The vast majority of eukaryotes belong to a diverse group of organisms referred to as protists. Protista was once considered a distinct Kingdom of life but we now know that protists aren’t necessarily closely related to each other.
Some protists are more closely related to plants, fungi or animals than they are to other protists. For convenience, the term protist is still often used to refer to all eukaryotes that aren’t animals, plants or fungi.
Most protists are extremely small microorganisms. They are mostly single-celled organisms and can have cells as small as prokaryotic cells between 0.5-2 µm
There is a huge diversity of protists. A large number of different species have only been discovered in the past 10 years using new technology to identify genetic differences. Some of the better known protists include algae, diatoms, slime molds, water molds and many parasites such as giardia.
Diversity of protists
The protists are a massively diverse group. Because they are mostly microorganisms we still know very little about them. Our knowledge of their evolution and the relatedness of different protist groups is still rapidly changing.
If plants, animals and fungi are each considered unique Kingdoms of life then the protists could arguably be separated into more than 20 different Kingdoms. This gives some indication of the amount of genetic diversity there is amongst protists.
One current classification separates all eukaryotes into five supergroups: Excavata, Chromalveolata, Rhizaria, Archaeplastida and Unikonta. Along with different groups of protists, animals and fungi are placed into the supergroup unikonta and plants are found in archaeplastida. The remaining three groups consist entirely of protists and the vast majority are microorganisms.
Structure and metabolism of protists
The structure of protists is hugely variable, far more so than the rest of the eukaryotes. Protists share only a few general characteristics.
A key feature of all protists are their eukaryotic cells. Because protists are eukaryotes, their cell or cells have a nucleus and membrane bound organelles.
The vast majority of protists are single-celled organisms. Some single-celled protists live in colonies with other cells of the same species. There are also a few protist groups that have evolved into multicellular organisms such as the brown algaes.
Different protists have different metabolisms. Some are able to produce cellular energy through photosynthesis (autotrophs) and others can only get energy by absorbing or engulfing organic molecules (heterotrophs). Certain groups are able to do both. These protists are known as mixotrophs because they are both an autotroph and a heterotroph.
Endosymbiosis has played an important role in the evolution of protists plus animals, plants and fungi. Multiple times over the history of life on Earth, a eukaryotic cell has engulfed another cell which has then become a part of the eukaryotic cell. This is endosymbiosis.
The engulfment of a cell by another cell has led to the evolution of far more complex cells. Various organelles for example are thought to have evolved from a eukaryotic cell engulfing another cell. Mitochondria are thought to have originated from a eukaryotic cell engulfing a prokaryotic cell. The chloroplasts found in plant cells and other photosynthesizing eukaryotes are believed to have evolved after a eukaryotic cell engulfed a cyanobacterium cell.
The engulfment of eukaryotic cells led to the evolution of more advanced protists. The ancestors of red and green algae were formed from a primary endosymbiosis event where a eukaryotic cell engulfed a cyanobacterium. More complex protists evolved when a eukaryotic cell engulfed red and green algal cells i.e. secondary endosymbiosis.
A trademark feature of a cell that has benefited from endosymbiosis are plastids. ‘Plastid’ is a general term for any organelle that has a double membrane. The double membrane is formed from the membrane of the engulfed cell plus a membrane from the larger ‘engulfer’ cell.
As one cell engulfs another, the membrane of the engulfer wraps around the smaller cell. Once the smaller cell is completely surrounded by the larger cell’s membrane, the membrane of the engulfer forms a bubble around the smaller cell called a ‘vesicle’. The vesicle creates the second membrane of a plastid.
The supergroup unikonta includes a range of protists plus animals and fungi. Many of the unikont protists are amoebas. An amoeba is any organism or cell that moves and feeds by extending out its plasma membrane. The extensions of the plasma membranes are known as ‘pseudopods’. The word ‘pseudopod’ can be translated into ‘false foot’.
