Protist in Microbiology Examples and Characteristics of Protista Kingdom

What is a Protist?

A protist is a member of the kingdom Protista which includes all eukaryotic organisms that are not plants, animals, or fungi. In microbiology Protists are simple unicellular eukaryotic organisms though some can be found as a colony of cells or in multicellular form.

The majority of protists inhabit the aquatic and damp terrestrial habitats and some exist in nature as parasites. Any environment that contains water is usually occupied by free-living protists. They can be seen in oceans, lakes, ponds, rivers, damp soil, snow, tree barks, etc.

The majority of protists are photosynthetic e.g algae and are crucial primary producers, especially in the aquatic ecosystem. Also, one of the major protista characteristics is that they can be autotrophic or heterotropic. They form a large portion of biomass in aquatic and terrestrial habitats. Other protists like the species of apicomplexan Plasmodium and kinetoplastid Trypanosoma brucei are not autotrophic or heterotrophic but parasitic. The former causes malaria and the latter causes sleeping sickness.

As eukaryotic organisms, they possess cells with a distinct and membrane-bound nucleus. Some of them have cells that have structures like cilia or flagella that aid in locomotion. Certain protist cells have cell walls and some lack cell walls. These organisms are neither plants, animals, or fungi though they have attributes that are similar to such organisms. This is why they are classified into 3 groups which include: Plant-like protists, animal-like protists, and fungi-like protists.

Protista is an example of a eukaryotic organism that combines various characteristics that are found in different organisms and yet it is neither an animal, plant, or fungus. Scientists are of the hypothesis that protista form a link between animals, plants, and fungi and that billions of years ago, the three kingdoms- Animalia, Plantae, and fungi may have diverged from a common protist-like ancestor. Some genes found in today’s plants and animals can be traced to protists even though this protist-like ancestor is a hypothetical organism. The protist is traditionally considered a predecessor to animals, fungi, and plants and as a result, appears to be the first eukaryotic form of life.

Habitat

The majority of protists inhabit the aquatic and damp terrestrial habitats and some exist in nature as parasites. Any environment that contains water is usually occupied by free-living protists. They can be seen in oceans, lakes, ponds, rivers, damp soil, snow, tree barks, etc. A few species of protists live on dead organisms and play a role in the decay.

Moreso, many protists live in a symbiotic relationship with other organisms. For instance, protists like the single-celled dinoflagellates called zooxanthellae have a symbiotic relationship with coral reefs. They live in the tissues of the coral polyps and provide the corals with essential nutrients such as amino acids, glucose, and glycerol and help in waste removal. The corals, in turn, provide the zooxanthellae with compounds needed for photosynthesis and a protected environment to live.

Another example is the parabasalids (a group of flagellated protists) that has a mutualistic relationship with termites where they live in the intestine of termites and sometimes cockroach. Also, another type of symbiosis known as parasitism is exhibited by a fungi-like protist, Phytophthora ramorum that inhabits oak trees and parasitizes them causing SOD (sudden oak death) disease.

Motility

Many protists are motile. However, protists of different types have evolved various means of movement. Some of them such as the euglena have one or more flagella that they whip or rotate in order to move. Whereas some other protists like the amoeba send out cytoplasmic extensions called pseudopodia from any part of their cell and anchor the pseudopodia to a surface or substrate, and then towards the anchor point, they pull the rest of the cell.

Also, some protists such as the paramecia are covered in tufts or rows of tiny cilia that they beat in coordination in order to swim. Moreso, some of these organisms have the ability to move towards the light (phototaxis) by coupling their locomotion mechanism with a light-sensing organ. Hence, Protista usually accomplishes motility by cilia, flagella, or amoeboid mechanisms.

Nutrition

Protista carries out many forms of nutrition. They could be aerobic or anaerobic, and autotrophic or heterotropic. Photoautotrophic protists such as algae are photosynthetic and are characterized by the possession of chloroplasts. Autotrophic protists are plant-like because they make use of photosynthesis to produce their own food. Examples of such protists include diatoms, euglena, dinoflagellates, brown algae, red algae, and green algae. Plants, however, are said to have evolved from green algae.

The heterotrophic protists, on the other hand, such as amoeba feed on organic material and other organisms. These heterotrophic protists ingest particles through a process called phagocytosis. This process involves the engulfing of food particles by the cell membrane which brings the food inward and pinches off a food vacuole (an intracellular membranous sac or vesicle).

