Protist (MM)

by Isabelle Levenson


The Kingdom Protista consists of unicellular eukaryotes, as well as their relatively simple multicellular relatives. This is not a monophyletic grouping, rather it is paraphyletic, although specialists now split most of the members into five or more newly designated kingdoms, as well as assigning certain groups that are a part of Protista to Plantae, Fungi, and Animalia. (In cladistics, a monophyletic group is a the group of organisms forming a clade, inclusive of all possible descendants. On the other hand, a polyphyletic group includes one whose last common ancestor is not a direct member of the group). (MB) The members of this kingdom vary in structure and function more than any other group of organisms. Of the approximately 60,000 known species of extant protists, most are unicellular, although some are multicellular or colonial. Protists are considered the simplest eukaryotic organisms, but at a cellular level, many of these organisms are quite complex. These eukaryotes must carry out, within the boundaries of a single cell, all the basic functions performed by specialized cells in the bodies of plants and animals. Most are aerobic, using mitochondria for cellular respiration, while few lack mitochondria and therefore they live in anaerobic environments or contain mutualistic respiring bacteria.
The word protist comes from the Greek word proto, meaning first. Protozoa combines proto and zoion, meaning animal. Scientists may have thought that protists were the first eukaryotes to evolve on Earth. (JS)


Protists are the most nutritionally divers of all eukaryotes. Some protists are photoautotrophs and contain chloroplasts, while others are heterotrophs, absorbing organic molecules or ingesting food particles. Others are mixotrophs, meaning that they combine photosynthetic and heterotrophic nutrition. Nutrition is therefore useful, in an ecological context, for understanding the adaptation of protists and the roles that they play in their communities and for dividing them into three catergories: those that are animal-like or ingestive (protozoa), those that are fungus-like are absoptive (there is no general name), and those that are plant-like or photosynthetic (algae).

Feeding mechanisms and their use are diverse among protists. They include the capture of living prey by the use of encircling pseudopodial extensions (in certain rhizopods), the trapping of particles of food in water currents by filters formed of specialized compound buccal organelles (in ciliates), and the simple diffusion of dissolved organic material through the cell membrane, as well as the sucking out of the cytoplasm of certain host cells (as in many parasitic protists). In the case of many symbiotic protists, methods for survival, such as the invasion of the host and transfer to fresh hosts, have developed through long associations and often the coevoltion of both parterners. (RW)


Most protists are motile; they either have flagella or cilia at some point in their life cycles. As a note, eukaryotic flagella and cilia are extensions of the cytoplasm with bundles of microtubules covered by the plasma membrance. Cilia are shorter and more numerous than flagella. The movement of cilia is often described as whip like whereas the movement of flagella is often described as propeller-like. Despite their differences in length and number, both cilia and flagella have the same underlying structure, consisting of ten bundles of paired microtubules.

Some protists also use pseudopodia to move. These are temporary elongations of cytoplasm that pull the cell along. Amoebas are famous for this mechanism of locomotion. (PS Source 12)

Protists with visible flagella (VN)
Protists with visible flagella (VN)

Life Cyles and Reproduction

The life cycles of protists are highly varied. Almost all experience mitosis, but there are variations in the process unknown to other eukaryotes. Some are exclusively asexual, others have the ability to reproduce sexually or they employ the sexual processes of meiosis and syngamy (the union of two gametes), therefore shuffling the genes between two individuals that can then go on to reproduce asexually. At some point during the life cycles of many protists, resistant cells or cysts form that can sruvive under harsh conditions.
Most protists are aquatic and can be found anywhere there is water. Protists are important constituents of plankton and phytoplankton, therefore contributing to the marine and freshwater communities. In addition to those protists that are free-living, many of the organisms are symbionts that inhabit the body fluids, tissues, or cells of a host in a symbiotic relationship ranging from mutualism to parasitism.

Slime molds (phylum mycetozoa) have a particularly strange form of reproduction. For instance, in the genus Physarum, there are over 29 variants of sex genes in eight different types of sex cells. Moreover, as long as two organisms are completely different from each other, any duo can reproduce! That's over 500 different combinations in the genus alone. (Matt B - Source 7)

Reproduction of Tetrahymena thermophila, a species of Protist.
Reproduction of Tetrahymena thermophila, a species of Protist.


Osmoregulation is the process by which cells maintain the balance of water content within their cells. This translates to cells actively regulating their water content using mechanisms within the cell. In the case of the Paramecium Caudatum, belonging to the protist, the hypotonic environment of the organism tends to diffuse the water from the environment into the organism. This process is balanced by the paramecium's contractile vacuoles actively pumping out the water back into the environment. The contractile vacuoles can be observed by using a simple light based microscope. The paramecium also balances internal water content through the use of salt crystals within itself. If the paramecium needs to absorb more salt, it will release these crystals. (RK)

Temperature control
Protists will decrease in size as the temperature decreases. Protists as mostly one celled organisms and largely are dependent on there environment for the temperature control.(RJ)

Brief Summaries of Different Types of Protists

As earlier discussed, the Kingdom protista is not a monophyletic grouping. What is problematic about the classification of “protista” is its paraphyletic nature. The necessity for a new classification system for these organisms arose due to the fact that virtually all motile unicellular organisms were categorized as being in a single phylum, with little regard to evolutionary relationships. In fact the common ancestor of the Protista Kingdom is exceedingly ancient. As a result the kingdom exhibits all types of symmetry, adaptations for all types of conditions, and a great range of structural complexity as noted by the many different types below. (ZJ- source 10)

The color red refers to a larger clade within Figure 28.8 on the right. The color blue refers to the divisions with in the clade as designated by the figure. Purple refers to groups not included in the diagram, while green refers to divisions within those groups.

