23: Protists Flashcards
Eukaryotic Origins, Characteristics of Protists, Groups of Protists, Ecology of Protists (157 cards)
1
Q
What are some examples of protists?
A
Protists range from the microscopic, single-celled Acanthocystis turfacea and the ciliate Tetrahymena thermophila, which can be seen using light microscopy, to the enormous, multicellular kelps (Chromalveolata) that extend for hundreds of feet in underwater “forests”.
2
Q
What is the origin of the term “protist”?
A
This name was first suggested by Ernst Haeckel in the late 19th century.
3
Q
What are some examples of protists with large, macroscopic cells?
A
Plasmodia (giant amoebae) of myxomycete slime molds, and marine green alga Caulerpa.
4
Q
What are some examples of multicellular protists?
A
Red, green, and brown seaweeds.
5
Q
What is endosymbiosis?
A
Engulfment of one cell within another such that the engulfed cell survives, and both cells benefit; the process responsible for the evolution of mitochondria and chloroplasts in eukaryotes.
6
Q
What is the endosymbiotic theory?
A
Theory that states that eukaryotes may have been a product of one cell engulfing another, one living within another, and evolving over time until the separate cells were no longer recognizable as such.
7
Q
What is a plastid?
A
One of a group of related organelles in plant cells that are involved in the storage of starches, fats, proteins, and pigments.
8
Q
In what kinds of environments are microscopic protists commonly found?
A
Soil, freshwater, brackish and marine environments, as well as the digestive tracts of animals and in the vascular tissues of plants. Others invade the cells of other protists, animals, and plants.
9
Q
What is the oldest fossils found?
A
The earliest fossils found appear to be Bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for prokaryotes, relatively large cells.
10
Q
When do the first eukaryotes appear in the fossil record?
A
Structures of size 10 micrometers and greater, which might be fossils, appear in the geological record about 2.1 billion years ago.
11
Q
Which characteristics must have been common in the most recent common ancestor of eukaryotic organisms?
A
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores.
- Mitochondria.
- A cytoskeleton containing the structural and motility components called actin microfilaments and microtubules.
- Flagella and cilia.
- Chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones.
- Mitosis.
- Sex.
- Cell walls.
12
Q
What is the prevalence of mitochondria in eukaryotes?
A
Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria.
13
Q
What is the prevalence of the cytoskeleton in eukaryotes?
A
All extant eukaryotes have actin microfilaments and microtubules.
14
Q
What is the prevalence of flagella and cilia in eukaryotes?
A
Some extant eukaryotes lack flagella and/or cilia, but they are descended from ancestors that possessed them.
15
Q
What is the prevalence of histones in eukaryotic chromosomes?
A
The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them.
16
Q
What is the prevalence of mitosis in eukaryotes?
A
Mitosis is universally present in eukaryotes.
17
Q
What is karyogamy?
A
A stage where two haploid nuclei fuse together to create a diploid zygote nucleus.
18
Q
What is the prevalence of cell walls in eukaryotes?
A
Members of all major lineages have cell walls, and it might be reasonable to conclude that the last common ancestor could make cell walls during some stage of its life cycle. However, not enough is known about eukaryotes’ cell walls and their development to know how much homology exists among them. If the last common ancestor could make cell walls, it is clear that this ability must have been lost in many groups.
19
Q
What was likely the origin of eukaryotes?
A
All extant eukaryotes are descendants of a chimeric organism that was a composite of a host cell and the cell(s) of an alpha-proteobacterium that “took up residence” inside it.
20
Q
Which metabolic processes are found in prokaryotes, and in eukaryotes?
A
Many important metabolic processes arose in prokaryotes, and some of these, such as nitrogen fixation, are never found in eukaryotes. The process of aerobic respiration is found in all major lineages of eukaryotes, and it is localized in the mitochondria. Aerobic respiration is also found in many lineages of prokaryotes, but it is not present in all of them, and many forms of evidence suggest that such anaerobic prokaryotes never carried out aerobic respiration nor did their ancestors.
21
Q
What is the history of atmospheric oxygenation?
A
Various metabolic processes evolved that protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. It became widely present among prokaryotes, including in a group we now call alpha-proteobacteria. Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Oxygen levels similar to today’s levels only arose within the last 700 million years.
22
Q
When were the first organelles observed?
A
Mitochondria were first observed by light microscopists in the late 1800s, where they appeared to be somewhat worm-shaped structures that seemed to be moving around in the cell.
23
Q
What is the history of the endosymbiotic theory?
A
In the 1960s, American biologist Lynn Margulis developed endosymbiotic theory. In 1967, Margulis introduced new work on the theory and substantiated her findings through microbiological evidence. Although Margulis’ work initially was met with resistance, the once-revolutionary hypothesis is now widely (but not completely) accepted, with work progressing on uncovering the steps involved in this evolutionary process and the key players involved.
24
Q
What are some features of mitochondria?