The vesicle fuses with a lysosome and the lysosome contains proteins that break down the food particles into smaller molecules. These molecules then diffuse into the cytoplasm and are used in the metabolism of the cell. The undigested food (undigested waste material) is then expelled from the cell via a process of exocytosis.

Also, some heterotrophs obtain their nutrients from dead organisms or their organic wastes. Generally, heterotrophic protists feed on bacteria, decaying organic matter, and other protists. Examples of heterotrophic protists are slime molds, water molds, amoebas, sporozoans, and paramecia.

Protist cells

The protist cell is among the most complex of all cells. This is because most protists are unicellular and microscopic whereas some of them exist in true multicellular forms. A few protists exist as colonies that act as a group of free-living cells. Also, in another way they act as multicellular organisms.

However, there are other protists that are composed of enormous, multinucleate single cells that resemble amorphous blobs of slime or like ferns in other cases. Generally, many protist cells are multinucleated. The nuclei in some species are of different sizes and carry put distinct roles in the function of the protist cell.

The size of a single protist cell range from less than a micrometer to the 3-meter lengths of the cells of the seaweed Caulerpa that are multinucleated. The protist cell may be enveloped by a cell membrane as in animal cells or by cell walls as in plant cells. Other protist cells are coiled up with pellicles of interlocking protein strips or enveloped in glassy silica-based shells. The pellicle serves as a flexible coat of armor that prevents the protist from being pierced or torn without its range of motion being compromised.

Many protists are motile. However, protists of different types have evolved various means of movement. Some of them have one or more flagella that whip or rotate. Whereas some other protists from any part of the cell extend lobe-like pseudopodia and anchor the pseudopodium to a substrate. Then, towards the anchor point, they pull the rest of the cell. Also, some protists are covered in tufts or rows of tiny cilia. In order to swim, they beat these tiny cilia in coordination. Moreso, some of these organisms have the ability to move towards light by coupling their locomotion mechanism with a light-sensing organ. Hence, they are usually single-celled and move by cilia, flagella, or amoeboid mechanisms.

Do protists have a nucleus?

Protists as eukaryotic organisms have a nucleus. The cell of protists is highly organized with a nucleus and organelles.

Do protists have cell walls?

Protists usually have no cell wall, though some of them may have.  The protists that usually have cell walls are the plant-like protists (algae) and the fungi-like protists (molds). Animal-like protists (protozoa) do not possess cell walls. The cell wall of fungi-like protists is composed of cellulose or such polymers.

Are protists unicellular or multicellular?

Most protists are unicellular and microscopic whereas some of them exist in true multicellular forms. A few protists exist as colonies that act as a group of free-living cells. Also, in another way they act as multicellular organisms. The giant kelp, for example, is a multicellular protist and is large compared to the many other protists that are microscopic. The protists that are multicellular do not show cellular specialization or differentiation into tissues. Hence, their cells all look the same and function the same.

Are protists prokaryotic or eukaryotic?

Protists are eukaryotic and not prokaryotic. The Protista kingdom covers all eukaryotic organisms that are not plants, animals, or fungi. The protist is traditionally considered a predecessor to animals, fungi, and plants and as a result, appears to be the first eukaryotic form of life.

Scientists are of the hypothesis that protists form a link between animals, plants, and fungi and that billions of years ago, the three kingdoms- Animalia, Plantae, and fungi may have diverged from a common protist-like ancestor. Some genes found in today’s plants and animals can be traced to protists even though this protist-like ancestor is a hypothetical organism.

Protista Kingdom

In 1866, a german zoologist, Ernst Haeckel first used the term Protista. Eukaryotic organisms that could not be categorized in the kingdoms Animalia, Plantae, or fungi were historically called protists and were grouped into the kingdom Protista. Hence, these organisms are diverse. They include single-celled eukaryotes that occupy a variety of aquatic and terrestrial habitats. However, not all of these organisms are single-celled and microscopic as some are very large and multicellular e.g kelps. Moreso, multicellular protists do not have tissues or organs that are highly specialized. This simple cellular-level organization is what distinguishes them from other eukaryotic organisms like plants, fungi, and animals.

