|Clickable Index of Protista|
You have learned that living organisms have at least seven unique properties listed in the index above. As Protista are living organisms, then they must possess these properties as well.
What are Protista?
We have learned that Biology can be illustrated as a tree of life:
The phylogeny above shows our previously-discussed Kingdoms Bacteria and Archaea, as well as the Kingdoms Plantae, Animalia, and Fungi. This diagram represents the reach of Kingdom Protista at its inception when biologists went from the three kingdom system (Monera, Plantae, Animalia) to the five kingdom system. At that time Bacteria and Archaea were together in Kingdom Monera.
The point of this diagram is that Protista is clearly a group of organisms from several areas of the tree of life. The Kingdom clearly included organisms that were unrelated to others. Biologists at the time were still dissecting out the tree and really were not sure of the relationships among these organisms. The kingdom was a convenient stop-gap measure the hold the mostly unicellular aquatic organisms until they could be fully studied and assigned to more natural groupings. After some additional research it was clear that protista needed to be refined...it contains several different lineages that are so different that they deserve kingdom status.
As you can see are some closely related groups that need to remain in Protista for further consideration (Archezoans, Euglenoids, and Protozoans, among others!). The chrysophytes and brown algae form a natural grouping for Kingdom Stramenopiles (aka Chromista). The red algae comprise yet another, Kingdom Rhodophyta. The green algae are really part of the clade of Kingdom Plantae. And the slime molds really are more closely affiliated with Kingdom Fungi and Kingdom Animalia. So it is in the refinement of Protista that biology moves from a five-kingdom system to eight or more kingdoms.
So the history of Protista can be summarized as a convenient catch-all kingdom in the transition from three to five kingdom systems...that has been refined into a much smaller group of species in the transition from five to eight or more kingdom systems.
For our study here, we shall take the more modern (more splitting than lumping) approach. So we will consider the Archezoans, Euglenoids, and Protozoans as major groups of Kingdom Protista...but there are other smaller groups as well. These organisms are related in terms of being on the one clade of living organisms, but their affinity as a coherent Kingdom is probably artificial.
|Protista Have Cellular Structure|
As previously mentioned, Protista was created to hold aquatic unicellular eukaryotic organisms. So obviously protists have eukaryotic cellular structure. As organisms branching out relatively low on the tree of life (perhaps 2 billion years ago), some of these have some primitive eukaryotic cell structure. This is particularly true of the archezoans. The archezoans are largely parasitic and what characterizes them as a group is that they lack mitochondria, golgi, and peroxisomes. Their ribosomes are 70S (prokaryotic size) rather than 80S (eukaryotic size). It is likely that archezoans are descendants of a group that was only part-way along the path to becoming truly eukaryotic. The step of taking in a bacterial endosymbiont to evolve into a mitochondrion or perhaps peroxisomes had not yet happened. The 400 species of archezoans then, are among the most-primitive of the eukaryotes. Examples include: Pelomyxa, Giardia and Nosema.
Pelomyxa is a free-living freshwater sediment organism. It uses phagocytosis to engulf its food and takes in endosymbionts. Most of the cells will have at least three different endosymbiont species, and two species known are methanogenic archaea! But these endosymbionts are taken up and can be released, so they are not yet evolved into the mutualistic associations we know as chloroplast or mitochondrion. However, like archezoans, the genome is held in a nuclear envelope, and the genome is contained in a collection of linear DNA molecules. The DNA is bound with histone proteins. The cell is amoeboid and has an anterior uroid (macropseudopodium) that assists in amoeboid movement. This is an organism I hope to have in the laboratory for our observations.
Giardia is a diplomonad intestinal parasite. It consists of two half-cells each with a nucleus. The cell has two anterior, four ventral, and two posterior flagella for motility. On the ventral surface of the cell are disc ridges that provide adhesion to the cells in the intestine of the host organism. One important host is beaver. The parasite forms a resting cyst and passes out of the digestive tract of the beaver into the pond where the beaver lives. Humans ingest the water with the cysts and the cyst grows into a trophozoite (feeding animal) and attaches to the intestinal wall of the human. The Giardia proliferates as digestion and reproduction continues. As large patches of the parasite are torn from the lining of the intestine by food passage, bleeding occurs. A major symptom of giardiasis is bloody diarrhea.
