Describe the role that protists play in the ecosystem

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.

Primary Producers

Figure 1. Coral polyps obtain nutrition through a symbiotic relationship with dinoflagellates.

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 dinoflagellates called zooxanthellae use sunlight to fix 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 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.

The protists themselves and their products of photosynthesis are essential—directly or indirectly—to 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 dinoflagellates, diatoms, and multicellular algae.

Figure 2. Virtually all aquatic organisms depend directly or indirectly on protists for food.

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.

Protists as Decomposers

Various organisms with a protist-level organization were originally treated as fungi, because they produce sporangia, structures producing and containing spores. These include chytrids, slime molds, water molds, and Labyrinthulomycetes. Many of these organisms were also treated as fungi due to a similar environmental role: that of a decomposer.

These 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.

Chytrids can be single or multi-cellular. There are about one thousand species, most living in water or soil. Most are decomposers. Some are parasites and can cause diseases in plants, including corn, alfalfa, and potatoes. One species, Batrachochytrium dendrobatidis, seems to be the cause of chytridiomycosis, a disease of frogs that is seriously affecting many wild frog populations around the world.

Figure 3. Slime mold on lawn, U.S.A

Slime molds are notable for their unusual life cycle. In some species, individual single-celled organisms come together and fuse to form a giant cell with thousands of nuclei. This body, called a plasmodium, can move around consuming bacteria, fungi, and decaying plant matter. (This is a different usage of the word plasmoidium from that used previously for the genus of parasitic protozoa.) Slime molds are found worldwide.

Water molds thrive in water and wet soil. They are considered to be more closely related to plants than fungi since they have cellulose cell walls. They are single-celled. Many are parasites and can cause diseases in plants, fungi, and animals. One species Phytophthora infestans causes the potato blight, which led to the Irish potato famine.

Labyrinthulomycetes form a network of tubes or filaments over which the single-celled organisms slide to gather food. They are mostly marine and are decomposers of dead plant material or parasites on plants and algae or some animals.

Human Pathogens

A pathogen is anything that causes disease. Parasites live in or on an organism and harm the organism. 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.

Plasmodium Species

Figure 4. Red blood cells are shown to be infected with P. falciparum. In this light microscopic image, the ring-shaped P. falciparum stains purple. (credit: modification of work by Michael Zahniser; scale-bar data from Matt Russell)

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 (Figure 4).

Of the four Plasmodium species known to infect humans, P. falciparum accounts for 50 percent 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 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 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.

Trypanosomes

Figure 5. Trypanosomes are shown among red blood cells. (credit: modification of work by Dr. Myron G. Shultz; scale-bar data from Matt Russell)

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 5). 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. 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.

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.

Plant Parasites

Protist parasites of terrestrial plants include agents that destroy food crops. The oomycete Plasmopara viticola parasitizes grape plants, causing a disease called downy mildew (Figure 6). 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 nineteenth century.

Figure 6. Both downy and powdery mildews on this grape leaf are caused by an infection of P. viticola. (credit: modification of work by USDA)

Figure 7. These unappetizing remnants result from an infection with P. infestans, the causative agent of potato late blight. (credit: USDA)

Phytophthora infestans is an oomycete responsible for potato late blight, which causes potato stalks and stems to decay into black slime (Figure 7). Widespread potato blight caused by P. infestans precipitated the 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.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.