A
Mitochondria arise from the division of existing mitochondria; they may fuse together; and they may be moved around inside the cell by interactions with the cytoskeleton. However, mitochondria cannot survive outside the cell.
25
How are mitochondria created?
Mitochondria divide independently by a process that resembles binary fission in prokaryotes. Specifically, mitochondria are not formed from scratch (de novo) by the eukaryotic cell; they reproduce within it and are distributed with the cytoplasm when a cell divides or two cells fuse. Their reproduction is synchronized with the activity and division of the cell.
26
Where are genes for respiratory proteins found?
Many of the genes for respiratory proteins are located in the nucleus, but in some eukaryotic groups, they are found in mitochondria.
27
What is some evidence from respiratory protein genes for the endosymbiont theory?
The genes of respiratory proteins in eukaryotes, when compared to those of other organisms, appear to be of alpha-proteobacterial origin. The fact that in many eukaryotes they are found in the nucleus and not mitochondria has been interpreted as evidence that the genes have been transferred from the endosymbiont chromosome to the host genome. This loss of genes by the endosymbiont is probably one explanation why mitochondria cannot live without a host.
28
Are mitochondria found in anaerobic eukaryotes?
Some anaerobic eukaryotes appear to lack organelles that could be recognized as mitochondria. Findings suggest that reduced organelles are found in most, if not all, anaerobic eukaryotes, and that all eukaryotes appear to carry some genes in their nuclei that are of mitochondrial origin.
29
What are some functions associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes?
The generation of clusters of iron and sulfur for use as cofactors.
30
Which type of molecule is found in plastids that is involved in the light-harvesting chemical reactions of photosynthesis?
Plastids are rich in the pigment chlorophyll *a* and a range of other pigments, called accessory pigments, which are involved in harvesting energy from light.
31
What is the primary endosymbiotic origin of plastids?
Plastids are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. Plastids of primary origin are surrounded by two membranes.
32
Why is it suggested that primary endosymbiosis has occurred twice in the history of photosynthetic eukaryotes?
In one case, the common ancestor of the major lineage/supergroup Archaeplastida took on a cyanobacterial endosymbiont; in the other, the ancestor of the small amoeboid rhizarian taxon, *Paulinella*, took on a different cyanobacterial endosymbiont. Almost all photosynthetic eukaryotes are descended from the first event, and only a couple of species are derived from the other.
33
What is one of the differences between cyanobacteria and most other prokaryotes?
Cyanobacteria have extensive, internal membrane-bound sacs called thylakoids.
34
What are some similarities between chloroplasts and cyanobacteria?
Chloroplasts of primary origin have thylakoids, a circular DNA chromosome, and ribosomes similar to those of cyanobacteria.
35
Which plastids have a relictual cyanobacterial wall?
Each chloroplast is surrounded by two membranes. In the group of Archaeplastida called the glaucophytes and in *Paulinella*, a thin peptidoglycan layer is present between the outer and inner plastid membranes. All other plastids lack this relictual cyanobacterial wall.
36
What are the origins of the two plasma membranes of plastids?
The outer membrane surrounding the plastid is thought to be derived from the vacuole in the host, and the inner membrane is thought to be derived from the plasma membrane of the symbiont.
37
What are some endosymbiotic characteristics of plastids?
There is, as with the case of mitochondria, strong evidence that many of the genes of the endosymbiont were transferred to the nucleus. Plastids, like mitochondria, cannot live independently outside the host. In addition, like mitochondria, plastids are derived from the division of other plastids and never built from scratch.
38
When did the endosymbiotic event that led to Archaeplastida occur?
Researchers have suggested that the endosymbiotic event that led to Archaeplastida occurred 1 to 1.5 billion years ago, at least five hundred million years after the fossil record suggests that eukaryotes were present.
39
What is secondary endosymbiosis?
A cell containing an endosymbiont to itself became engulfed.
40
What are some characteristics of secondary plastids?
Secondary plastids are surrounded by three or more membranes, and some secondary plastids even have clear remnants of the nucleus of the endosymbiotic alga. Others have not "kept" any remnants.
41
Are all plastids in eukaryotes derived from primary endosymbiosis?
Not all plastids in eukaryotes are derived directly from primary endosymbiosis. There are cases where tertiary or higher-order endosymbiotic events are the best explanations for plastids in some eukaryotes.
42
What is a chlorarachniophyte?
Chlorarachniophytes are rare algae indigenous to tropical seas and sand that can be classified into the rhizarian supergroup. Chlorarachniophytes extend thin cytoplasmic strands, interconnecting themselves with other chlorarachniophytes, in a cytoplasmic network. Molecular and morphological evidence suggest that the chlorarachniophyte protists are derived from a secondary endosymbiotic event, where a eukaryote engulfed a green alga, the latter of which had already established an endosymbiotic relationship with a photosynthetic cyanobacterium.
43
What is the evidence that chlorarachniophytes evolved from secondary endosymbiosis?