It is said that there are protist species between 60,000 and 200,000 and many are yet to be identified. Nevertheless, the field of molecular genetics in the past two decades has demonstrated organisms in the Protista kingdom to be more related to plants, animals, or fungi than they are to other protists. This is the reason why protist lineages that were originally classified in the Protista kingdom have been reassigned into other existing or new kingdoms. However, the evolutionary lineages of the protists are still examined and debated. Therefore, for now, the term Protist is still used informally to refer to these diverse groups of eukaryotic organisms that are not plants, animals, or fungi.

This collective group, ‘Kingdom Protista’ portrays an astounding diversity of physiologies, morphologies, and ecologies. There is not much in common amongst the protists besides a relatively simple organization. Thus, protists can look very different from each other. Traditionally, protists in the Protista kingdom based on their similarities with plant, animal, or fungus were subdivided into one of several groups which are plant-like protists (protophyta), animal-like protists (protozoa), and fungus-like (slime mold and water mold).

These traditional subdivision groups were based on non-scientific characteristics. Hence, this classification has been replaced by another classification that is based on phylogenetics which involves the evolutionary relatedness among these organisms. The older classification, however, is still in use as informal names to describe the general characteristics of the various protists.

What is Protoctista?

The ‘Protoctista’ is an interchangeable term for the Protista, which is a kingdom of simple eukaryotic organisms that are usually composed of a single cell or a colony of similar cells.

In the five-kingdom system of Lynn Margulis (which included Animalia, Plantae, Fungi, Protista, and Prokaryota, or Monera), the term protist is reserved for microscopic organisms. While the comprehensive kingdom Protoctista (or protoctists) entailed certain large multicellular eukaryotic organisms, like slime molds, kelp, and red algae.

However, others interchangeably use the term protist with Margulis’s protoctist, to entail both the unicellular and multicellular eukaryotic. This also includes those organisms that form specialized tissues but do not fit into any of the other traditional kingdoms. Listed below are some Protoctista examples.

Protoctista examples

• Giant kelp
• Forams
• Paramecium caudatum
• Marimo
• Forams
• Euglena viridis
• Sea lettuce
• Brain-eating amoeba
• Rhizaria

Protists Characteristics

• One of the major Protists characteristics is the possession of a distinct and membrane-bound nucleus.
• Members of the Kingdom Protista are all eukaryotic.
• The majority of protist has mitochondria.
• They inhabit aquatic and moist environments.
• Most protists are single-celled and very few are multicellular.
• A protist may be an autotroph or heterotroph.
• Symbiosis is one of the Protista characteristics as some protists are seen in symbiotic relationships with other organisms.
• Some protists exist as parasites and can cause disease in humans and other organisms.
• Protista exhibit mobility via flagella, pseudopodia, or cilia.
• These organisms reproduce asexually and very few reproduce sexually.

Protist Examples (Protista kingdom examples)

• Amoebas (including nucleariids and Foraminifera)
• Choanoflagellates
• Ciliates
• Paramecium
• Diatoms
• Dinoflagellates
• Marimo
• Green algae
• Forams
• Giardia
• Brown algae
• Plasmodium
• Rhizaria
• Oomycetes
• Red algae
• Water mold
• Slime molds

How do protists reproduce?

• Binary fission
• Multiple fission
• Fragmentation
• Budding
• Plasmotomy
• Spore formation
• Syngamy
• Conjugation

The majority of protists reproduce via asexual reproduction where the protist cell divides through binary fission, multiple fission, and fragmentation. Also, they reproduce asexually via budding. However, not all protists are asexual some of them reproduce sexually through syngamy and conjugation.

Asexual reproduction in Protist

Protists reproduce through a variety of asexual mechanisms. The majority of them are capable of binary fission where they produce two daughter cells. Some undergo multiple fission where they divide simultaneously to produce many daughter cells. Whereas, others reproduce via budding where they produce tiny buds that further divide and grow to the size of the parent protist.

Binary fission

Protists reproduce via binary fission by a single protist dividing its nucleus and itself to produce two new individual protists. The duration of the process of binary fission in protists varies among protist species and with environmental conditions. Generally, under optimal conditions, fission could take a few hours and under other conditions, it could take days. Typical examples of protist species that undergo binary fission are paramecium, amoeba, and euglena.

