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Ecology of Protists
∗
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
Abstract
By the end of this section, you will be able to:
•
•
Describe the role that protists play in the ecosystem
Describe important pathogenic species of protists
Protists function in various ecological niches. Whereas 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.
1 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.
For instance, photosynthetic dinoagellates called zooxanthellae use sunlight to x inorganic carbon. In this
symbiotic relationship, these protists provide nutrients for coral polyps (Figure 1) 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. This type of symbiotic relationship
is important in nutrient-poor environments. Without dinoagellate 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: insucient light reaches those depths for dinoagellates to photosynthesize.
∗ Version
1.5: Apr 12, 2013 9:42 am -0500
† http://creativecommons.org/licenses/by/3.0/
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Figure 1:
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Coral polyps obtain nutrition through a symbiotic relationship with dinoagellates.
The protists themselves and their products of photosynthesis are essentialdirectly or indirectlyto
the survival of organisms ranging from bacteria to mammals (Figure 2).
As primary producers, protists
feed a large proportion of the world's aquatic species. (On land, terrestrial plants serve as primary producers.) In fact, approximately one-quarter of the world's photosynthesis is conducted by protists, particularly
dinoagellates, diatoms, and multicellular algae.
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Figure 2:
Virtually all aquatic organisms depend directly or indirectly on protists for food.
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(credit
mollusks: modication of work by Craig Stihler, USFWS; credit crab: modication of work by David
Berkowitz; credit dolphin: modication of work by Mike Baird; credit sh: modication of work by
Tim Sheerman-Chase; credit penguin: modication of work by Aaron Logan)
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Protists do not create food sources only for sea-dwelling organisms.
For instance, 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.
2 Human Pathogens
A pathogen is anything that causes disease. Parasites live in or on an organism and harm the organism.
A signicant 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, eecting massive destruction of
food crops.
Plasmodium Species
Members of the genus Plasmodium must colonize both a mosquito and a vertebrate to complete their life
2.1
cycle.
In vertebrates, the parasite develops in liver cells and goes on to infect red blood cells, bursting
Plasmodium
P. falciparum accounts for 50 percent of all malaria cases and is the primary
from and destroying the blood cells with each asexual replication cycle (Figure 3). Of the four
species known to infect humans,
cause of disease-related fatalities in tropical regions of the world.
In 2010, it was estimated that malaria
caused between one-half and one 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 inammatory response with episodes of delirium-inducing fever as
P. falciparum is transmitted to
Anopheles gambiae. Techniques to kill, sterilize, or avoid exposure
parasites lyse red blood cells, spilling parasite waste into the bloodstream.
humans by the African malaria mosquito,
to this highly aggressive mosquito species are crucial to malaria control.
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Figure 3:
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Red blood cells are shown to be infected with
P. falciparum, the causative agent of malaria.
P. falciparum
In this light microscopic image taken using a 100× oil immersion lens, the ring-shaped
stains purple. (credit: modication of work by Michael Zahniser; scale-bar data from Matt Russell)
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This movie
:
depicts the pathogenesis of
Plasmodium falciparum,
the causative agent of malaria.
2.2 Trypanosomes
Trypanosoma brucei,
the parasite that is responsible for African sleeping sickness, confounds the human
immune system by changing its thick layer of surface glycoproteins with each infectious cycle (Figure 4).
The glycoproteins are identied by the immune system as foreign antigens, and a specic antibody defense
is mounted against the parasite.
1 http://openstaxcollege.org/l/malaria
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However,
T. brucei
has thousands of possible antigens, and with each
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subsequent generation, the protist switches to a glycoprotein coating with a dierent molecular structure.
In this way,
T. brucei is capable of replicating continuously without the immune system ever succeeding in
Without treatment, T. brucei attacks red blood cells, causing the patient to lapse
clearing the parasite.
into a coma and eventually die. During epidemic periods, mortality from the disease can be high. Greater
surveillance and control measures lead to a reduction in reported cases; some of the lowest numbers reported
in 50 years (fewer than 10,000 cases in all of sub-Saharan Africa) have happened since 2009.
2
This movie
:
discusses the pathogenesis of
Trypanosoma brucei, the
causative agent of African sleeping sickness.
In Latin America, another species,
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.
An estimated 10
million people are infected with Chagas disease, and it caused 10,000 deaths in 2008.
Figure 4:
Trypanosomes are shown among red blood cells. (credit: modication of work by Dr. Myron
G. Shultz; scale-bar data from Matt Russell)
2 http://openstaxcollege.org/l/African_sleep
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3 Plant Parasites
The oomycete Plasmopara
viticola parasitizes grape plants, causing a disease called downy mildew (Figure 5). Grape plants infected
with P. viticola appear stunted and have discolored, withered leaves. The spread of downy mildew nearly
Protist parasites of terrestrial plants include agents that destroy food crops.
collapsed the French wine industry in the nineteenth century.
Figure 5:
Both downy and powdery mildews on this grape leaf are caused by an infection of
P. viticola.
(credit: modication of work by USDA)
Phytophthora infestans is an oomycete responsible for potato late blight, which causes potato stalks and
P. infestans precipitated the
stems to decay into black slime (Figure 6). Widespread potato blight caused by
well-known Irish potato famine in the nineteenth 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 percent of crops
when no pesticides are applied.
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Figure 6:
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These unappetizing remnants result from an infection with
P. infestans, the causative agent
of potato late blight. (credit: USDA)
4 Agents of Decomposition
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. Indeed, without saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all
organic carbon became tied up in dead organisms.
5 Section Summary
Protists function at several levels of the ecological food web: as primary producers, as direct food sources,
and as decomposers. In addition, many protists are parasites of plants and animals that can cause deadly
human diseases or destroy valuable crops.
6 Review Questions
Exercise 1
An example of carbon xation is _____________.
a. photosynthesis
b. decomposition
c. phagocytosis
d. parasitism
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(Solution on p. 10.)
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Exercise 2
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(Solution on p. 10.)
Which parasitic protist evades the host immune system by altering its surface proteins with each
generation?
a.
b.
c.
d.
Paramecium caudatum
Trypanosoma brucei
Plasmodium falciparum
Phytophthora infestans
7 Free Response
Exercise 3
How does killing
Anopheles mosquitoes aect the Plasmodium protists?
Exercise 4
(Solution on p. 10.)
(Solution on p. 10.)
Without treatment, why does African sleeping sickness invariably lead to death?
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Solutions to Exercises in this Module
to Exercise (p. 8)
A
to Exercise (p. 9)
B
to Exercise (p. 9)
Plasmodium parasites infect humans and cause malaria. However, they must complete part of their life
Anopheles mosquitoes, and they can only infect humans via the bite wound of a mosquito. If
the mosquito population is decreased, then fewer Plasmodium would be able to develop and infect humans,
cycle within
thereby reducing the incidence of human infections with this parasite.
to Exercise (p. 9)
The trypanosomes that cause this disease are capable of expressing a glycoprotein coat with a dierent
molecular structure with each generation. Because the immune system must respond to specic antigens to
raise a meaningful defense, the changing nature of trypanosome antigens prevents the immune system from
ever clearing this infection. Massive trypanosome infection eventually leads to host organ failure and death.
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