The chloroplasts contained within the green algal endosymbionts are still capable of photosynthesis, making chlorarachniophytes photosynthetic. The green algal endosymbiont also exhibits a stunted vestigial nucleus. Additionally, the plastids of chlorarachniophytes are surrounded by four membranes.
44
Why is it suggested that chlorarachniophytes are the products of an evolutionary recent secondary endosymbiotic event?
Their plastids are surrounded by four membranes, whereas in other lineages that involved secondary endosymbiosis, only three membranes can be identified around plastids, which is explained as a sequential loss of a membrane during the course of evolution.
45
How are the four membranes of chlorarachniophytes explained?
The first two correspond to the inner and outer membranes of the photosynthetic cyanobacterium, the third corresponds to the green alga, and the fourth corresponds to the vacuole that surrounded the green alga when it was engulfed by the chlorarachniophyte ancestor.
46
What are some examples of secondary endosymbiosis, excluding chlorarachniophytes?
Secondary endosymbiosis of green algae led to euglenid protists, whereas secondary endosymbiosis of red algae led to the evolution of dinoflagellates, apicomplexans, and stramenopiles.
47
What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts?
All eukaryotic cells have mitochondria, but not all eukaryotic cells have chloroplasts.
48
What is a mixotroph?
Organism that can obtain nutrition by autotrophic or heterotrophic means, usually facultatively.
49
What is a pellicle?
Outer cell covering composed of interlocking protein strips that function like a flexible coat of armor, preventing cells from being torn or pierced without compromising their range of motion.
50
What is a phagolysosome?
Cellular body formed by the union of a phagosome containing the ingested particle with a lysosome that contains hydrolytic enzymes.
51
How diverse are protists?
There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live as commensals or parasites in other organisms and these relationships are often species-specific, there is a huge potential for protist diversity that matches the diversity of the hosts. As the catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists.
52
What are some characteristics of protist cell structure?
The cells of protists are among the most elaborate of all cells. Most protists are microscopic and unicellular, but some true multicellular forms exist. A few protists live as colonies that behave in some ways as a group of free-living cells, and in other ways as a multicellular organism. Still other protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In fact, many protist cells are multinucleated; in some species, the nuclei are different sizes and have distinct roles in protist cell function.
53
What is the size and protective features of protists?
Single protist cells range in size from less than a micrometer to three meters in length to hectares. Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls. Others are encased in glassy silica-based shells or wound with pellicles of interlocking protein strips.
54
What are the stages of phagocytosis?
The engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.
55
What is a saprobe?
A subtype of heterotrophs, which absorb nutrients from dead organisms or their organic wastes.
56
What modes of movement have protists evolved for motility?
The majority of protists are motile, but different types of protists have evolved varied modes of movement. Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or tufts of tiny cilia that they coordinately beat to swim. Others form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward.
57
What is taxis?
The movement toward or away from a stimulus.
58
What is phototaxis?
Movement toward light, which is accomplished by coupling a locomotion strategy with a light-sensing organ.
59
What are examples of protists which use different transportation methods?
*Paramecia* waves hair-like appendages called cilia to propel itself; amoebas use lobe-like pseudopodia to anchor itself to a solid surface and pull itself forward; *Euglena* use a whip-like tail called a flagellum to propel itself.
60
What are the two modes of binary fission in protists?
Transverse or longitudinal, depending on the axis of orientation.
61
What is an example of a protist which exhibits binary fission?
*Paramecium*.
62
What is an example of a protist which exhibits multiple fission?
True slime molds.
63
What are some different modes of replication among protists?
Asexual reproduction, such as binary fission, multiple fission, or budding which divide and grow to the size of the parental protist; or sexual reproduction involving meiosis and fertilization.
64
Do protists reproduce sexually or asexually?
Many protist species can switch from asexual to sexual reproduction when necessary. Sexual reproduction is often associated with periods when nutrients are depleted or environmental changes occur. Sexual reproduction may allow the protist to recombine genes and produce new variations of progeny that may be better suited to surviving in the new environment.
65
What is a microbial cyst?
Sexual reproduction is often associated with resistant cysts that are a protective, resting stage. Depending on their habitat, the cysts may be particularly resistant to temperature extremes, desiccation, or low pH. This strategy allows certain protists to "wait out" stressors until their environment becomes more favorable for survival or until they are carried (such as by wind, water, or transport on a larger organism) to a different environment, because cysts exhibit virtually no cellular metabolism.
66
What are some life cycle modes used by protists?
Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect different host species at different developmental stages to complete their life cycle. Some protists are unicellular in the haploid form and multicellular in the diploid form, a strategy employed by animals. Other protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations that is also used by plants.
67
What are the habitats that protists live in?
Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Several protist species are parasites that infect animals or plants. A few protist species live on dead organisms or their wastes, and contribute to their decay.
68
What is a biological carbon pump?