Mode of division

Also, the mode in which these diverse groups of protists divide varies. For instance, the flagellates exhibit a longitudinal type of binary fission. On the other hand, the ciliates exhibit a transverse or perkinetal type of fission where they divide in a point-by-point correspondence of parts. Protists like the amoeba exhibit no clear-cut body symmetry and portray an irregular type of fission. The fission occurs in any plane and is usually perpendicular to the plane of karyokinesis (a division of chromosomes).

Multiple fission

This type of asexual reproduction is seen in protists where the parent protist divides into several daughter individuals. It is similar to binary fission, except that the nucleus of the protist divides to form many daughter nuclei in multiple fission. This reproduction is very common in parasitic protists. The number of offspring produced from a multiple division vary from four to dozens or even hundreds which are produced in a short period of time.

There are various types of multiple fission which usually correlates with stages and phases in the life cycle of a given protist species. Multiple fission can occur in the form of budding where the daughter nucleus that is produced, splits from the parent with some of the cytoplasm surrounding it. Another form of multiple division is sporogony where sporozoites are produced by repeated divisions of a zygote. In malaria parasites, a form of multiple fission known as schizogony occurs. Schizogony and sporogony mainly occur in many protists that are obligate parasites of eukaryotic organisms. Typical examples of protists that carry out multiple fission are amoeba and plasmodium.

Budding

This is a form of asexual reproduction where the parent protist body develops a small outgrowth that separates and develops into a new individual. Arcella (sarcodine) is a typical example of such protists that exhibit budding.

Plasmotomy

This type of reproduction occurs in Opalina. In the process of plasmotomy, the multinucleate protist divides into two or more multinucleate offspring without nuclear division.

Spore formation

Some protists form spores for reproduction which has some sort of covering in order to withstand conditions that are unfavorable. A typical example is seen in slime molds. As it germinates, each spore produces a new individual.

Fragmentation

Reproduction in some unicellular algal protists takes place via fragmentation. This is a form of cloning whereby the organism splits into fragments. It is seen in protists such as molds.

Sexual reproduction in protist

Some protists reproduce sexually via conjugation and syngamy. Meiosis and fertilization are involved in sexual reproduction and many protist species can alternate between asexual and sexual reproduction when necessary. Meiosis functions in sexual reproduction to reduce the number of chromosomes to half in gametes in order for the number of chromosomes to be kept constant in a species after fertilization. Usually, protists carry out sexual reproduction during periods where there is depletion of nutrients or changes in the environment.

Reproducing sexually, allow protists to be able to recombine genes. This eventually gives rise to a new variation of progeny that can be well suited for survival in new habitats. Also, sexual reproduction is usually associated with protective cysts. These cysts are resting stages and can be resistant to extreme temperature, low pH, or desiccation, depending on their habitat. This mechanism allows protists to wait out stressors till they are carried to a different environment by water, wind, or by larger organisms. Also, the cysts enable them to wait out stressors until the environment becomes more favorable for them to survive. This mechanism functions so because cysts exhibit no cellular metabolism.

Types of sexual reproduction in protists

There are virtually two kinds of sexual reproduction in protists:

1. Syngamy
2. Conjugation

Syngamy

Many unicellular species form gametes under appropriate conditions which fuse to produce a new genetically unique offspring. Syngamy which is a sexual reproduction that involves the fusion of two gametes is the most common type of sexual reproduction form among protists.

For instance, Syngamy is seen among pseudopods, plasmodium, and various flagellated organisms. In such protists, two gametes or sex cells unite to give rise to diploid zygotes.

Types of syngamy

There are three types of syngamy which include:

1. Oogamy
2. Isogamy
3. Anisogamy

Oogamy

In this type of syngamy, small motile gametes fertilize the large non-motile gametes. This type is seen in plasmodium.

Isogamy

In this syngamy type, the two gametes that unite are similar. This is seen in Monocystis.

Anisogamy

In this type of syngamy, the two gametes that unite are dissimilar. This is seen in Ceratium.

Conjugation

This is the second major sexual form of reproduction that occurs in protists. It gives genetic and evolutionary results that are similar to syngamy. Conjugation occurs mainly in ciliated protists and involves the fusion of gametic nuclei instead of independent sex cells. This is seen in paramecium.

A true zygote is not produced from this process. Rather, a fusion or zygotic nucleus is produced. This nucleus undergoes a series of meiotic divisions that give rise to a number of haploid pronuclei. Then, all except one of the pronuclei in each organism disintegrates; the one pronuclei remaining divide mitotically. From each organism, one pronucleus is exchanged and new micronuclei and macronuclei of the next generation are produced.