Willimantic's water supply comes from Mansfield Hollow Dam Reservoir. There are beaver living in that watershed and so the water supply is very likely to contain Giardia cysts. Fortunately, the Willimantic Water Works has installed new water filtration equipment that can remove the cysts before the water gets in the water mains of the town.
One other interesting member of this group is Trichonympha. This organism is part of the gut fauna of termites (insects that "eat" wood). This symbiont lacks mitochondria too and so survives anaerobically by fermentation. The protist, in turn, has its own bacterial endosymbionts that produce cellulase (the enzyme that breaks down cellulose in wood). The termite chews and swallows the wood bits and the protist takes the particles in by phagocytosis (endocytosis). The bacterial endosymbionts digest the cellulose into simple sugars that are fermented by the protist for its metabolism. The fermentation products leaking out of the protist are used by the termite for its metabolism.
An interesting twist on this siutation is that what appear to be flagella on the Trichonympha gut protist are also symbionts. These are ectosymbiotic spirochete bacteria that provide motility for the eukaryotic protist!
Nosema is a genus of microsporidian parasites of various arthropods. The cysts can be spread into the environment as a form of pesticide on crops. The Nosema proliferate in the insect, reducing its growth and ability to feed...and certainly inhibits its ability to reproduce. Because these parasites seem host-specific, they might be used to control specific pests. There is a species specific to grasshoppers, another specific to mosquitoes. Unfortunately there is a species of Nosema that is specific to honeybees, and this one causes diarrhea and winter-kill of entire colonies! Fortunately this parasite can be controlled by fumagillin-b, a natural product of a fungus (Aspergillus fumigatus) to help cure affected bee colonies.
A second major group of protists are the Euglenophyta/Euglenozoa which we will just call euglenoids here. These flagellated protists can be photosynthetic, but can live heterotrophically too. Their body is flexible as the internal cell wall is a protein pellicle rather than some rigid material on the exterior. The euglenoid mitochondrion has very odd mitochondria, with discoid cristae...but they DO have mitochondria!). The chloroplast has chlorophyll a and b but it has three external membranes. This evidence indicates that the photosynthetic euglenoids obtained their chloroplast from a secondary endosymbiosis...they took up a green alga not a cyanobacterium. In this way the photosynthetic euglenoids are actually advanced, and we do not have to explain photosynthesis evolving at a second place in the tree of life (in other words it is not a homoplasy). Their 400 species are found in aquatic environments, but some are also parasites.
One example parasitic euglenoid is Trypanosoma gambiense the causative agent of sleeping sickness. It is a blood-borne parasite showing flagellated undulating cells. It is vectored by the TseTse fly from one host to another. It is a chemical produced by the parasite the brings on the disease symptoms.
There is a small group of sarcodine protists that have amoeboid properties. The cell lacks any kind of external or internal surface adornment, but the cytoplasm is surrounded by a simple membrane. The cytoplasm does have an internal cytoskeleton of microtubules and microfilaments that flexibly allow for movement of the cell and for strong changes in shape. Living in fresh water (hypotonic), they have internal pressure regulated by contractile vacuoles. But the distinguishing feature of sarcodines is their basic amoeboid cell form. The protoplasm can extend outward in what are called pseudopodia (false legs). and can use these to ooze and flow around objects. The Amoeba shown here is engulfing a green alga (Staurastrum).
Marine relatives of Amoeba are known as foraminifera. These secrete an external shell called a test that provides some defense of the soft cell body. The tropical foraminifera in the Caribbean, Gulf of Mexico, and Bermuda have a calcified test of a bright red color. As these organisms die and their tests become part of the sediments, they make the sand of these shores quite pink. So if you have heard of the pink sands of Bermuda and the Bahamas, you can remember that this is a contribution from Kingdom Protista!