Process by which inorganic carbon is fixed by photosynthetic species that then die and fall to the sea floor where they cannot be reached by saprobes and their carbon dioxide consumption cannot be returned to the atmosphere.
69
What is bioluminescence?
Generation and emission of light by an organism, as in dinoflagellates.
70
What is a contractile vacuole?
Vesicle that fills with water (as it enters the cell by osmosis) and then contracts to squeeze water from the cell; an osmoregulatory vesicle.
71
What is cytoplasmic streaming?
Movement of cytoplasm into an extended pseudopod such that the entire cell is transported to the site of the pseudopod.
72
What is a hydrogenosome?
Organelle carried by parabasalids (Excavata) that functions anaerobically and outputs hydrogen gas as a byproduct; likely evolved from mitochondria.
73
What is a kinetoplast?
Mass of DNA carried within the single, oversized mitochondrion, characteristic of kinetoplastids (phylum: Euglenozoa).
74
What is a mitosome?
Nonfunctional organelle carried in the cells of diplomonads (Excavata) that likely evolved from a mitochondrion.
75
What are plankton?
Diverse group of mostly microscopic organisms that drift in marine and freshwater systems and serve as a food source for larger aquatic organisms.
76
What is a raphe?
Slit in the silica shell of diatoms through which the protist secretes a stream of mucopolysaccharides for locomotion and attachment to substrates.
77
What is a test?
Porous shell of a foram that is built from various organic materials and typically hardened with calcium carbonate.
78
What are the six supergroups of the Eukarya domain?
Excavata, Chromalveolata, Rhizaria, Archaeplastida, Amoebozoa, Opisthokonta
79
How stable is the classification scheme for Eukarya?
The classification of eukaryotes is still in flux, and the six supergroups may be modified or replaced by a more appropriate hierarchy as genetic, morphological, and ecological data accumulate.
80
What are the kingdoms of Excavata?
Diplomonads, Parabasalids, Euglenozoans
81
What are the kingdoms of Chromalveolata?
Dinoflagellates, Apicomplexans, Ciliates, Diatoms, Golden algae, Brown algae, Oomycetes
82
What are the kingdoms of Rhizaria?
Cercozoans, Forams, Radiolarians
83
What are the kingdoms of Archaeplastida?
Red algae, Chlorophytes (green algae), Charophytes (green algae), Land plants
84
What are the kingdoms of Amoebozoa?
Slime molds, Gymnamoebas, Entamoebas
85
What are the kingdoms of Opisthokonta?
Nucleariids, Fungi, Choanoflagellates, Animals
86
What are Excavata?
Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms with a feeding groove "excavated" from one side. This supergroup includes heterotrophic predators, photosynthetic species, and parasites.
87
What are Diplomonads?
Among the Excavata are the diplomonads, which include the intestinal parasite, *Giardia lamblia*. These protists were originally believed to lack mitochondria, but mitochondrial remnant organelles, called mitosomes, have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. Each diplomonad cell has two identical nuclei and uses several flagella for locomotion.
88
What is *Giardia lamblia*?
A mammalian intestinal parasite; a waterborne protist that causes severe diarrhea when ingested. In the Diplomonad kingdom.
89
What are Parabasalids?
An Excavata subgroup that exhibits semi-functional mitochondria. They function anaerobically and are called hydrogenosomes because they produce hydrogen gas as a byproduct. Parabasalids move with flagella and membrane rippling.
90
How do certain protist species contribute to the digestion of cellulose in some insects?
Certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute an essential step in the digestion of cellulose ingested by these insects as they bore through wood.
91
What is *Trichomonas vaginalis*?
A parabasalid that causes a sexually transmitted disease in humans.
92
How does *T. vaginalis* move?
*T. vaginalis* employs mechanisms of flagella and membrane rippling to transit through the male and female urogenital tracts.
93
*T. vaginalis* is the causative agent of which disease, and how prevalent is it?
*T. vaginalis* causes trichomoniasis, which appears in an estimated 180 million cases worldwide each year.
94
What are the symptoms and effects of trichomoniasis?
Whereas men rarely exhibit symptoms during an infection with this protist, infected women may become more susceptible to secondary infections with HIV and may be more likely to develop cervical cancer. Pregnant women infected with *T. vaginalis* are at increased risk of serious complications, such as pre-term delivery.
95
What are Euglenozoans?
An Excavata subgroup that includes parasites, heterotrophs, autotrophs, and mixotrophs, ranging in size from 10 to 500 micrometers. Euglenoids move through their aquatic habitats using two long flagella that guide them toward light sources sensed by a primitive ocular organ called an eyespot.
96
What are *Euglena*?
A genus of the Euglenozoans that encompasses some mixotrophic species that display a photosynthetic capability only when light is present. In the dark, the chloroplasts of *Euglena* shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their environments.
97
What is Kinetoplastid?
Subgroup of the Euglenozoans that includes several parasites, collectively called trypanosomes.