There is a series of asexual fission which occurs together with mitotic divisions of the new diploid micronuclei in each exconjugant line. This actually takes place after the exchange of the pronuclei and the formation of new macronuclei and micronuclei. In the initial process of these divisions, the new polyploid macronuclei are passively distributed. Then, the macronuclei in subsequent fission, replicate themselves through a kind of mitosis.

Conjugation is mainly limited to ciliates and there is variation in the manner in which they exhibit it amongst themselves. For instance, there may be a noticeable difference in the size of the two ciliates (i.e macroconjugants and microconjugants). Also, the number of pre-divisions of the micronuclei, as well as the number of nuclear divisions that occur after the formation of the zygotic nucleus, may vary.

Furthermore, in conjugation, before a pair of protists unite they give out or exchange chemical signals (gamones). These gamones attract and bring different mating types together. However, it is not known if these gamones should be referred to as sex pheromones which are common in many animals like some kind of insects.

In conclusion, conjugation is a temporary union of two individuals to exchange their haploid pronuclear in order to form a zygote nucleus. This kind of reproduction can be seen as a process of reciprocal fertilization. However, a parallel sexual condition takes place in ciliates occurring in single unpaired individuals which can be considered as a process of self-fertilization. This type of fertilization is called autogamy where complete homozygosity is gotten in the lines derived from the single parent.

How are protists classified?

1. Animal-like protists (Protozoa)
2. Plant-like protists (Protophyta or algae)
3. Fungi-like Protists (Molds)

The protist based on their similarities with plant, animal, or fungus was subdivided into the above-listed groups. These traditional subdivision groups were based on non-scientific characteristics. Hence, this classification has been replaced by another classification that is based on phylogenetics which involves the evolutionary relatedness among these organisms which is seen as follows in each group.

Animal-like protist (Protozoa)

Protozoans are heterotrophic unicellular organisms that have the ability to move. They are called animal-like because of their motility and heterotrophic nature. However, they are some protozoans that are parasitic and live in the cells of larger organisms.

Many animal-like protists do not have a predefined shape e.g the amoeba. However, a paramecium has a definite shape that resembles a slipper. Common examples of protozoans include paramecium, euglena, and amoeba. Euglena, however, is different from other members of this group because it is a free-living protozoan that possesses chlorophyll. This means it is photosynthetic and can make its own food.

Furthermore, these protists are classified based on their ability to move. Thus, protozoa are divided into several phyla.

Types of animal-like protists

1. Sarcodina
2. Mastigophora
3. Ciliophora
4. Sporozoa

Sarcodina

This phylum is the group of protists that move using a false foot (bloblike structure) called pseudopod (pseudopodia) which helps them in capturing and engulfing food. It also aids them to change their shape. The false feet are temporary membrane-bound cytoplasm projections that control the movement of the sarcodines. Sarcodines can assume virtually any shape as a result of the innate flexibility of the pseudopod. These protists mostly inhabit both fresh and saline water bodies.

Amoeba is a typical example of a protist and they are called amoeboid protozoans. They are typical sarcodines and use pseudopods to locate and engulf food. Another typical example of Protista in this group includes the foraminifera. These are aquatic protists that are known mostly for the calcium carbonate shells they secrete. These carbonate shells sometimes accumulate in large deposits when these protists die. Foraminifers only inhabit warm water and when their fossilized shells are discovered in strata, the climate for that aquatic environment as at that time can be estimated.

Mastigophora

This is a subphylum for the group of protists that move using flagella. They are called flagellated protozoans and can be free-living, colonial, or parasitic. These protists are known for their specialized flagella that looks like a whip. This whip-like structure propels these protists through their aquatic habitats.

Generally, flagellated protozoans possess only one flagellum though they may have up to four that work together. The majority of flagellates are harmless. However, some are parasites that parasitize humans. A typical example of Protista that is a parasitic flagellate is the Trypanosoma that causes the African sleeping sickness.

This disease is associated with symptoms such as fever, chills, and skin rash. Humans that are affected usually become very weak, unconscious, and may fall into a fatal coma.
This protist (Trypanosoma) lives in the bloodstream of its host and is transmitted by the tsetse fly. In order to be invisible to the immune system of their host and to remain undetected, they continually change their surface molecular structure. The nervous system of the infected human faces attacks during the course of the disease.

