Protists are simple microscopic unicellular microorganisms. They are made up of only one cell but the simple cell structure is highly organized and consists of a nucleus and organelles or specialized cellular machinery.
Whatever scientists can’t classify under animal, plant, fungi, and bacteria are classified under the category of kingdom Protista. As a result, you’ll find that most of the protists share very little similarities.
Get to know the microscopic unicellular microorganisms known as protists and discover its many different types. Be able to identify their characteristics and cite examples of these microorganisms.
Types
Archaeplastida
In Archaeplastida, eukaryotic life forms containing chloroplasts are enclosed by two membranes. It is generally thought that this is a type of structure that hints at the endosymbiosis of cyanobacteria. Most Archaeplastida have cells with walls, and, more often than not, those walls are made of cellulose.
Their cell organization, however, varies widely and can range from isolated cells to colonies to filaments to multi-celled organisms. The earliest cells were all unicellular, and today many groups are still unicellular.
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Eventually, however, multi-cellularity evolved across several groups, and these groups include ulvo-phyte green algae and red algae, and the green algae have given rise to stoneworts and land plants.
Archaeplastida, or primoplantae, comprise green and red algae, the land plants, and a small group known as the glaucophytes. They have two major evolutionary lines. The red algae have chlorophyll which is pigmented and phycobiliproteins, just like most cyanobacteria.
The green algae and the land plants have chlorophyll that is pigmented but lack phycobiliproteins. Some studies have suggested that other groups – including haptophytes, katablepharids, and cryptophytes – might be related more closely to red and green algae than the glaucophytes are.
Chromalveolata
Chromalveolata are types of eukaryotes and are also single-celled organisms. Most of their members have morphologies that resemble those of terrestrial plants, and they have cell walls. These kinds of protists also have the plant-like ability to perform photosynthesis. They are usually not considered a kingdom and are not given any formal taxonomical classification.
However, they do include groups such as heterokontophyte, alveolate, dinoflagellate, apicomplexa, ciliophoran, haptophyte, and cryptophyta. Most groups are autotrophic, although some no longer have the ability to photosynthesize, including the water molds and ciliates. All chromalveolates that are photosynthetic use chlorophylls a and c; many also use accessory pigments.
Currently, it is believed that species classified as chromalveolates are derived from a common ancestor that engulfed a photosynthetic red algal cell, and this had since evolved chloroplasts from an endosymbiotic relationship with a photosynthetic prokaryote.
Hence, the chromalveolates’ ancestor is thought to have resulted from a secondary endosymbiotic event, even though some chromalveolates appear to have lost plastid organelles that have derived from red alga and also lack plastid genes altogether. Because of this, the supergroup is best considered a hypothesis-based working group subject to change.
Chromalveolates include photosynthetic organisms that are considered very important, including brown algae, diatoms, and significant disease agents in both plants and animals. Chromalveolates can be further divided into stramenopiles and alveolates.
Excavata
Excavata are made up of single-celled eukaryotic organisms that might be symbiotic as well as free-living. Most of these life forms lack typical mitochondria, and most of them have two or more flagella. The excavates that do have mitochondria usually have it in the form of tubular, discoidal, or laminar cristae.
Most in this supergroup are single-celled and asymmetrical organisms that have a feeding groove that is “excavated” from one side. The group includes parasites, heterotrophic predators, and photosynthetic species. There are three main subgroups of Excavata – parabasalids, diplomonads, and euglenozoans.
Rhizaria
These consist only of single-celled eukaryotic organisms with mitochondria and tubular cristae. Most of them are also like an amoeba, because they have pseudopodia or false feet. The Rhizaria is a supergroup consisting mostly of unicellular eukaryotes, even though recently a multicellular form has been described.
This subgroup has only been around since 2002, and for the most part, it consists of amoeba-like structures with pseudopods. Many of them produce skeletons or shells that have structures that are complex. The vast majority of protozoan fossils consist of rhizaria, and nearly all have mitochondria with folds.
In the past, many rhizarians were considered animals, in part because of their movement and their heterotrophy. Later, when the five-kingdom system started being used, rhizarians were put into the Protista kingdom. After Carl Woese’s three-domain system was published, however, rhizarians were put into a monophyletic group.
Unikont
With a genetic structure of three genes that are fused with one another, the unikont has a cellular structure that is eukaryotic, and most of them are either organisms that have just one protruding flagellum or amoeboid without flagella.
This group includes Amoebozoa and Obazoa, the latter of which contains the Opisthokonta, which encompasses animals, fungi, and choanoflagellates.
With a triple-gene fusion missing in the bikonts, the genes are fused together, and the three genes encode enzymes for synthesis of the pyrimidine nucleotides, which include dihydroorotase, carbamoyl phosphate synthase, and aspartate carbamoyltransferase.
This likely involved a double fusion, which are rare, that support the shared ancestry of Amoebozoa and Apisthokonta.
