotrophs. Different sources of carbon are utilized by mixotrophs thus they are considered as a combination of phototroph and phagotrophic. Based on dominancy and roll of phototrophy and phagotrophy, Harriet Jones classified mixotrophs into four groups:

• Heterotrophy –phagotrophy is the standard mode of nutrition.

• Phototrophy-phototrophy is the standard mode of nutrition.

• Phototrophy is preferred for growth and ingestion, however, phagotrophy is employed in limited light.

• A common mode of nutrition is phototrophy, however, phagotrophy is employed during the prolonged dark period.

Later, Diane K. Stoeker classified mixotrophs into three types:

Type 1- Known as ideal mixotrophs that utilize sunlight and prey equally.

Type 2- Phagotrophy is used as supplementary for phototrophy.

Type3- They can change according to sunlight availability. They are primarily heterotrophic organisms.

Protists are divided into two major groups by Aditee Mitra et al. named constitutive mixotrophs and Non- constitutive mixotrophs.

Based on motility, protists can be divided as follows-

• Diplomonads: the organisms that contain two nuclei and mitosomes are called diplomonads. They also contain flagella.

• Parabasalids: they contain a parabasal body with hydrogenosomes.

• Euglenozoans: contain flagella with a flagellar crystalline rod.

• Alveolates: The alveoli are present in the cytoplasmic membrane of these protists. They are further divided into ciliates, dinoflagellates, and Apicomplexans.

• Stramenopiles: they are the members of oomycetes that reside in water. Examples include diatoms and brown algae.

• Cercozoans: They possess pseudopodia that helps in the movement of these organisms. E.g. Amoeba

• Amoebozoa: They also possess pseudopodia but they differ from cercozoans in having lobe-shaped pseudopodia.

Reproduction and Life Cycle of Protists

Reproduction can occur by both sexual and asexual methods, however, the major mode of reproduction is an asexual method.

i. Asexual Mode of Reproduction in Protists

The method involves only one parent in which the parent cell produces genetically similar offspring by dividing itself. The offspring have the same genetic composition as that of the parent cell thus it is called a clone. Asexual reproduction can occur by various methods, For example-

• Binary Fission– The parent cell undergoes mitosis and divides itself into two daughter cells. Example- Amoeba

• Multiple Fission– Several daughter cells are produced by a single mother cell. Example- Plasmodium, Amoeba

• Plasmotomy– Usually, multinucleate protists reproduce by this method. Here, two or more daughter cells are produced by dividing the parent cell. The nucleus is not divided in this process. Example- Opalina

• Spore formation– In unfavorable conditions, some protists produce spores that are exposed and germinate in favorable conditions. Example- Slime molds

• Budding– A small outgrowth or bud formation occurs on the body of the parent cell. The bud pinches off and forms a new organism. Example- Arcella 

ii. Sexual Mode of Reproduction in Protists

There are two basic processes involved in sexual reproduction-

• Meiosis– It is also called reduction division that is an essential part of sexual reproduction. The step reduces the chromosome from 2n to n (haploid), which is necessary to keep the constant number of chromosomes in the progeny.

• Fertilization or Fusion– It involves the fusion of male and female gametes that results in the formation of a diploid zygote.

Protists include two methods for sexual reproduction-

• Syngamy: Here, the complete fusion of two gametes results in the formation of a diploid zygote. Different types of syngamy include isogamy (fusion of two similar gametes), anisogamy (fusion of two dissimilar gametes), and oogamy (fusion of a non-motile, large gamete with motile small gamete).

• Conjugation: The mode of sexual reproduction includes the temporary union of two individuals to exchange their genetic material. Eventually, both the parents undergo binary fission and produce offspring. Example- Paramecium

Life Cycle of Slime Molds

i. Plasmodial Type– The plasmodial type of slime molds is formed by multinucleated cells. They are heterotrophic organisms that engulf food particles and bacteria. During stress, they produce sporangia or fruiting bodies over a stalk. The sporangia possess haploid spores that germinate and produce progeny in favorable conditions.

ii. Cellular Types– A plenty of nutrients are present in these amoeboid cells. However, depletion of food sources results in the formation of slugs in cellular slime molds. Asexual fruiting bodies are formed on the top of the slug that bears haploid spores. The spores germinate and produce new progeny after getting the optimum temperature and moist environment.

