By Jarrod Stein
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Diagnostic Characteristics
Protists are a unique group of eukaryotic organisms because they defy the characteristics that would traditionally group them into one kingdom. Recent phylogenetic studies have shown that they are too diverse for specific classification since they vary in structure and function more than any other group. Usually characterized as the eukaryotes that are not plants, fungi, or animals, protists are mostly unicellular and microscopic; however, many are complex since they must perform all the basic functions of specialized plant and animal cells in the boundaries of a single cell. There are multicellular exceptions as well. Some protists are photoautotrophic, primary producers that undergo photosynthesis, and others are heterotrophs, consumers of other organisms. There are even those who are mixotrophs, which means that they act as both photoautotrophs and heterotrophs. In terms of motility, most have flagella or cilia at some point in their life cycle. Unlike those of prokaryotes, a protist's flagella, long whip-like cellular appendages specialized for locomotion, and cilia, short hair-like cellular appendages specialized for locomotion, are extensions of the cytoplasm rather than attached to the cell surface. Although flagella are much longer than cilia, they are made out of a similar material. Generally if a cell has flagella, it has only a couple, whereas if a cell has cilia, it will have tons of them.Some protists such as those in the kingdom Amoebozoa (includes the amoeba) locomote via pseudopods formed by internal cytoplasmic flow. Some protists are pathogens of animals and plants. For example, Plasmodium falciparum causes malaria in humans and Phytopthora infestans causes potato blight.

Major Types
Protists exhibit many different types and morphologies.
Protists exhibit many different types and morphologies.
Protista as a whole group is considered to be paraphyletic, a term describing a taxon that excludes some members that share a common ancestor with included members.The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. .Protists have traditionally been divided into three categories: ingestive animal-like protists, absorptive fungus-like protists, and photosynthesetic plant-like protists. Recently, discoveries have been made that illustrate how some types of protists that are closely related to plants, fungi, and animals and others are very distant. Therefore, we can divide the protistan "kingdom" into different clades, evolutionary branches.

This phylogenetic tree of the diversity of protists shows us the relationships among protists and with other eukaryotes as well.
This phylogenetic tree of the diversity of protists shows us the relationships among protists and with other eukaryotes as well.

Diplomonadia and Parabasala:
Giardia lamblia
Giardia lamblia
The main distinction of these two unicellular groups is that they lack mitochondria, organelles where cellular respiration takes place. A recent discovery has shown that their ancestors had lost mitochondria as they evolved. Diplomonads are unique in baring two separate nuclei. They have multiple flagella and a simple cytoskeleton. One example is Giardia lamblia, the parasite that causes Giardia by infecting the human intestine. Parabasalids differ with their undulating membrane that enables them to move through reproductive and urinary tracts that are coated with mucous. Trichomonas vaginalis is a parasitic parabasalid that causes the known sexually transmitted disease Trichomaniasis by infecting the human vagina and urethra.

This is a group made up of flagellates, organisms with flagella that function in motion. They can be photosynthesetic, heterotrophic, or mixotrophic. Kinetoplastids, one division, are symbiotic and associated with a kinetoplast, a unique organelle that houses extranuclear DNA. Trypanosoma are pathogenic kinetoplastids that cause African sleeping sickness. Euglenoids, the other division that includes the autotrophic Euglena, use the glucose polymer paramylon as a storage molecule. They also have one or two flagella that emerge from an anterior pocket. In the Euglenoid line individuals divide longitudinally in which division begins with the duplication of the basal body at the base of the flagellum creating a cell with two flagella that then splits right down the middle.

The unicellular alveolata are characterized by their subsurface alveoli, which are membrane-bound cavities. Dinoflagellates make up one group. They have internal plates of cellulose that cause their flagella to spin. Many are components of phytoplankton. Dinoflagellate blooms cause red tide in which their toxins kill many fish. Apicomplexans, another group, are all parasites. The Plasmodium, in particular, causes malaria and spends most of its time inside the human liver and blood cells. Ciliates, the third group of alveolata, use cilia for movement and feeding. These very complex protists, which live a solitary life in freshwater, have a large macronucleus and several tiny micronuclei. The Sentor and Paramecium are both found in this group.

