You should have a working knowledge of the following terms:
Introduction and Goals
Figure 1. Giardia lamblia, a protistan intestinal parasite.(Click to enlarge)
This tutorial will introduce protists, which are relatively simple eukaryotic organisms. The taxonomy of this group is currently under extensive revision, therefore, no two textbooks present the same taxonomic scheme. Indeed, the emerging science of comparative genomics is almost daily revising the structure of the tree of life. For more on this emerging field, read this Discover Magazine blog post entitled "Festooning the Tree of Life".
This course will present a scheme that uses three domains, and five kingdoms will be discussed. Recall, the non-mitochondrial/chloroplast DNA in a eukaryotic cell is located within the nucleus, which is surrounded by the nuclear envelope. Eukaryotic cells also possess extensive systems of intracellular membranes that provide elaborate internal compartmentalization. Additionally, they generally have organelles (e.g., mitochondria and, in some cases, chloroplasts). In contrast, prokaryotic DNA is concentrated in the nucleoid region, which is analogous to the eukaryotic nucleus, but is not membrane-bound (discussed in Tutorial 18). Prokaryotes do not have mitochondria or chloroplasts, and they generally possess internal membrane systems that are far less complex than those found in the eukaryotes.
This tutorial contains a number of links that have been included for your interest. The material in these Web sites need not be memorized unless the material is covered directly in the text of the tutorial.
By the end of this tutorial you should have a working knowledge of the following:
- Three of the many protistan kingdoms: Archaezoa, Euglenozoa, and Alveolata
- The role of protists in various mammalian diseases (e.g., giardiasis, trypanosomiasis, and malaria)
- Dinoflagellates and their ecological roles in various habitats (e.g., coral reefs and red tides)
Protists are, for the most part, single-celled organisms. Like other eukaryotes, they have membrane-bound nuclei and organelles. Protists are relatively simple organisms, only because most are unicellular. However, a closer inspection reveals that they really are surprisingly sophisticated. All of the essential metabolic functions that occur in multicellular organisms (e.g., respiration, photosynthesis, digestion, and excretion) also occur in protists. Moreover, they can show sophisticated locomotion. The ubiquitous Euglena, found in most freshwater ponds, illustrates such protistan sophistication.
Figure 2. The golden alga, Mallomonas sp. (Click to enlarge)
Euglena possess (within a single cell): a flagellum for locomotion; an eyespot and light detector which, when used in concert, allows them to discern the direction of the most intense light; chloroplasts and mitochondria; granules of the polysaccharide paramylum for storage of surplus carbohydrates from photosynthesis; a contractile vacuole for osmoregulation; a nucleus; and an extensive endomembrane system.
Figure 3. A drawing of a Euglena. (Click to enlarge)
Protists are a metabolically and ecologically diverse group. Metabolically, they range from photosynthetic autotrophic species to heterotrophic species. They occupy diverse ecological habitats ranging from marine, to freshwater, to terrestrial. They have diverse relationships with other organisms, and are found free-living or as members of various symbioses.
With all of this diversity, protistan taxonomy is uncertain and the field is currently undergoing major changes. Some of these changes are confusing and reflect the uncertainty of how to best classify these organisms.
Many scientists still place all protists together into one kingdom, Protista. Although there are arguments for this classification scheme, in this course we will use the three-domain system. This newer system accommodates the great diversity of protists and recognizes many kingdoms within this group. Scientists working with protists generally agree that these organisms, as a group, are polyphyletic. That is, based on genetic sequence analyses and investigation of cellular components, protists arose from several distinct ancestors; therefore, they should not be placed in a single kingdom. For the sake of convenience we will use the term "protist," but keep in mind that this is not a taxonomic term; rather, protist describes a diverse group of mostly single-celled eukaryotes.