Unikonta is the most diverse group of eukaryotes, largely thanks to the extreme diversity of insects in the animal kingdom. This supergroup is separated into two major groups: the Amoebazoans and the Opisthokonts. The amoebozoans contain only protists while the opisthokonts include the fungi and animal kingdoms plus some closely related protists.
Amoebozoans are amoeba protists that have lobed or tube-shaped pseudopodia. This group includes the slime molds, gymnamoebas and entamoebas.
Slime molds were originally grouped in with fungi because they produce fruiting bodies similar to how fungi produce mushrooms. Fruiting bodies are an important for the reproduction of slime molds because they help to disperse thousands and thousands of spores.
Gymnamoebas are found in soil, freshwater and marine habitats. They feed on bacteria and other protists. Entamoebas live as parasites inside animals.
The opisthokont eukaryotes include nucleariids (protists), fungi, choanoflagellates (protists) and animals.
Nucleariids are the most closely related organisms to the fungi kingdom. They are single-celled organisms that feed on algae and bacteria.
Choanoflagellates are the closest relatives of animals. They are also single-celled protists and have a flagella at one end of their cell. Their flagella is densely surrounded by long, thin growths called microvilli. The flagella is used to pump water across the microvilli and the microvilli then filter out food particles from the water.
The supergroup archaeplastida includes the red algae, green algae and land plants. Each of these three groups have multicellular species and the green and red algae have many single-celled species. The land plants are not considered protists.
The archaeplastida evolved over 1 billion years ago. The first ancestors of this group were formed by endosymbiosis when a eukaryotic cell engulfed a cyanobacterium. These groups form the base of food webs in many different ecosystems.
Most red algae are found in marine ecosystems but some are also found on land. Their red colour comes from a photosynthetic pigment called phycoerythrin. They also have chlorophyll but the green of chlorophyll is masked by the red of the phycoerythrin.
The color of red algae often changes with the depth of water. In shallow water they are often green, at moderate depths they are typically red and in deep water they tend to be black. Red algae can sometimes be found growing in water deeper than 200 m (650 ft.).
Green algae are split into two groups – chlorophytes and charophytes. Both groups contain both single-celled and multicellular algae. Charophytes are the most closely related organisms to land plants and are found in freshwater environments. Chlorophytes are found in marine, freshwater and land based ecosystems but are most commonly found in shallow freshwater.
The supergroup chromalveolata includes many important photosynthetic protists. This group includes many of the organisms that make up the phytoplankton and seaweeds in oceans and lakes such as brown algae, diatoms and dinoflagellates. Some chromalveolates are serious pathogens such as Plasmodium which causes malaria and Phytophthora which caused the potato famine in Ireland.
There is evidence to suggest that chromalveolates evolved over a billion years ago after a eukaryotic cell engulfed a red algae cell. This theory is not fully supported because some chromalveolates don’t contain plastids or plastid DNA. This supergroup is split into two main divisions – alveolates and stramenopiles.
The alveolates includes dinoflagellates, apicomplexans and ciliates. These protists are distinguished by the presence of membrane bound sacs called alveoli. The alveoli are found just inside the cell’s plasma membrane and biologist are yet to work out the function of these sacs.
Dinoflagellates are characterised by having two flagella and a hard cellulose shell. They are found in water, mostly in marine ecosystems and are an important member of the photosynthetic plankton that drifts in the surface water of the ocean.
The hardened external cover that surrounds the cell of a dinoflagellate is made from multiple cellulose plates. The plates contain grooves. Along the groove is where the two flagella are located. The flagella help dinoflagellates to move through water.
Apicomplexans are a group of protists that are almost entirely parasitic. Protists such as Plasmodium, which causes malaria, penetrate the cells of animals. Once they enter a host cell they typically reproduce multiple times before bursting the host cell open. The cells of apicomplexans protists have a collection of organelles at one end of the cell which enables it to penetrate into a host cell.
Ciliates are a group of single-celled protists that have many short, thin growths on the outside of their cell. These short structures are called cilia and are used to help cells move and collect food.