Types of flagellated protozoans

1. Phytomastigophorea
2. Zoomastigophorea

Flagellates can be grouped taxonomically into two groups:

Phytomastigophorea

These flagellates (also called phytoflagellates) resemble plants and are groups of chlorophyll-containing protozoans that can produce their own food via photosynthesis. E.g euglena and dinoflagellates. There is an obscure distinction between algae and phytomastigophorea.

Zoomastigophorea

These flagellates (also called zooflagellates) resemble animals and are colorless. E.g symbiotic hypermastigids and parasitic Trypanosoma.

Ciliophora

This phylum includes protists that move using cilia. They are called ciliated protozoans and have cilia all over their body which help them move and aid nutrition. Ciliates are unicellular or colonial organism. They are free-living, non-parasitic, and inhabit fresh or saltwater bodies. The cilia in these protists direct their movement towards a food source or away from unfavorable territories.

Paramecium is a typical example of a ciliated protozoan. A paramecium possesses two nuclei. The large macronucleus takes care of the everyday cell activities whereas, the smaller micronucleus functions during the exchange of gamete. Under normal conditions, the paramecium reproduces asexually via binary fission. However, during periods of stress, they reproduce sexually via conjugation and exchange haploid micronuclei with another paramecium.

However, sexual reproduction doesn’t necessarily occur as no offspring or fertilized eggs are produced. Rather, gametes are exchanged by mature adults which results in a new genetic complement for both paramecia. Paramecia also possess most organelles that more advanced organisms use. For example, they possess food vacuole, anal pore, and contractile or water vacuoles in addition to the mitochondria and nucleus that they possess. They utilize the anal pore for waste removal and the food vacuole has digestive enzymes for the breaking down of food. The water vacuoles function in the transportation of water.

Sporozoa

This phylum includes protists that form spores. Their life cycle involves a spore-like stage and this is why they are called sporozoans. This group of protists is well known because they are all parasitic. They live in the host body and reproduce spores. These spores are dormant cells that are enclosed in a protective membrane. The spores are able to enter an appropriate host once it lands on the host. They enter through various means and grow into maturity as parasites.

As parasites, their cells possess specialized organelles that penetrate the host cell membranes. The Sporozoa that involve two hosts in their life cycle are the more exotic Sporozoa. A typical example of Protista that is a sporozoan is the plasmodium. This parasite causes malaria in humans by entering the red blood cells and digesting the nutritional content of the cells till it becomes nonfunctional. It then grows, reproduces, and infects other neighboring red blood cells. Malaria spreads with the help of the female anopheles mosquitoes which withdraw some blood infected with plasmodium during her normal dietary meal and then transfer the plasmodium to another unsuspecting human.

Plant-like protist (Protophyta or algae)

Protophyta is autotrophic and could be unicellular or multicellular. They are called plant-like because of their autotrophic nature. These protists can manufacture their own food due to the presence of chloroplast. Scientists are of the thought that plants evolved from algae.

These protists inhabit aquatic habitats and do not possess true stems, roots, or leaves.

Types of Plant-like protist

1. Euglenophyta (euglenids)
2. Chrysophyta (diatoms)
3. Pyrrophyta (dinoflagellates)
4. Chlorophyta (green algae)
5. Phaeophyta (brown algae)
6. Rhodophyta (red algae)

The plant-like protists are divided into several phyla:

Euglenophyta (euglenids)

Euglenids are structurally similar to Zoomastigina (subclass of Mastigophora). This is because euglenophytes and zoomastigina both utilize flagella and have common structures. These protists lack a cell wall but undergo photosynthesis and possess chloroplasts. A typical example of a euglenophyte is a euglena. This protist has an eyespot that doesn’t focus as an eye. However, the eyespot differentiates light from dark. For greater photosynthetic opportunities, the eyespot allows the euglena to be able to move towards a light source.

A protein-rich layer called a pellicle covers the outer body of these protists e.g Trachelomonas, and Euglena. Euglenids are swimmers with two flagella which is crucial because they can inhabit diverse aquatic habitats. They are quite versatile in their habitat requirements because they can live as both a photosynthetic autotrophic organism in the presence of sunlight and as a nutrient absorbing heterotroph when sunlight is absent. They can feed on small organisms in the absence of sunlight. Also, reproduction doesn’t limit their range as they can be sexual when needed and are usually asexual the rest of the time.