Figure 4. The Three-Domain System for Classifying Life. (Click to enlarge)
You will learn about three of the five kingdoms that have been best characterized: Archaezoa, Euglenozoa, and Alveolata. In Tutorial 30, two more kingdoms will be examined: Stramenopila and Chlorophyta. There are many protists that do not fit into these five kingdoms, and many more kingdoms are emerging as work on these groups progresses. We will briefly cover one group of unclassified protists, the slime molds.
The kingdom Archaezoa consists of diplomonads, trichomonads, and microsporidians. These three light micrographs show representatives of each of these three groups. All three groups form parasitic symbioses with a variety of other organisms (including humans). They retain primitive characteristics, and therefore are considered to be the most primitive of protists.
Figure 5. A diplomonad. (Click to enlarge) Giardia lamblia, an intestinal parasite.
Figure 6. A Trichomonad: Trichomonas vaginalis. This specimen was obtained from an in vitro culture.
Figure 7. A microsporidian. (Click to enlarge) Enterocytozoon, an intestinal parasite.
Unlike other protists, the archaezoans lack mitochondria. There is some debate about what this means. It is unclear whether this lineage split off from other protists before the acquisition of mitochondria, or whether the archaezoans secondarily lost their mitochondria because these organelles were not needed in the anaerobic environments characteristic of many parasitic lifestyles. It is even possible that certain groups within the kingdom Archaezoa lost their mitochondria and that other groups never had them. In other words, there is a good chance that this group is polyphyletic.
One archaezoan, the diplomonad Giardia, is an intestinal parasite responsible for the disease giardiasis or "beaver fever," which is generally transmitted via contaminated drinking water. Giardia attaches to the intestinal epithelium, or lining, in humans and other mammals (e.g., dogs, cats, bears, and beavers), creating severe diarrhea and intestinal cramps. Unlike bacterial infections, in which a large number of organisms must be ingested to cause illness, a few Giardia individuals can cause a severe case of giardiasis. The parasite is endemic to many wilderness areas; infection can be prevented by treating the water either with a biofilter or with chemicals such as bleach. While it is a decidedly unpleasant disease to have, giardiasis is fairly easily treated with drugs.
Figure 8. Giardia intestinalis in culture. (Click to enlarge) In these preparations, the flagella (four pairs per cell) are clearly visible.
It is easy to speculate about why Giardia and its fellow parasites in the kingdom Archaezoa do not require mitochondria. Most of them spend the active portion of their lives under anaerobic conditions, deep within the body cavities of other organisms; aerobic respiration may not always be possible under these circumstances, nor is it necessary since parasites acquire ample energy directly from their hosts. Giardia and other archaezoans must spend a portion of their lives outside their host during transmission between individuals. In the case of Giardia, cysts offer protection from an unpredictable environment during the protists' passage from the intestine of one host to the mouth of another. Cysts can exist outside of the body for months without losing their ability to infect new hosts.
Figure 9. Cysts of Giardia intestinalis from a human patient. (Click to enlarge) Note that each Giardia cell has two nuclei, as do all diplomonads.
Although the euglenozoans are monophyletic (derived from a single ancestor), they are a diverse group in terms of their feeding strategies. Kingdom Euglenozoa includes heterotrophs, such as Trypanosoma, and autotrophs, such as Euglena (shown here). This kingdom can be divided into two groups. The first consists of Euglena and its relatives, collectively called the euglenoids. These organisms are primarily photosynthetic. All euglenoids have flagella (structurally different from those of prokaryotes) and all store surplus carbohydrates from photosynthesis as paramylum. Click here to see movies of Euglena at the Website called "Molecular Expressions."
Figure 10. A Euglena sp. (Click to enlarge)
The second group of euglenozoans is the kinetoplastids, which includes Trypanosoma and its relatives, all of whom have one large mitochondrion and an organelle called a kinetoplast, which stores extranuclear DNA. All kinetoplastids live in symbiosis with host organisms, and a number of them are pathogenic. For example, the disease African trypanosomiasis, or African sleeping sickness, is caused by euglenozoans from the genus Trypanosoma. African Trypanosoma species are transmitted between mammalian hosts by the tsetse fly, a biting insect that transfers the parasite from the blood of infected individuals to the blood of uninfected hosts. The symptoms of African sleeping sickness include fatigue and confusion, which results after Trypanosoma invades the central nervous system. This disease, although fatal if left unchecked, can be treated.