There are currently over 100,000 known species of stramenopila, and most are diatoms, a common type of phytoplankton. This group of protists is named for the hair-like projections on their flagella. Oomycotes, one category known as "egg fungi," include water molds that grow as cottony masses on dead algae and animals that they decompose. Some include fungus-like hyphae, branching filaments that absorb nutrients. Another group, bacillariophyta, includes diatoms, yellow and brown algae characteristic of glassy, two-part walls. They reproduce aesexually by mitotic cell division. The other two categories are also algae: chrysophytes, golden algae that are mainly unicellular, and phaeophytes, brown algae that are all multicellular, such as kelp.

red algae
red algae
Also known as red algae, rhodophyta are mostly multicellular, lack flagella, and reproduce sexually. They can become fairly large like seaweed. These usually grow attached to rocks or other algae, but there are a few free floating forms, found either in unicellular or colonial forms. Red algae lack flagella and centrioles; however, they have phycobiliproteins as accessory photosynthetic pigments, which is why they are red. Red algae are ecologically significant as primary producers, providers of structural habitata for other marine organisms, and their important role in the primary establishment and maintainence of coral reefs.

Also known as green algae, chlorophyta have plant-type chloroplasts, hence the name. Some examples draw from a wide range of sizes. For instance, Chlamydomonas are simple, biflagellated (two flagella) unicells; Volvox are colonial, which means they contain many individual cells held together in one colony; and Ulva, also known as "sea lettuce," is multicellular. Ulva is macroscopic, its body is 2 cells thick, and there is a division of labor among the cells. Chloyophyta consists of both unicellular and multicellular organisms. Most species of Chlorophyta lives in freshwater habitats and some others live in marine habitats. However, some, such as the Watermelon Snow (Chlamydomonas nivalis), are adapted to a wide range of enviroments.

Pseudopod-equipped protists:
Amoeba Proteus
Amoeba Proteus
Although there is no yet determined name for this clade, these protists are characterized by their pseudopodia, cellular extensions used for movement and feeding. Their phylogeny is uncertain, but we do know that they are mostly heterotrophs that actively seek and consume bacteria, detritus, and other protists. Three categories of these organisms include rhizopodia, the group with the famous Amoeba that is unicellular and has lobe-like pseudopodia; actinopodia, planktonic protists with axopodia (ray-like slender pseudopodia); and foramnifera, which have porous shells of calcium carbonate.In environments which are potentially lethal to the cell, an amoeba may become dormant by forming itself into a ball and secreting a protective membrane to become a microbial cyst. The cell remains in this state until it encounters more favorable conditions. While in cyst form the amoeba will not replicate and may die if unable to emerge for a lengthy period of time.

slime mold
slime mold
This group of protists contains slime molds reminiscent of fungi. What sets them apart from the fungi kingdom, however, are their complex life cycles with amoeboid stages. They too use pseudopodia for movement and feeding. One category, myxogastrids, includes plasmodial slime molds, which are all heterotrophic and brightly pigmented. In their feeding stage, they form a net-like amoeboid mass. Another category, dictyostelids, includes cellular slime molds, which function as individual amoeboid cells in their feeding stage but form an aggregate of reproductive colonies.

This is a video displaying 5 hours of slime mold formation in 15 seconds. The process is shown via fluorescence microscopy.

The general habitat for most protists is in aquatic areas, whether marine or freshwater. This includes damp soil and even polar bear hairs. In any case, certain niches pertain to specific protists. For instance, red algae is the most abundant large algae in warm coastal waters. Some protists like only moist spots in dirt or shore sand. Other protists are parasitic and require a host as their basic habitat for nourishment and reproduction. Protists can be commensalistic as well; for example, certain protists perform vital digestive processes in the intestinal tracts of their hosts.