Habitats of Various Protists

The kingdom includes over 100,000 living species. Most of the protists are aquatic organisms found in freshwater, marine, damp soil, and some are found in the snow. A most common research organism, Paramecium is also an aquatic protist. Some protists also reside as the parasite is host organisms such as Amoeba that is a human parasite. Some species are scavengers and reside on the dead organic material. The slime molds live on bacteria and fungi.

Evolutionary History of Protists

They are believed to be the first eukaryotic cells due to the presence of a nucleus in simple protists such as amoeba. It is assumed based on the endosymbiotic theory that describes the evolution of the eukaryotic cell. The theory concludes that the larger prokaryotic cell engulfs a small prokaryote and both establish a symbiotic association between them. The endosymbiont gets protection and nutrients from the larger cells while the larger cell receives energy from the endosymbiont. Over time the cell engulfed by a larger cell evolved into a cellular organelle that is the present-day mitochondria. Thus, kingdom Protista consists of a diverse group of organisms, depending upon the endosymbiotic cell.

Classification of Protists

Protists are divided into major three types-

1. Animals like protists or Protozoa

2. Plants like protists or algae

3. Fungi like protists or molds

Protozoa: They are single-celled, motile, ad heterotrophic organisms therefore they are called animal-like protists. Based on their motility they are further classified into different types-

Amoeboid: consist peudopodium

Ciliate: consists of cilia

Flagellate: contain flagella S

porozoan: Adults are immobile

Algae: They can be single-celled or multicellular organisms. They contain photosynthetic pigments and perform photosynthesis therefore they are called plant-like protists. The higher plants are believed to be evolved from algae. Algae are also classified into different groups according to the pigments present in them. For example- Red algae, Green algae, brown algae, dinoflagellates, etc

Molds: They are also heterotrophic organisms that feed upon the dead or decaying material. The organisms also reproduce via spores thus they are called fungi like protists. The cell wall of molds in made up of cellulose whereas fungi contain a cell wall made up of chitin. They are also classified into two classes: Slime molds- fungus-like organisms that feed upon the dead and decaying material. In the case of the unavailability of dead material, they feed upon their secretion and form a slimy mass.

Water Molds: The water molds are mostly aquatic organisms found in surface water and moist soil. They also act as plant pathogens and cause disease in grapes, lettuce, corn, and potatoes. In 2005, all the protists are categorized into six major categories by a group of 28 scientists.

1. Amoebozoa: include slime molds, Acanthamoeba, Dictyostelium

2. Opisthokonta: it includes fungi and the metazoan.

3. Rhizaria: include amoeba-like organisms.

4. Archaeoplastida: include autotrophic organisms. Example- red algae, green algae, etc.

5. Chromalveolata: It includes ciliates, diatoms, and photosynthetic protists such as brown algae.

6. Excavata: includes parasites such as trypanosomes, Euglena, etc.

Ecological importance of Protists

• Autotrophic protists form the foundation of the food chain.

• Controls bacterial and microbial populations by feeding upon them.

• Almost 40% of total photosynthesis is carried out by autotrophic protists, thus helps to reduce carbon dioxide.

• Molds feed upon decaying material and act as a primary decomposer.

• Mixotrophs are an important component of the aquatic food web.

• Phytoplankton is the primary producer in the marine food chain. • Some of the species are pathogens that cause infections in plants as well as animals.

Economic Importance of Protists

• The photosynthetic protists have the potential to produce biofuel.

• The red alga named Porphyra and many other species has medicinal properties. It is used for the treatment of hypertension, arthritis, ulcers, and joint pains.

• Seaweeds are edible and also used as fertilizers.

• Diatomite is produced in the cell wall of diatoms that are used in manufacturing cement, stucco, plaster, grouting, and pesticides.

• Algal species Gelidium, Gracilaria are used to obtain agar-agar.

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