Figure 10. Trypanosoma brucei (blue) and red blood cells (gray) of an individual infected with African sleeping sickness. (Click to enlarge)
Another parasite from this same genus causes Chagas disease, or American trypanosomiasis, which is transmitted by insects known as "kissing bugs." Chagas disease, which occurs primarily in rural areas of Central and South America, affects between 8 and 11 million people and kills nearly 40,000 every year. It tends to be a chronic infection that causes acute symptoms (e.g., gastrointestinal discomfort and heart enlargement or other cardiac problems) between 10 and 20 years after infection.
Figure 11. Trypanosoma cruzi (purple) among human red blood cells (beige). (Click to enlarge) Trypanosoma cruzi is responsible for Chagas disease.
There is some speculation that the debilitating fatigue and gastrointestinal distress that plagued Charles Darwin for the better part of his life after his return to England from South America was actually due to a chronic infection of Chagas disease.
Preventive measures for both African sleeping sickness and Chagas disease are largely focused on vector control, which ranges from insecticide-treated bednets (ITNs) to the "sterile insect technique", which is a controlled release of sterilized male insects into the vector population in an effort to reduce total fertility rate.
Introduction to Kingdom Alveolata
The kingdom Alveolata is another diverse group. It includes some of the most familiar and ecologically interesting protists in existence, including those that build coral reefs, the organism that causes malaria, and the algae that poison fish during red tide events. This group is monophyletic and recognizable by their alveoli (small cavities enclosed in membranes that hug the internal cell surface). There are three distinct groups of alveolates: dinoflagellates, apicomplexans, and ciliates. The remainder of this tutorial will concentrate on the dinoflagellates and apicomplexans.
Figure 13. A dinoflagellate. (Click to enlarge) Peridinium sp.
Figure 14. An apicomplexan. Plasmodium falciparum.
Figure 15. A ciliate. (Click to enlarge) Stentor sp.
Kingdom Alveolata: Dinoflagellates
Dinoflagellates typically possess distinct shapes due to "frames" of cellulose within their cell walls. Their cell surface is generally ridged with perpendicular grooves that house a pair of flagella (shown left). These flagella, the defining characteristic of this group, beat within their grooves and cause dinoflagellates to rotate as they move forward. The word dinoflagellate is derived from the Greek word dinos, which means "rotation" or "whirling," and the Latin flagellum, which means "whip." Many dinoflagellates are photosynthetic; accordingly, they comprise a significant proportion of the phytoplankton that floats near the surface of the ocean, making them a critical component of the food web. Phytoplankton are an essential food resource for many other organisms, ranging from heterotrophic protists to baleen whales and many other organisms in between (most of whom serve as food themselves for creatures at higher trophic levels).
Figure 16. A dinoflagellate. (Click to enlarge) Ceratium tripos.
Not all dinoflagellates are photosynthetic; many are heterotrophic. Some of these heterotrophs exploit chloroplasts from photosynthetic protists, becoming autotrophic themselves for a time. Some dinoflagellates live in symbiosis with different species, as parasites in some cases and as mutualists in others.
Some dinoflagellates, such as those in the genus Noctiluca, have the ability to bioluminesce (make their own light). This is accomplished with the compound luciferin, which is the same chemical that makes fireflies glow. Noctiluca floats just under the surface of the ocean, and when individuals number in the millions they can produce spectacular glowing tides (pictured below). The red border at the advancing wave front (tide line) as it washes onto the beach is a real visible glow that is triggered by the tumbling dinoflagellates as they hit the sand. If you walk along the tide line of such a beach, your footprints actually glow with each step when your foot disturbs these bioluminescent protists. How bioluminescence evolved is not completely understood. The Burglar Alarm theory posits that the bioluminescent glow attracts predators of dinoflagellate predators and this allows the glowing protist to escape predation.