Basic Anatomy
Since protists vary in type, they are not all anatomically similar. The unicellular protists share all the characteristics of eukaryotic cells of other organisms: membrane-bound organelles, a cytoskeleton, and multiple chromosomes bound by proteins. Some protists are ambiguous in having no mitochondria or plastids whereas others have many of each. A few types, such as the paramecium in the diagram below, even have more than one nucleus. Many bare cilia or flagella for motility. As for the multicellular algae, their anatomical structures are analagous to plants, fungi, and animals. Refer to the section Environmental Adaptations for further information on analagous structures.
A diagram of a Paramecium's anatomy pictured with a view under a microscope
A diagram of a Paramecium's anatomy pictured with a view under a microscope

Transport Of Materials
Despite the compexities of a single-celled protist, they transport materials in a very simple fashion. Flagellates filter feed, which means that they strain suspended matter and food particles from the water. The most common method of nutrition consumption is endocytosis, the process by which cells absorb the food by engulfing it with their cell membrane. There are two major types of endocytosis; pinocytosis and phagocytosis. Pinocytosis is when a liquid material is absorbed or engulfed by a cell, and phagocytosis is when a solid material is engulfed by a cell. Some, like the Amoeba, use phagocytosis (seen below), the cellular uptake of large substances using (in this case) pseudopods, as a means of getting food. Any with plastids bring in materials using photosynthesis. Fungus-like protists, such as water molds, use hyphae to absorb necessary nutrients.

Helping exemplify the diversity of protists, reproduction varies in accordance with certain types. At some point, almost all undergo mitosis but in various forms. Asexual reproduction, in which one parent splits into two or more genetically identical offspring, is common among the single-celled protists. Even during asexual reproduction, some protists show signs of conjugation, the sexual shifting of genes, which require meiotic processes and syngamy, the union of two gametes. Other protists are sexual or include a sexual process in their reproductive mechanism, such as in a Plasmodium during the spread of
This diagram exemplifies the unique reproductive diversity of protists in slime molds, which cycle between sexual and asexual reproduction.
This diagram exemplifies the unique reproductive diversity of protists in slime molds, which cycle between sexual and asexual reproduction.
malaria. For most protists, the zygote (fertilized egg) is diploid, which means containing two sets of chromosomes (one from each parent), and the adult is haploid, which means containing only one set of chromosomes. An anomaly in reproduction is found in the life cycle of slime molds, which combine sexual and asexual mechanisms as shown to the left.

Environmental Adaptations
Protists have made some significant adaptations to their environment during their evolution. Algae, notably multicellular algae, have adapted to live on the coast in both high and low tides. Some anatomical structures analagous to plants have aided in these adaptations. For example, the seaweed body that is plant-like, the thallus, lacks true roots, stems, and leaves typical of a plant. The thallus is made up of a holdfast, a root-like structure that anchors the organism, and a stipe, a stem-like structure that supports leaf-like blades. Yet another adaptation, cellulose and gel-forming polysaccharides in the cell walls help cushion the thalli against the agitation of waves. Important adaptations have occurred in single-celled protists as well. Many have the capacity of become cysts, cells resistant to harsh conditions. Another example of a protist that adapts under harsh conditions is that of slime molds. In the plasmodial stage, sporangia, stalked fruiting bodies that produce haploid spores, will form to send its life elsewhere.

Some protists, notably algal and parasitic protists, have periods of dormancy during their life cycles. They enter these periods when environmental conditions are unfavorable, such as when temperatures are too high or the food supply is low.

Review Questions:
1. Protists often exhibit two forms of endocytosis: pinocytosis and phagocytosis. What are these two forms and how do they allow protists to obtain necessary nutrients?
2. Why is protista unable to be specifically classified as a kingdom?
3. What is the most common method of nutrition consumption?
4. What are some of the adaptations that protists have made to their environment?
5. Describe some of the major types of protists (red algae, green algae, mycetoza). How are they different from similar organisms of separate classifications?
6. What is unique about slime molds, especially in regard to their reproductive cycle?

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