98
What are trypanosomes?
Parasites of the kinetoplastid subgroup that cause devastating human diseases and infect an insect species during a portion of their life cycle.
99
What is *Trypanosoma brucei*?
A human parasite of the subgroup kinetoplastid, and the causative agent of African sleeping sickness.
100
What is African sleeping sickness?
A disease associated with severe chronic fatigue and coma, which can be fatal if left untreated.
101
Where is *T. brucei* found, geographically?
*T. brucei* is common in central Africa.
102
What is the life cycle of *T. brucei*?
1. Tsetse fly takes a blood meal and injects *T. brucei* into the bloodstream.
2. *T. brucei* multiplies by binary fission in blood, lymph, and spinal fluid.
3. Tsetse fly takes a blood meal and ingests *T. brucei*.
4. In the midgut of the fly, *T. brucei* multiplies by binary fission.
5. *T. brucei* transforms into an infectious stage.
6. *T. brucei* enters the salivary gland and multiplies.
103
How does *T. brucei* defeat human immune response?
*T. brucei* confounds the human immune system by changing its thick layer of surface glycoproteins with each infectious cycle. The glycoproteins are identified by the immune system as foreign antigens, and a specific antibody defense is mounted against the parasite. However, *T. brucei* has thousands of possible antigens, and with each subsequent generation, the protist switches to a glycoprotein coating with a different molecular structure. In this way, *T. brucei* is capable of replicating continuously without the immune system ever succeeding in clearing the parasite. Without treatment, *T. brucei* attacks red blood cells, causing the patient to lapse into a coma and eventually die. During epidemic periods, mortality from the disease can be high.
104
What are Chromalveolata?
Current evidence suggests that species classified as chromalveolates are derived from a common ancestor that engulfed a photosynthetic red algal cell, which itself had already evolved chloroplasts from an endosymbiotic relationship with a photosynthetic prokaryote. Therefore, the ancestor of chromalveolates is believed to have resulted from a secondary endosymbiotic event. However, some chromalveolates appear to have lost red alga-derived plastid organelles or lack plastid genes altogether. Therefore, this supergroup should be considered a hypothesis-based working group that is subject to change. Chromalveolates include very important photosynthetic organisms, such as diatoms, brown algae, and significant disease agents in animals and plants. The chromalveolates can be subdivided into alveolates and stramenopiles.
105
What are alveolates?
A subgroup of chromalveolates. A large body of data supports that the alveolates are derived from a shared common ancestor. The alveolates are named for the presence of an alveolus, or membrane-enclosed sac, beneath the cell membrane. The exact function of the alveolus is unknown, but it may be involved in osmoregulation. The alveolates are further categorized into some of the better-known protists: the dinoflagellates, the apicomplexans, and the ciliates.
106
What are dinoflagellates?
A subgroup of alveolates that exhibit extensive morphological diversity and can be photosynthetic, heterotrophic, or mixotrophic. Many dinoflagellates are encased in interlocking plates of cellulose. Two perpendicular flagella fit into the grooves between the cellulose plates, with one flagellum extending longitudinally and a second encircling the dinoflagellate. Together, the flagella contribute to the characteristic spinning motion of dinoflagellates. These protists exist in freshwater and marine habitats, and are a component of plankton.
107
What are some characteristics of dinoflagellates in marine habitats?
Some dinoflagellates generate light, called bioluminescence, when they are jarred or stressed. Large numbers of marine dinoflagellates (billions or trillions of cells per wave) can emit light and cause an entire breaking wave to twinkle or take on a brilliant blue color. For approximately 20 species of marine dinoflagellates, population explosions (also called blooms) during the summer months can tint the ocean with a muddy red color. This phenomenon is called a red tide, and it results from the abundant red pigments present in dinoflagellate plastids. In large quantities, these dinoflagellate species secrete an asphyxiating toxin that can kill fish, birds, and marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume these protists may become poisoned.
108
What are apicomplexans?
A subgroup of alveolates that are so named because their microtubules, fibrin, and vacuoles are asymmetrically distributed at one end of the cell in a structure called an apical complex. The apical complex is specialized for entry and infection of host cells. Indeed, all apicomplexans are parasitic. This group includes the genus *Plasmodium*, which causes malaria in humans. Apicomplexan life cycles are complex, involving multiple hosts and stages of sexual and asexual reproduction.
109
What are ciliates?
A subgroup of alveolates which includes *Paramecium* and *Tetrahymena*, and is a group of protists 10 to 3000 micrometers in length that are covered in rows, tufts, or spirals of tiny cilia. By beating their cilia synchronously or in waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-based structures that function like paddles, funnels, or fins. Ciliates are also surrounded by a pellicle, providing protection without compromising agility.
110
What is *Paramecium*?
A genus of ciliates that includes protists which have organized their cilia into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria. Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes. Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called the anal pore. In addition to a vacuole-based digestive system, *Paramecium* also uses contractile vacuoles, which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell.