Figure 17. A dinoflagellate. (Click to enlarge) Noctiluca scintillans is one dinoflagellate responsible for red tides.
Figure 18. A bioluminescent algal bloom. (Click to enlarge)
This image shows a bloom of bioluminescent Noctiluca scintillans.
Dinoflagellates and Coral Reefs
One of the mutualistic associations formed by dinoflagellates is their symbiotic relationship with reef-building cnidarians, animals in the same phylum as sea anemones and jellyfish. Corals engulf dinoflagellates but do not digest them; rather, these dinoflagellates, called zooxanthellae, live out their lives within the tissues of corals. What does each of the partners in this mutualistic symbiosis have to gain? Corals provide dinoflagellates with a relatively safe refuge from predators and fluctuating environmental conditions. In return, photosynthetic dinoflagellates provide the chief source of food (photosynthate or fixed carbon) for coral-building cnidarians.
Figure 19. Individual cnidarian polyps (coral-building organisms). (Click to enlarge)
Coral reefs are actually composed of the exoskeletons (secreted calcium carbonate shells) from countless generations of organisms, each of which anchored itself onto the remains of the previous generation, secreting more calcium carbonate and building up the reef incrementally over time.
While some corals are capable of existing without symbiotic protists, they do not thrive and the rate of calcium carbonate deposition (reef-building) is far slower when dinoflagellates are absent. As tiny as dinoflagellates are, they cooperate to build reefs that are thousands of miles long.
Furthermore, reefs are complex vital ecosystems that support many life forms that simply would not exist if reefs disappeared; therefore, a great many organisms are indirectly dependent on the activity of dinoflagellates.
Recently many reefs have begun to suffer, becoming dramatically bleached in appearance as corals die or lose their zooxanthellae. There have been a number of hypotheses advanced to explain this phenomenon (e.g., pollution, an increase in ultraviolet light due to the widening hole in the ozone, and global temperature changes), and all of these factors can be traced back to the impact of humans on the environment. Others suggest that natural weather fluctuations or disease outbreaks may be responsible. Whatever the ultimate cause, loss of symbiotic dinoflagellates appears to be the immediate source for the bleaching. For more information on coral bleaching check out the NOAA (National Oceanic and Atmospheric Administration) Coral Reef Site, the Australian Institute of Marine Science, and the NOAA Coral Reef Watch Site.
Dinoflagellates and Red Tide
Dinoflagellates are the organisms responsible for red tide events, or "harmful algal blooms" (HABs). Blooms (population explosions) of dinoflagellates are sometimes called "red tides" because dinoflagellates can reach such high densities that they actually change the color of the water in which they reside. Depending on the pigments present in these dinoflagellates, these tides actually appear brown, red, orange, or yellow.
Figure 20. A red tide in Hong Kong. (Click to enlarge)
A number of dinoflagellate species release toxins into the water, killing many aquatic animals during major algal blooms and poisoning others with sublethal doses of toxins. The primary function of these compounds is probably defensive, warding off or poisoning grazers, and perhaps preventing competing algal species from gaining a foothold in monospecific mats of plankton (minute animal and plant life in the ocean).
A recent study suggests that these toxins, known as brevetoxins, can be aerosolized during HAB events, increasing the incidence of respiratory infections among inland populations by 54%.
In any case, filter feeders (e.g., shellfish) are particularly vulnerable to brevetoxins because they actively ingest plankton. In turn, heterotrophs (e.g., humans) that eat poisoned animals are often themselves unintended victims of the dinoflagellates' defensive chemicals, contracting such conditions as paralytic shellfish poisoning. The compounds involved are neurotoxins, poisons that attack the central nervous system, so they are particularly nasty, producing symptoms such as delirium and respiratory paralysis.