Diplomonads and parabasalids (including trichomonads) lack mitochondria. Diplomonads have multiple flagella, two separate nuclei, a simple cytoskeleyon, and no plastids. These two groups are on the phylogenetic branch which diverged earliest in eukaryotic history.

For euglenozoa, there are two groups included in the clade: euglenoids and kinetoplastids.
--------Euglenoids are characterized by an anterior pocket or chamber from which one or two flagella emerge. Paramylon, a glucose polymer that functions as a storage molecule, is also characteristic of euglenoids, which can be autotrophic but are mainly mixotrophic or heterotrophic.
--------Kinetoplastids have a single, large mitochondrion associated with a unique organelle called the kinetoplast that houses extranucleur DNA. These can be symbio
Figure 28.8: A tentative phylogeny of eukaryotes. Those that are yellow correspond to Protista.

tic, although also can be pathogenic.

Alveolata are known for have small membrane-bounded cavities or alveoli under their cell surfaces, and although the function of alveoli is unknown, many speculate that they stabilize the cell surgace or regulate water and ion content. The three types of protists in alveolata are dinoflagellates, apicomplexans, and ciliophora or ciliates.
--------Dinoflagellates are mostly unicellular, although some are colonial, with each species having a characteristic shape reinforced in some species by internal plates of cellulose. The action of two flagella in perpendicular grooves within the "armor" make a spinning movement. These organisms are abundant components of populations of phytoplankton and blooms of dinoflagellates cause "red tides" in coastal waters. Also some dinoflagellates are carnivorous, while others are bioluminescent.
--------Apicomplexans are parasites of animals which disseminate as tiny infectious cells called sporozites. The apex of this sporozite cell will contain a complex of organelles specialized for penetration host cells and tissues. These organisms experience complex life cycles, with both asexual and sexual stages. They also often require two or more different host species.
--------Ciliophora use cilia to move and feed, and most of them live as solitary cells in freshwater. Some may be covered by cilia while others have fewer cilia or tufts of cilia... these adaptions are for the diverse lifestyles of the organisms. Ciliates have two types of nuclei - a large macronucleus (which contains 50 or more copies of the genome and controls everyday functions of the organism) and several tiny micronuclei. Genes for these are not distributed in typical chromosomes; instead they are packaged into a larger number of small units with hundreds of copies of just a few genes. Ciliates usually divide by binary fission with sexual shuffling of genes occuring during conjugation and the sexual mechanisms of meiosis and syngamy acting separately to reproduction.

Stramenopila include several heterotrophic groups and a variety of photosynthetic protists (algae). These organisms are known for numerous fine, hairlike projections on their flagella. In most cases, this "hairy" flagellum is paired with a smooth, non-hairy flagellum. For most life cycles of these organism, only flagellated stages are for motile reproductive cells.
--------Oomycotes, which include water molds, white rusts, and downy mildews, are heterotrophic. Some are unicellular. Others have hyphae (or fine, branching filaments) that are multi-nucleated. Water molds and their relatives have cell walls made of cellulose and also a diploid condition, biflagellated cells, and sexual reproduction (egg fertilized by "sperm nucleus" produce a resistant zygote) prevail in the life cycles of these organisms. Many are decomposers, but could also be parasitic or pathogenic. Oomycotes may be similar to fungi in their filamentous bodies with extensive surface areas, but they are not closely related.
--------Diatoms are yellow or brown with glasslike walls made of hydrated silica embedded in an organic matrix. These organisms spend most of the year asexual by mitosis and the regeneration of the second half of the cell wall, although sexual stages are common and cysts can be formed by some in resistant stages. Diatoms also store food reserves in the form of a glucose polymer called laminarin or in the form of oil.
--------Golden algae or chrysophytes are named for their color, which is created by accessory pigments. They are typically biflagellated, some are mixotrophic, most are unicellular, some are colonial, and if the cell density reaches a certain high level than many form cysts.
Chrysophytes are mostly autotrophic and perform photosynthesis. Gold algae are not considered completely autotrophic. This is because when there is not enough light present or a surplus of food and nutrients, Gold algae will become heterotrophic, meaning they will consume other organism, like bacteria and diatoms. They are very important in lake ecosystems, for they are the base of the food chain and are eaten by zooplankton. (MS 11)
--------Brown algae or phaeophytes are all multicellular, most are marine, and have a plastid structure and pigment composition homologous to the photosynthetic equipment of golden algae and diatoms. Brown algae also undergo the alternation of generations where diploid sporophytes become haploid gametophytes and continue alternating between the two.