111
How do *Paramecium* reproduce?
*Paramecium* has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential for sexual reproduction, whereas the macronucleus directs asexual binary fission and all other biological functions. The process of sexual reproduction in *Paramecium* underscores the importance of the micronucleus to these protists. *Paramecium* and most other ciliates reproduce sexually by conjugation. This process begins when two different mating types of *Paramecium* make physical contact and join with a cytoplasmic bridge. The diploid micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. A similar process occurs in bacteria that have plasmids. Fusion of the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell. This pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform multiple rounds of DNA replication and go on to become new macronuclei. Two cell divisions then yield four new *Paramecia* from each original conjugative cell.
112
What are stramenopiles?
The other subgroup of chromalveolates, which includes photosynthetic marine algae and heterotrophic protists. The unifying feature of this group is the presence of a textured, or "hairy", flagellum. Many stramenopiles also have an additional flagellum that lacks hair-like projections. Members of this subgroup range in size from single-celled diatoms to the massive and multicellular kelp.
113
What are diatoms?
A subgroup of stramenopiles which are unicellular photosynthetic protists that encase themselves in intricately patterned, glassy cell walls composed of silicon dioxide in a matrix of organic particles. These protists are a component of freshwater and marine plankton. Most species of diatoms reproduce asexually, although some instances of sexual reproduction and sporulation also exist. Some diatoms exhibit a slit in their silica shell, called a raphe. By expelling a stream of mucopolysaccharides from the raphe, the diatom can attach to surfaces or propel itself in one direction. Diatoms range in size from 2 to 200 micrometers.
114
What is the role of diatoms in the biological carbon pump?
During periods of nutrient availability, diatom populations bloom to numbers greater than can be consumed by aquatic organisms. The excess diatoms die and sink to the sea floor, removing carbon dioxide from the atmosphere.
115
What are golden algae?
A subgroup of stramenopiles, golden algae are largely unicellular, although some species can form large colonies. Their characteristic gold color results from their extensive use of carotenoids, a group of photosynthetic pigments that are generally yellow or orange in color. Golden algae are found in both freshwater and marine environments, where they form a major part of the plankton community.
116
What are brown algae?
A subgroup of stramenopiles, brown algae are primarily marine, multicellular organisms that are known colloquially as seaweeds. Giant kelps are a type of brown algae. Some brown algae have evolved specialized tissues that resemble terrestrial plants, with root-like holdfasts, stem-like stipes, and leaf-like blades that are capable of photosynthesis. The stipes of giant kelps are enormous, extending in some cases for 60 meters.
117
What is the life cycle of brown algae?
A variety of algal life cycles exist, but the most complex is alternation of generations, in which both haploid and diploid stages involve multicellularity. In the brown algae genus *Laminaria*, haploid spores develop into multicellular gametophytes, which produce haploid gametes that combine to produce diploid organisms that then become multicellular organisms with a different structure from the haploid form. Certain other organisms perform alternation of generations in which both the haploid and diploid forms look the same.
118
What are oomycetes?
A subgroup of stramenopiles, the water molds, oomycetes ("egg fungus"), were so-named based on their fungus-like morphology, but molecular data have shown that the water molds are not closely related to fungi. The oomycetes are characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake. As diploid spores, many oomycetes have two oppositely directed flagella (one hairy and one smooth) for locomotion. The oomycetes are nonphotosynthetic and include many saprobes and parasites. The saprobes appear as white fluffy growths on dead organisms. Most oomycetes are aquatic, but some parasitize terrestrial plants.
119
What is an example of an oomycete plant pathogen?
One plant pathogen is *Phytophthora infestans*, the causative agent of late blight of potatoes, such as occurred in the 19th century Irish potato famine.
120
What is Rhizaria?
The Rhizaria supergroup includes many of the amoebas, most of which have threadlike or needle-like pseudopodia. Pseudopodia function to trap and engulf food particles and to direct movement in rhizarian protists.
121
How is cytoplasmic streaming used?
Cytoplasmic streaming is used by several diverse groups of protists as a means of locomotion or as a method to distribute nutrients and oxygen.
122
What are forams?
A subgroup of rhizaria, foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length, and occasionally resembling tiny snails. As a group, the forams exhibit porous shells, called tests that are built from various organic materials and typically hardened with calcium carbonate. The tests may house photosynthetic algae, which the forams can harvest for nutrition. Foram pseudopodia extend through the pores and allow the forams to move, feed, and gather additional building materials. Typically, forams are associated with sand or other particles in marine or freshwater habitats. Foraminiferans are also useful as indicators of pollution and changes in global weather patterns.
123
What are radiolarians?
A subtype of Rhizaria that exhibits intricate exteriors of glassy silica with radial or bilateral symmetry. Needle-like pseudopods supported by microtubules radiate outward from the cell bodies of these protists and function to catch food particles. The shells of dead radiolarians sink to the ocean floor, where they may accumulate in 100 meter-thick depths. Preserved, sedimented radiolarians are very common in the fossil record.