Figure 21. Alexandrium sp. (Click to enlarge) A red tide organism.
One red tide organism, Pfiesteria, is unusual in that it uses toxins specifically to kill fish, stunning them and then feeding on their tissues (or on other protists that thrive on decaying fish). Pfiesteria also produce some of the most ecologically severe red tides, sometimes killing enormous numbers of fish. It is not clear how Pfiesteria affect humans, but there does seem to be considerable evidence that people exposed to its toxin by physical contact with affected fish or water containing Pfiesteria suffer various symptoms (e.g., skin lesions and loss of cognitive function).
Figure 22. Pfiesteria sp. (Click to enlarge)
HABs seem to be on the rise. Scientists believe that the enrichment of oceans and estuarine waters from fertilizer runoff has created an aquatic environment that favors dinoflagellate blooms. Research is underway to learn more about the causes of HABs, with the ultimate goal of designing viable control measures. This link offers a more thorough explanation of HAB causes and control strategies.
Can protists really be seen from space? They can if they cause red tides. NOAA, the National Oceanic and Atmospheric Administration, sponsors many programs to monitor HABs using satellite imagery. The image shown here is from a study that is focused on the dinoflagellate Gymnodinium breve, which causes HABs in the Gulf of Mexico.
Figure 23. A satellite image from NOAA showing hot spots (in red) of Gymnodinium breve. (Click to enlarge)
Kingdom Alveolata: Apicomplexans
Apicomplexans are parasites, specialized for living and reproducing within the tissues of animals. The apical complex is a cluster of microtubules and organelles located in the apex of cells that are in the sporozoite (infectious stage). This structure's function is to facilitate penetration of host cells. A number of apicomplexans require more than one host to complete their complicated life cycles. The infamous Plasmodium, which causes malaria in humans and other animals, is an apicomplexan transmitted by mosquitoes.
Figure 24. Plasmodium falciparum, an apicomplexan.
More people are killed each year by malaria than by any other transmissible disease (besides tuberculosis); over one million people die each year from malaria (of approximately 515,000,000 infections every year), with the overwhelming majority of cases occurring in sub-Saharan Africa (and many others in Southeast Asia and South America). What's more, malaria can strike a person multiple times per year, contributing to a major deficit in productivity, depriving families of income, and plunging many into deeper poverty. We have discussed the role that the sickle-cell allele plays in resistance to malaria; given the severity of the symptoms, it should not surprise you that this disease affects human microevolution.
Many apicomplexan life cycles are quite complex, and Plasmodium is no exception. The life stage that is transferred from an infected human to an Anopheles mosquito is called a gametocyte (see figure). Within the gut of the mosquito, male and female gametocytes form gametes; male gametes then fertilize female gametes, creating a zygote. The zygote develops into an oocyst on the gut wall of the mosquito and ruptures to release thousands of sporozoites, which then migrate through the body of the mosquito and take up residence in its salivary glands. When the mosquito bites an animal, the sporozoites leave the salivary glands and migrate into the new host through the mouthparts of the mosquito, then through the new host's bloodstream to the liver. In the liver, sporozoites transform to merozoites, which penetrate red blood cells in the host using the apical complex. Once in red blood cells, merozoites reproduce rapidly, producing many more merozoites, which burst out of the blood cells and penetrate new cells. Some of these merozoites form gametocytes, which can then be picked up by Anopheles mosquitoes that bite the newly infected host, beginning the cycle again.
Figure 25. The life history of Plasmodium, the apicomplexan that causes malaria. (Click to enlarge)
Individuals who are heterozygotic for the sickle-cell allele have red blood cells that do not support the life cycle of the merozoites. This is also true for individuals who are homozygotic wild type. Hence, heterozygotes can be infected, but they do not exhibit the extreme symptoms.