Subcategories of Protists
Archaeplastida
Glaucophyte
As an example, microscopic freshwater alga characterized by the inclusion of a murein layer and is thought to be a sign of endosymbiosis of cyanobacterial plasmids. The glaucophyte is often used as a synonym for Plantae, even though the most accurate use of this term includes only the land plants and green algae, and even though the relationship among green algae, red algae, and the glaucophytes is unclear.
An important feature of glaucophytes is a chloroplast with a peptidoglycan layer, which is a signal of a possible remnant of the endosymbiotic origin of plastids from cyanobacteria. The glaucophytes consist of four major genera.
Rhodophyta
These are Eukaryotes which lack flagella, and they are a great example of that which is red alga; in fact, this is often a synonym for red algae. In the classification with five kingdoms, this is a kingdom that is comprised of a variety of unicellular and simple multinuclear and multicellular eukaryotic organisms.
Red algae are photosynthetic organisms, and they were previously classified as thallophytes, which is a primitive subdivision in the plant kingdom and lack sterns, leaves, true roots, and flowers. There are approximately 4,000 species of red algae, and nearly all of them are marine; however, a few of the species occur in freshwater.
Red algae are found in all oceans but are most common in those with warm temperatures and tropical climates, and they can also be found at greater depths than other photosynthetic organisms.
Red algae are multicellular and characterized by an enormous amount of branching, although it takes place without differentiation into complex tissues.
The cell wall of the red algae has an inner layer that’s firm and contains cellulose, as well as a mucilaginous or gelatinous later on the outside. The cells can have one or more nuclei, and this depends on the species itself. In addition, cell division is mitosis.
The red algae’s life cycle is very complex and involves one haploid phase and two diploid phases. Although most marine red algae have delicate, soft bodies (thalli), the coralline algae have thalli that get stronger due to calcification and contribute to the growth of coral reefs located in tropical seas.
Because they produce structures that are permanent, coralline algae have a rich fossil record dating back as far as 700 million years.
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Chromalveolata
Alveolate
Alveolate has cortical alveoli, mitochondria, flagella that are distinctly structured, and flattened vesicles.
Cryptomonad
These include most chloroplast that contain algae.
Haptophyte
These are eukaryotes with pigmented chloroplasts, and an example is the coccolithophore alga.
Heterokont
These are diatomic organisms – mostly kelp and other algae – and they are characterized by motile cells and chloroplast content.
Excavata
Euglenozoa
These are life forms with protozoa and flagella, and they are unicellular, with some being parasitic and others being free-living.
Metamonads
These are anaerobic flagellate protozoa that are mostly either parasitic or in a symbiotic relationship.
Percolozoa
These are protozoan organisms, and some have the ability to transform into encysted forms, amoeboid, or flagellate. They also lack color pigmentation.
Diplomonads
This group includes the Giardia lamblia, which is an intestinal parasite. Until recently, they were believed to lack mitochondria, and mitosomes, which are mitochondrial remnant organelles, are now identified in diplomonads, although they are essentially nonfunctional.
Diplomonads are found in anaerobic environments and utilize alternative pathways to generate energy, including a pathway called glycolysis. In addition, every diplomonad cell has two nuclei that are identical and also uses several flagella for locomotion.
Parabasalids
The parabasalids exhibit semi-functional mitochondria. The structures function anaerobically and produce hydrogen gas as a byproduct, which is why they are called hydrogenosomes. Parabasalids move with the help of flagella and membrane rippling.
There are numerous types of parabasalid, including a sexually transmitted disease in humans called Trichomonas vaginalis. With T. vaginalis, women can be at risk of serious complications if they are pregnant, and both men and women are more susceptible to infections such as HIV and certain types of cancer.
Euglenozoans
This subgroup includes heterotrophs, autotrophs, parasites, and mixotrophs, which range in size from 10 to 500 um. Euglenoids move through their aquatic habitats with the use of two long flagella that help guide them to light sources that are sensed by an eyespot, which is a primitive ocular organ.
The genus Euglena includes some mixotrophic species displaying a photosynthetic capability only when there is light, because when it is dark, the euglena’s chloroplasts shrink up and cease functioning temporarily. In addition, the cells will instead take up organic nutrients from the environment that they are in.
Rhizaria
Cercozoa
These are flagellates and amoeboids that can form pseudopods if there is no mouth well-defined enough for feeding.
Foraminifera
Amoeboids which possess cytoplasmic strands that are very fine and which branch out and merge in order to give the protist the appearance of a microscopic net that has a nucleus.
Radiolaria
Amoeboid protists that possess complex skeletons that are made of various minerals, some of which can be seen as marine zooplankton.
Unikont
Amoebozoa
Generic amoeboids that have movements which are dependent on their internal cytoplasmic flow.
Choanozoa
Animal-like protists that are mostly parasitic.