A second distinctive feature of ciliates is that they have two nuclei. One nucleus is much smaller than the other and is called the micronucleus. The second is a macronucleus and contains multiple copies of the cell’s DNA.
Stramenopiles are the second group of chromalveolates. This group of protists have flagella with many short hair-like structures along the length of the flagella. A ‘hairy’ flagellum is often paired with a shorter, smooth flagellum. Many important photosynthesizers are stramenopiles and not all stramenopiles are single-celled.
Diatoms are a single-celled algae with a unique silicon-based cell wall. Their galss-like cell walls are made from two plates called frustules that overlap with each other. Diatoms are another important part of marine and freshwater phytoplankton.
Brown algae, or phaeophyta, are a group of complex, multicellular algae. They include many of the algae commonly referred to as seaweeds. Brown algae will often form dense ‘forests’ in the sub-tidal zone in marine ecosystems. Their brown color is caused by a photosynthetic pigment called fucoxanthin.
Oomycetes are a group of chromalveolates that were once considered to be fungi. They cannot however be fungi because their cell walls are made from cellulose. Fungal cell walls are made from chitin.
Oomycetes includes a number of decomposers and parasites such as water molds, white rusts and downy mildews. Water molds are a group of oomycetes that feed by helping to decompose dead animals. White rusts and downy mildews are most commonly parasites that exploit plants.
Rhizaria are a supergroup of protists that includes many species of amoeba. Rhizaria amoeba differ from other groups of amoebas because they have thread-like pseudopods.
The pseudopods of rhizaria enable the cell to move by extending the thread-like pseudopodia out from the cell, anchoring the tip to a surface and then moving the contents of the cell towards the tip of the pseudopodia.
Rhizarians are a morphologically diverse group that have been placed into their own supergroup based on DNA evidence. Some biologists believe they should be included as a part of the chromalveolates. The supergroup is split into three separate groups: radiolarians, forams and carcozoans.
The radiolarians have internal skeletons made from silica. These protists use their pseudopods for food collection and are mostly found in marine environments.
Forams, or foraminifera, are unicellular protists with porous shells. Their pseudopods extend out through the holes in their shells. Although they are single-celled organisms, some forams can be several centimeters in diameter. This group of protists are found in marine and freshwater environments usually attached to a substrate such as rock or algae. Some species live as plankton.
Cercozoans are the final group of rhizarians. They are found in salt and fresh water and in soil on land. Species from this group live as parasites, predators, autotrophs and mixotrophs. Cercozoans are important predators of bacteria.
Excavata are a group of single-celled protists that are distinguished by an ‘excavated’ groove along one side of their cell. They are a group of primitive eukaryotes and can be parasites, photosynthetic organisms or predators. The scientific support for this group is still quite weak and there is a good chance it will be reconstructed in the future.
There are three main groups within the supergroup excavata – diplomonads, parabasalids and euglenozoans. Diplomanads and parabasalids have modified mitochondria and euglenozoans have unusual flagella.
Diplomanads are single-celled protists with modified mitochondria called mitosomes. They are anaerobic protists because mitosomes are unable to use oxygen for respiration. A diplomonad cell has two nuclei rather than the usual one nucleus that most cells have.
Many species of diplomonads are parasites. Probably the best known diplomonad is the genus Giardia. Giardia lives and reproduces in the intestines of humans and many other animals.
Parabasalids also have unique mitochondria. The mitochondria of parabasalids are called hydrogenosomes. Hydrogenosomes respire anaerobically and release hydrogen as a byproduct.
The remaining excavates are known as euglenozoans. The cells of euglenozoa protists have unique flagella because the internal structure of their flagella are either crystalline or spiralled.
Euglenozoans are separated into two main groups – the kinetoplastids and the euglenids. Kinetoplastid cells contain one large mitochondria and a mass of DNA called a kinetoplast.
Euglenids are distinguished by a pocket at one end of their cell that two flagella protrude from. At the base of one of the flagella there is a pigmented eyespot and light detector. The eyespot and light detector work together to direct which way the flagella should move the cell.
Last edited: 13 March 2016