More information on apicomplexans and malaria is available from UC-Berkeley. If you are interested in parasitic disease organisms, check out the CDC Disease Database for thorough, reliable information, along with some worthwhile images. The United States Agency for International Development (USAID) also offers an in-depth tutorial on the causes, treatments, and preventive measures for malaria at its Global Health e-Learning Center.
Figure 26. A myxomycete, or plasmodial slime mold. (Click to enlarge)
Figure 27. A plasmodial slime mold, growing on a conifer. (Click to enlarge)
Figure 28. A plasmodial slime mold, growing on pine bark mulch. (Click to enlarge)
Slime molds used to be classified with the fungi, due to their heterotrophic feeding habits and appearance; we know these similarities likely arose independently in this group and hence the similarities between fungi and slime molds are an example of convergent evolution. We now know the slime molds are more closely related to amoeboid protists. Myxomycetes, or plasmodial slime molds, are among the most bizarre and beautiful creatures on Earth, often brilliantly colored in shades of yellow, orange, or pink. They are quite enormous and have given many a hapless gardener a good scare when found growing on a damp pile of bark mulch (see image above). Despite their size, plasmodial slime molds are unicellular; basically, they are large bags of cytoplasm. However, within each "cell" are many nuclei. The synchronous division of these nuclei has made slime molds very useful research organisms for the study of mitosis; additionally, many scientists study cytoplasmic streaming (the circular flow of cytoplasm that aids in moving materials within the cell) in myxomycetes because it is a relatively easy phenomenon to observe due to the size of these organisms.
Acrasiomycetes, or cellular slime molds, are also very unusual organisms. They spend most of their lives as amoeba-like single cells, but when resources are scarce they converge, joining with other cells to form units that function as multicellular organisms. Unlike plasmodial slime molds, the cells within the acrasiomycetes retain their cell membranes and their ability to live independently. Cellular slime molds have become something of a model system for scientists studying the behaviors of individual cells and coordination among groups of cells. For more information on research involving cellular slime molds, visit the "DictyBase," a resource for the hundreds of researchers working on the cellular slime mold Dictyostelium.
Figure 29. Dictyostelium, an acrasiomycete or cellular slime mold.
The protists are comprised of very diverse organisms that are polyphyletic in origin. Earlier taxonomic schemes that grouped all of these organisms into one giant kingdom were not very informative because they were not based on the evolutionary relationships between different protistan groups. The modern classification scheme presented here is considered superior because it does take into account these ancestral relationships.
As you learned, not all eukaryotes have mitochondria. Members of the kingdom Archaezoa lack these organelles. It is unclear if they branched off before these organelles were acquired (recall the endosymbiotic theory of mitochondrial evolution) or if they lost these organelles secondarily, after acquisition. In either case, keep in mind that their lifestyle (as parasitic microsymbionts) does not require these organelles because they are quite successful living in the manner that they do (albeit, at the expense of their hosts).
Members of the kingdom Euglenozoa do have mitochondria, but are a diverse group in terms of structure. All euglenoids have a flagellum, whereas the kinetoplastids have a kinetoplast. The euglenoids are typically free-living (and can be photoautotrophic), whereas the kinetoplastids are all microsymbionts (some, like the trypanosomes, are pathogenic).
Members of Kingdom Alveolata are even more diverse. They include the dinoflagellates that are involved in such diverse activities as coral reef building and that cause various forms of red tide. They play a major role in the ecology of deep-ocean communities because they comprise a significant portion of the phytoplankton that a variety of ocean species ingest (including whales).
The apicomplexans all form parasitic symbioses, therefore, they have a major impact on other life. The causative agent of malaria is caused by Plasmodium. More people are killed each year by malaria than by any other transmissible disease (besides tuberculosis). Keep in mind the importance of understanding a pathogen's life cycle when public health decisions need to be made.
We also briefly introduced the slime molds. Taxonomists really don't know how to classify these organisms. In a hike through the woods, you might encounter a brightly colored, fungus-like structure spreading over the surface of a decaying tree trunk; most likely, you are observing a slime mold.