An example of a colonial euglenophyte is the volvox. It consists of individual cells that are capable of producing daughter colonies. This colonial euglenophyte resembles a hollow ball. Volvox cells as well as their cousin Chlamydomonas have features that are common with more complex plants. It is because of the common features such as cell walls that are made of cellulose, chloroplasts, and energy-storing compound that there are suggestions that they may have evolved into today’s plants.

Chrysophyta (diatoms)

Diatoms as an example of a protist are the main species in the phylum Chrysophyta. They are a crucial food source in freshwater and marine water. Diatoms and golden algae are typical examples of chrysophytes. These protists can store food in a light less-dense oil form that permits them to float on water in order to be close to sunlight. They are photosynthetic and these protists are characterized by the possession of a hard siliceous cell wall.

Diatoms, in their cell wall substitute carbohydrate pectin for cellulose. Carbohydrate pectin is rich in the element silicon which is the main element in a glass. The shells of diatoms usually accumulate in large strata called diatomaceous earth which is used for filtering purposes and as an abrasive. Diatomaceous earth is produced as a result of the accumulation of cell wall deposits.

Pyrrophyta (dinoflagellates)

Dinoflagellates are the main member of the phylum Pyrrophta. They are unicellular photosynthetic algae with unique features. Several species of this phylum are bioluminescent and when disturbed give off light. According to the pigment present in them, they can be seen in various different colors. Also, they are different from other eukaryotes as their DNA is not packed with histones.

Furthermore, when the conditions are favorable, these protists are subject to population explosions which are sometimes called red tide. This bloom is called red tide because the algae are so numerous and the algae being red in color gives the water a reddish look. They are so thick that they hinder less-mobile aquatic life like crabs and flounders from absorbing oxygen.

Chlorophyta (green algae)

Green algae is a typical example of a protist and could exist as unicellular or colonial organisms. They have diverse body forms and move by flagella. Their body could be made of filaments or sheets. The green algae have a cell wall of cellulose and stores energy as starch in the vacuole. These protists inhabit aquatic and terrestrial habitats. Some of them even form lichens with fungi.

Phaeophyta (brown algae)

These protists are found in marine waters that are cool most especially in intertidal areas. In southeast Asian countries, they are used as food. All brown algae species are multicellular and their body is differentiated into blade, floats, stipe, and holdfast.

Rhodophyta (red algae)

The majority of red Algae are mostly multicellular and by holdfast, their body of filaments or sheets are attached to a substrate. Some of them excrete calcium carbonate such as the coralline algae. They are found in warm tropical oceans and possess thick starchy cell walls that yield commercial quantities of agar and carrageenan.

Fungi-like protist (Molds)

These protists are fungi-like and reproduce by forming spores. They are heterotrophic and their cells have cell walls. Molds feed on organic decaying matter and share similarities with fungi.

Like fungi, they feed on organic matter and reproduce via spore formation. However, they have a cellulosic cell wall while fungi have a chitinous cell wall. Molds, in a certain part of their life cycle, can have motility.

Types of fungi-like protist

1. Slime molds
2. Water molds

Fungi-like protists can be divided into:

Slime molds

Slime molds feed on the dead and decaying matter such as compost and rotting logs which means they are saprophytes. These protists are tiny and have many nuclei. Usually, they are characterized by having aggregates referred to as plasmodium which can be seen with the naked eye.

They are fungus-like and when organic matter is less, they agglomerate to form a slimy mass. Hence, they move slowly by sliding upon their own secretion and eating the organic matter.

Types of slime mold

1. Cellular molds
2. Acellular (plasmodial) molds

There are two types of slime mold which include:

Cellular Slime mold

A cellular slime mold lives as distinct individual single-cell protists. However, these slime molds can assemble into a cluster when a chemical signal is secreted. This cluster then acts as a single (one) organism.

Acellular (plasmodial) slime mold

The acellular molds agglomerate in order to produce a single cell that has multiple nuclei. This slime mold is surrounded by a single membrane without walls. It is one large cell called a syncytium which is a bag of cytoplasm that contains thousands of individual nuclei.