Some of the foraminfera are quite beautiful as you can see here. The main cell has very delicate pseudopodia radiating out from its test for feeding. You can also see that this one...either Orbulina universa or globigerina bulloides...receives nutrients from endosymbionts.
A freshwater relative of Amoeba is Arcella. This protist secretes a chitinous test (similar to fungal cell walls or insect exoskeletons). The cell shows hyaline connections between the test and the cytoplasm. You can see food vacuoles and that the animal has been feeding upon algae. Below the test you can see the pseudopodia extending out like the tentacles of an octopus for feeding and locomotion. In this micrograph it appears to be closing in a green algal cell as its next meal.
Actinopods and radiolarians have pseudopodia extending out through a test also, but for these protists, the test is neither carbonate nor chitin...instead these organisms accumulate silicates from the water. Their silicate tests can form interesting geological layers in sediments. Here you see Actinosphaerium eichhorni showing its silicate-stiffened axipods (not soft pseudopodia). Also here is a test of a radiolarian showing the openings in the test where living pseudopodia would emerge.
A major group of protists are the 9000 species of alveolates (ciliates). The alveolate cell structure involves some kind of funnel-like opening, often involved in feeding. This means that the cell has sort of a blind-ended depression in it that traps and engulfs the food particles from the environment. The particles are loaded into food vacuoles (phagocytosis) in the cytoplasm where digestion occurs.
These protists also have hair-like cellular projections known as cilia in various locations on the cell surface that are involved in either locomotion or feeding or both. The cells have contractile fibers, some of which are striated, to alter the shape of the cell. Some have stiffened plates just beneath the cell membrane. Some have toxic extrusomes (trichocysts) that can discharge into predators or prey; so they are used for defense or hunting or both! Inside the cell is usually one polyploid macronucleus that might be 860-ploid! This macronucleus is involved with normal cell functions such as homeostasis. The cell also has diploid micronuclei for sexual reproduction and for producing the macronucleus. These cells can conjugate for sexual reproduction.
Yet another group of protists are the dinoflagellates. These have cellulosic plates and their alveolus is a deep groove between the plates. They have two flagella for motility coming out at right angles from two parts of the groove. This gives them a kind of tumbling swimming motion. The dinoflagellates are aquatic and responsible for "red tides." Their red/brown pigments can make the water look like blood sometimes. But worse, saxitoxin, produced by dinoflagellates can cause widespread fish death, and can make poisonous the shellfish and otherwise saxitoxin-resistant species to the rest of the food chain that might feed upon them.
Other species of dinoflagellates are endosymbionts in coral polyps and are responsible for feeding the coral animals during the daylight hours. So they are the primary producers of coral reef ecosystems.
Another group of interesting protists, the apicomplexans, includes Plasmodium falciparum, the causative agent of malaria. This parasitic organism lives part of its life cycle in certain species of mosquito, (Anopheles gambiae). When the female mosquito bites a human for the blood meal for its brood, the sporozoites from the salivary gland of the mosquito go into the bloodstream of the human. These migrate to the liver and become parasitic merozoites in liver cells. These break out and infect red blood cells and undergo meiosis. The resulting gametocytes are shed into the serum and enter the blood meal of the next mosquito, form zygotes, and the oocysts form sporozoites that take up residence in the salivary glands. In this way, the malaria parasite can move from one mammmal host to another by means of the mosquito alternate host. For the human with this protist, the anemia and loss of toxin processing in the liver leads to the general malaise of malaria.
|Protists Have Homeostasis|
While archaezoa may live in microaerophilic conditions, their metabolism and that of the other protists are at least facultatively aerobic and their nutritional mode is chemoheterotrophic. So they carry out the usual sequence of glycolysis, TCA cycle, electron transport and oxidative phosphorylation. The euglenoids do have chloroplasts and can carry out photosynthesis; so they are uniquely photoautotrophic in this kingdom. However, euglenoids are capable of chemoheterotrophic growth just as well!