124
What is Archaeplastida?
Red algae and green algae are included in the supergroup Archaeplastida. It was from a common ancestor of these protists that the land plants evolved, since their closest relatives are found in this group. Molecular evidence supports that all Archaeplastida are descendants of an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium. The red and green algae include unicellular, multicellular, and colonial forms.
125
What are red algae?
A subgroup of Archaeplastida, AKA rhodophytes.
126
What is the size and cellular morphology of red algae?
Rhodophytes are primarily multicellular, lack flagella, and range in size from microscopic, unicellular protists to large, multicellular forms grouped into the informal seaweed category.
127
What is the red algae lifecycle?
The red algae life cycle is an alternation of generations.
128
What makes red algae red?
Some species of red algae contain phycoerythrins, photosynthetic accessory pigments that are red in color and outcompete the green tint of chlorophyll, making these species appear as varying shades of red. Other protists classified as red algae lack phycoerythrins and are parasites.
129
Where can red algae be found?
Red algae are common in tropical waters where they have been detected at depths of 260 meters. Other red algae exist in terrestrial or freshwater environments.
130
What are green algae?
A subgroup of Archaeplastida, and the most abundant group of algae. The green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. That this group of protists shared a relatively recent common ancestor with land plants is well supported. The green algae are subdivided into the chlorophytes and the charophytes.
131
What are charophytes?
A subgroup of green algae, the charophytes are the closest living relatives to land plants and resemble them in morphology and reproductive strategies. Charophytes are common in wet habitats, and their presence often signals a healthy ecosystem.
132
What are chlorophytes?
A subgroup of green algae, the chlorophytes exhibit great diversity of form and function.
133
Where are chlorophytes found?
Chlorophytes primarily inhabit freshwater and damp soil, and are a common component of plankton.
134
What is an example of a unicellular chlorophyte?
*Chlamydomonas* is a simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior flagella that guide this protist toward light sensed by its eyespot. More complex chlorophyte species exhibit haploid gametes and spores that resemble *Chlamydomonas*.
135
What are some examples of multicellular or multinucleate chlorophytes?
True multicellular organisms, such as the sea lettuce, *Ulva*, are represented among the chlorophytes. In addition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus *Caulerpa* exhibit flattened fern-like foliage and can reach lengths of 3 meters. *Caulerpa* species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.
136
What is an example of a colonial chlorophyte?
The chlorophyte *Volvox* is one of only a few examples of a colonial organism, which behaves in some ways like a collection of individuals, but in other ways like the specialized cells of a multicellular organism. *Volvox* colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion. Individual *Volvox* cells move in a coordinated fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell specialization in this organism.
137
What is Amoebozoa?
The amoebozoans characteristically exhibit pseudopodia that extend like tubes or flat lobes, rather than the hair-like pseudopodia or rhizarian amoeba. The Amoebozoa include several groups of unicellular amoeba-like organisms that are free-living or parasites.
138
What are slime molds?
A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into spore-generating fruiting bodies, much like fungi. The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types.
139
What are plasmodial slime molds?
Plasmodial slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of slime during their feeding stage. Food particles are lifted and engulfed into the slime mold as it glides along. Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Haploid spores are produced by meiosis within the sporangia, and spores can be disseminated through the air or water to potentially land in more favorable environments. If this occurs, the spores germinate to form amoeboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.
140
What is the plasmodial slime mold life cycle?
Germination of 1*n* spores gives rise to cells that can convert between amoeboid and flagellated forms. Fertilization by fusion of either cell type results in a 2*n* zygote. Mitosis without cytokinesis results in a single-celled multinucleate mass called a plasmodium. The plasmodium feeds, and matures, and sporangia form. Meiosis within the sporangium results in haploid spores. Repeat.
141
What are cellular slime molds?
The cellular slime molds function as independent amoeboid cells when nutrients are abundant. When food is depleted, cellular slime molds pile onto each other into a mass of cells that behaves as a single unit, called a slug. Some cells in the slug contribute to a 2-3 millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment.
142
What is an example of a cellular slime mold?
One representative genus of the cellular slime molds is *Dictyostelium*, which commonly exists in the damp soil of forests.
143
What is the cellular slime mold life cycle?
Germination of 1*n* spores gives rise to solitary 1*n* amoeboid cells. These are fertilized to produce 2*n* zygotes, which reproduce asexually via mitosis and meiosis to become solitary 1*n* amoebas. These can then be fertilized, and repeat the cycle. Mitosis without cytokinesis results in multiple solitary amoebas that may aggregate. The aggregated amoebas are able to migrate. A stalk forms with a fruiting body at the top. Cells migrate up the stalk to form spores. The spores disperse. Repeat.
144
What are choanoflagellates?