Water molds

The water mold is an example of a protist. These protists are commonly seen in surface water and moist soil. Some of the members of water molds are plant pathogenic and infect and destroy crops such as corn, grapes, lettuce, and potatoes. Whereas, others parasitize fish and other marine animals.

Economic Importance of Protist

• Protists serve as the basis of the food chain in aquatic environments.
• A symbiont protist has a close relationship with another species in which, one benefits.
• Some protists produce oxygen during photosynthetic activities and can be used to produce biofuel.
• Many animals depend on protists as their primary sources of food.
• There are rare occasions where humans harvest protists for food and for other industrial use.

Frequently Asked Questions

What are protists?

Protists are members of the kingdom Protista which includes all eukaryotic organisms that are not plants, animals, or fungi. They are simple unicellular eukaryotic organisms though some can be found as a colony of cells or in multicellular form. The majority of protists inhabit the aquatic and damp terrestrial habitats and some exist in nature as parasites. Any environment that contains water is usually occupied by free-living protists. They can be seen in oceans, lakes, ponds, rivers, damp soil, snow, tree barks, etc.

The majority of protists are photosynthetic e.g algae and are crucial primary producers, especially in the aquatic ecosystem. Also, a protist can be autotrophic or heterotropic. They form a large portion of biomass in aquatic and terrestrial habitats. Other protists are not autotrophic or heterotrophic but parasitic.

Is algae a protist?

Yes, algae is a plant-like protist. It is autotrophic and could be unicellular or multicellular. They are called plant-like because of their autotrophic nature. These protists can manufacture their own food due to the presence of chloroplast. Scientists are of the hypothesis that plants evolved from algae.

Are protists eukaryotic?

Protists possess cells with a distinct and membrane-bound nucleus. Hence, they are eukaryotic and not prokaryotic. Their kingdom- Protista covers all eukaryotic organisms that are not plants, animals, or fungi. The protist is traditionally considered a predecessor to animals, fungi, and plants and as a result, appears to be the first eukaryotic form of life.

Scientists are of the hypothesis that protists form a link between animals, plants, and fungi and that billions of years ago, the three kingdoms- Animalia, Plantae, and fungi may have diverged from a common protist-like ancestor. Some genes found in today’s plants and animals can be traced to protists even though this protist-like ancestor is a hypothetical organism.

Do protists have cell walls?

Protists usually have no cell wall, though some of them may have.  The protists that usually have cell walls are the plant-like protists (algae) and the fungi-like protists (molds). Animal-like protists (protozoa) do not possess cell walls. The cell wall of a fungi-like protist is composed of cellulose or such polymers.

What are the types of protists?

Protists are basically of three types: protozoa, algae, and molds. The protozoans are heterotrophic unicellular organisms that have the ability to move. They are called animal-like because of their motility and heterotrophic nature. However, they are some protozoans that are parasitic and live in the cells of larger organisms.

The algae or protophyta is autotrophic and could be unicellular or multicellular. It is called a plant-like protist because of its autotrophic nature. These protists can manufacture their own food due to the presence of chloroplast. Scientists are of the thought that plants evolved from algae.

Molds are fungi-like and reproduce by forming spores. They are heterotrophic and their cells have cell walls. Molds feed on organic decaying matter and share similarities with fungi.

Are protists autotrophs or heterotrophs?

Protists could be autotrophic or heterotropic. Photoautotrophic protists such as algae are photosynthetic and are characterized by the possession of chloroplasts. Autotrophic protists are plant-like because they make use of photosynthesis to produce their own food. Examples of such protists include diatoms, euglena, dinoflagellates, brown algae, red algae, and green algae. Plants, however, are said to have evolved from green algae.

The heterotrophic protists, on the other hand, such as amoeba feed on organic material and other organisms. These heterotrophic protists ingest particles through a process called phagocytosis. This process involves the engulfing of food particles by the cell membrane which brings the food inward and pinches off a food vacuole (an intracellular membranous sac or vesicle).

How are protists classified?

The protist is classified based on their similarities with plant, animal, or fungus into the following groups: Animal-like protist (Protozoa), Plant-like protist (Protophyta or algae), and Fungi-like Protist (Molds).

How do protists reproduce?

The majority of protists reproduce via asexual reproduction where the protist cell divides through binary fission, multiple fission, or fragmentation. Also, they reproduce asexually via buddy. However, not all protists are asexual some of them reproduce sexually through syngamy and conjugation.