Red algae or rhodophyta do not have flagellated stages and are reddish due to accessory pigments, although not all red algae is actually red. Water depth tends to affect the proportions of accesory pigments in the species inhabiting there. Most are multicellular. The thalli (plantlike seaweed body) are filamentous while the base is differentiated as a simple holdfast (rootlike and achors the algae). These organisms have diverse life cycles, depend on water currents for reproduction, and commonly experience the alternation of generations.

For the clade chlorophyta, members are green algae (or chlorophytes and charophyceans). They have green chloroplasts, similar in structure and pigment composition to plants. There are more than 7,000 identified species of green algae, most of which are freshwater. Many live as plankton, symbiotically within other eukaryotes, or with fungi in lichens. These organisms can either be unicellular, multicellular, or colonial. They have complex life histories with both asexual and sexual life cycles.

Mycetozoa are slimemolds. Their resemblance to fungi is analogous, and all slime molds use pseudopodia for movement and feeding. There are two types: plasmodial and cellular.
---------Of plasmodial slime molds or myxogastrids, many are brightly pigmented, all are heterotrophic, and most are diploid. The feeding stage is an ameoboid mass called a plasmodium, which is large but not multicellular and is a single mass of cytoplasm undivided by membranes and filled with many nuclei are a result of mitotic divisions without cytokinesis. Cytoplasmic streaming helps to distribute nutrients and oxygen.
--------For cellular slime molds or dictyostelids, the feeding stage consists of solitary ells that function individually. When food is depleted, all the cells will form an aggregaate that will function as a unit. This type of slime mold is different from plasmodial slime molds because the cells of cellular slime molds retain their identity and remain separated from each other by membranes. These organsims are haploid with no flagellated stages and with fruiting bodies that function in asexual reproduction.
Three groups were not included on the phylogenetic tree because little is known about their phylogeny, although they represent several distinct eukaryotic lineages. Most of these groups are heterotrophs that actively seek and consume bacteria, other protists, and detritus.
---Rhizopods, or amoebas, are all unicellular and utilize pseudopodia (cellular extensions) for movement and for feeding. These pseudopodia can bulge from virtually anywhere on the cell surface with the cytoskeleton consisting of microtubules and microfilaments functioning in amoeboid movement. This movement is often directed although it appears chaotic, and also some amoebas live inside a secreted protein shell. Amoebas inhabit both freshwater and marine environments while also being abundant in soils, with a majority of the organisms being free-living although some are parasites.
---Actinopoda have slender pseudopodia called axopodia that radiate from the organism and that are reinforced by a bundle of microtubules covered by a thin layer of cytoplasm. Most are planktonic and their projections are then utilized to help float and feed. Heliozoans live in fresh water, and their skeletons consist of siliceous, or glassy, chitinous unfused plates.
Heliozoans are spherical actinopods. They may float or have stalks. Usually, theyr’e surrounded by a shell that is silica or organic material (like scales). They radiate “pseudopods, radiating cytoplasmic masses” (Encyclopedia Britannica). Surprisingly, these function mostly for feeding purposes, moreso than transportation. The pseudopods vary in shape, making each heliozoan distinctive. Many of them are very beautiful to look at under a microscope. Some types have flagellated gametes. (LPE)
Radiolarians are the several groups of mainly marine actinopods that have skeletons commonly made of silica that are fused into one delicated piece.
---Foraminiferans, or forams, are mostly marine and named for their porous shells that are generally multichambered and consist of organic material hardened with calcium carbonate. Pseudopodia extend through the pores and are useful in swimming, shell formation, and feeding.

external image euglena2.gif

The chart above provides an excellent summary of the different types of protist phyla mentioned above in greater detail. (MR; Source 13)
The chart above provides an excellent summary of the different types of protist phyla mentioned above in greater detail. (MR; Source 13)
Review Questions:
1) Some scientists believe that protists were the first Eukaryotic cells on Earth. What characteristics of different types of protists are characteristics of later, more common, eukaryotic species?
2) Describe feeding mechanisms in protists. (CM)
3) How do organisms in the kingdom protista move (i.e describe the mechanisms and their functions)? (SP)
4) What is/are the difference between diplomonads and parabasalids? (JP)

Campbell, Neil A., and Jane B. Reece. "Chapter 28: The Origins of Eukaryotic Diversity." Biology. San Francisco: Benjamin Cummings, 2002. 545-71. Print. (LPE)

"Osmoregulation in Paramecium." JCCC Staff and Faculty Pages. Web. 30 Oct. 2011. <>. (JS) (TB) (RW) (MM)
7. (Matt B) (picture of reproduction of protists) (AR)

8. Photograph. Protists. Web. 11 Nov. 2011. <>. (VN)

9. (AP)

10. Barnes, Robert. Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.
"Introduction to the Chrysophyta Golden Algae." Web. 13 Nov. 2011. <>.

Protist Kingdom. N.d. Biology Web Links//. Central Okanagan, School District No. 23, n.d. Web. 3 Dec. 2011.<>.