The opisthokonts include the animal-like choanoflagellates, which are believed to resemble the common ancestor of sponges and, in fact, all animals. These organisms exhibit a single, apical flagellum that is surrounded by a contractile collar composed of microvilli. The collar uses a similar mechanism to sponges to filter out bacteria for ingestion by the protist. The morphology of choanoflagellates was recognized early on as resembling the collar cells of sponges, and suggesting a possible relationship to animals.
145
How many species of choanoflagellates are there?
Choanoflagellates include unicellular and colonial forms, and number about 244 described species.
146
What is Mesomycetozoa?
The Mesomycetozoa form a small group of parasites, primarily of fish, and at least one form that can parasitize humans. Their life cycles are poorly understood. These organisms are of special interest, because they appear to be closely related to animals. In the past, they were grouped with fungi and other protists based on their morphology.
147
What are the general ecological niches in which protists function?
Some protist species are essential components of the food chain and generators of biomass, others function in the decomposition of organic materials. Still other protists are dangerous human pathogens or causative agents of devastating plant diseases.
148
In what ways are protists primary producers/food sources?
Protists are essential sources of nutrition for many other organisms. In some cases, as in plankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of nutrition for other organisms.
149
Explain the symbiotic relationship between zooxanthellae and coral polyps.
Photosynthetic dinoflagellates called zooxanthellae use sunlight to fix inorganic carbon. In this symbiotic relationship, these protists provide nutrients for coral polyps that house them, giving corals a boost of energy to secrete a calcium carbonate skeleton. In turn, the corals provide the protist with a protected environment and the compounds needed for photosynthesis. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching, and they eventually die. This explains why reef-building corals do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates to photosynthesize.
150
What is the importance of photosynthetic protists as primary producers?
Protists themselves and their products of photosynthesis are essential - directly or indirectly - to the survival of organisms ranging from bacteria to mammals. As primary producers, protists feed a large proportion of the world's aquatic species. In fact, approximately ¼ of the world's photosynthesis is conducted by protists, particularly dinoflagellates, diatoms, and multicellular algae.
151
In what ways do some protists act as pathogens?
A significant number of protists are pathogenic parasites that must infect other organisms to survive and propagate. Protist parasites include the causative agents of malaria, African sleeping sickness, and waterborne gastroenteritis in humans. Other protist pathogens prey on plants, effecting massive destruction of food crops.
152
What is *Plasmodium*?
Members of the genus *Plasmodium* must colonize both a mosquito and a vertebrate to complete their life cycle. In vertebrates, the parasite develops in liver cells and goes on to infect red blood cells, bursting from and destroying the blood cells with each asexual replication cycle.
153
What is *Plasmodium falciparum*?
Of the four *Plasmodium* species known to infect humans, *P. falciparum* accounts for 50% of all malaria cases and is the primary cause of disease-related fatalities in tropical regions of the world. In 2010, it was estimated that malaria caused between ½ and 1 million deaths, mostly in African children. During the course of malaria, P. falciparum can infect and destroy more than one-half of a human's circulating blood cells, leading to severe anemia. In response to waste products released as the parasites burst from infected blood cells, the host immune system mounts a massive inflammatory response with episodes of delirium-inducing fever as parasites lyse red blood cells, spilling parasite waste into the bloodstream. *P. falciparum* is transmitted to humans by the African malaria mosquito, *Anopheles gambiae*. Techniques to kill, sterilize, or avoid exposure to this highly aggressive mosquito species are crucial to malaria control.
154
What is *Trypanosoma cruzi*?
In Latin America, *T. cruzi* is responsible for Chagas disease. *T. cruzi* infections are mainly caused by a blood-sucking bug. The parasite inhabits heart and digestive system tissues in the chronic phase of infection, leading to malnutrition and heart failure due to abnormal heart rhythms.
155
What is *Plasmopara viticola*?
A protist that parasitizes grape plants, causing a disease called downy mildew. Grape plants infected with *P. viticola* appear stunted and have discolored, withered leaves. The spread of downy mildew nearly collapsed the French wine industry in the 19th century.
156
What is *Phytophthora infestans*?
*Phytophthera infestans* is an oomycete responsible for potato late blight, which causes potato stalks and stems to decay into black slime. Widespread potato blight caused by *P. infestans* precipitated the well-known Irish potato famine in the 19th century that claimed the lives of approximately 1 million people and led to the emigration of at least 1 million more from Ireland. Late blight continues to plague potato crops in certain parts of the United States and Russia, wiping out as much as 70% of crops when no pesticides are applied.
157
What is the importance of saprobes?
The fungus-like protist saprobes are specialized to absorb nutrients from nonliving organic matter, such as dead organisms or their wastes. For instance, many types of oomycetes grow on dead animals or algae. Saprobic protists have the essential function of returning inorganic nutrients to the soil and water. This process allows for new plant growth, which in turn generates sustenance for other organisms along the food chain. Without saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all organic carbon became "tied up" in dead organisms.