As you recall, growth, defined as an irreversible change in size, can be a function of both increases in cell size and cell number. After cell division, the growth of protists is like that of bacteria and archaea. The cells can double in volume between cell divisions. Probably because of the complexity of reproducing the endosymbionts in some synchronous way, the protist cell cycle is a bit slower. The presence of the nuclear membrane and the multiple linear chromosomes in eukaryotes generally means that the complexity of the genomic duplication and separation is slower too.
The cell division process in eukaryotes involves mitosis. The DNA is associated with histone proteins and is supercoiled. This supercoiling becomes more intense as the cell division time approaches. The chromosomes condense and can be observed individually with nuclear stains. The nuclear envelope disappears and the chromosomes, now free in the cytosol, are attached by their centromeres to microtubules. The chromosomes are pulled apart and the sister chromatids of each chromosome are pulled to opposite ends of the cell. Finallly the cell pinches off in furrowing.
We have observed that these organisms have either flagella and/or cilia for locomotion. Their internal myonemes (contractile fibers) allow the cell to change shape and alter the motility. Some protists move by the pressure of the cell altering the shape and direction of flow of cytoplasm in the various volumes of an amoebic cell.
Inside all protist cells, whether amoebic or not, materials are in motion as well. So these organisms have cyclosis just as all other living cells.
The movements are directed in chemotaxis for food. The euglena are phototactic when in a photosynthetic mode.
As discussed previously, protists can divide to produce more cells. This is a kind of reproduction that does not involve sex we call it asexual reproduction. In contrast to the bacteria and archaea, is not binary fission...indeed it is called mitosis. What you have learned about mitosis, such as prophase, metaphase, anaphase, or telophase applies very well to protists.
In interphase, the genome is relicated. In prophase, the chromosomes condense and the nuclear envelope breaks down. In metaphase, the chromosomes are attached to the spindle fibers and are pushed to the middle of the cell. In anaphase, the spindle fibers pull the chromosomes apart, the centromeres separate, the sister chromatids are pulled to opposite ends of the cell. In telophase, the chromosomes are resurrounded by a nuclear envelope, and the chromosomes decondense.
The euglenoids are unique among these organisms in their cell division. The chromosomes remain condensed for much of the cell cycle and the nuclear envelope never really disappears; instead the spindle operates around the through the envelope. Would that be transitional from prokaryotic to eukaryotic? Does this feature (among others) place them between the archezoans and the ciliates?
The ciliates also have a process for recombination of genomic information. We might call this sexual, but it works in a way that is very different from the usual concept for sexual reproduction.
Two cells join in conjugation, the micronucleus of each cell undergoes meiosis. Of the four haploid meiotic products, three disintegrate and the remaining one undergoes mitosis. The two cells exchange one of their two haploid micronuclei. Each cell now has two haploid micronuclei (one of its own and one from the partner). These fuse together in syngamy to produce a diploid micronucleus. As this process is complete, the old macronucleus disintegrates in each cell and is replaced by mitotic divisions of the new diploid micronucleus.
As you might gather from this description, sex here does not produce any offspring. The two partners conjugate, exchange DNA, and both are genetically altered by contributions of the other. Now that is strange sex...you find a beautiful mate, you conjugate, and now you have some of your mates genes and are perhaps more like the mate than you were before!
Can you tell from the sketch here, what stage of the conjugation process you are seeing? Would you say that the process in both cells is completely synchronous?
Throughout the material above, you have read about archaea responding to the environment, to the presence of chemicals, light, or other organisms. I don't think we need to amplify this too much right here. But in your course on genetics, you will learn a bit more about how eukaryotic genes are turned on and expressed which is quite different from the ways prokaryotic organisms transcribe and translate that result in responses!
Kingdom Protista as we have cast it here is still very likely to be paraphyletic (composed of unrelated groups). It requires further study and considerable refinement. We have examined some major groups but there are many other minor groups and how these relate to each other is unclear. How precisely these species relate to each other phylogenetically remains to be seen. Because of this, we cannot show an acceptable cladogram here.
This page © Ross E. Koning 1994.
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