Ecology of Protists

Protists as Primary Producers, Food Sources, and Symbionts

Protists function as sources of food for organisms on land and sea.

Primary Producers/Food Sources

Protists function in various ecological niches. Some protist species are essential components of the food chain and are generators of biomass.

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 the coral polyps that house them, giving corals a boost of energy to secrete a calcium carbonate skeleton. In turn, the corals provide the protists 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. 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.

Protists do not only create food sources for sea-dwelling organisms. 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 Human Pathogens

Many protists exist as parasites that infect and cause diseases in their hosts.

Human Pathogens

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

Plasmodium Species

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. 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 and one-half 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 parasitic 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

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. 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; with each subsequent generation, the protist switches to a glycoprotein coating of 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; it caused 10,000 deaths in 2008.

Protists as Plant Pathogens

Many protists act as parasites that prey on plants or as decomposers that feed on dead organisms.

Plant Parasites

Protist parasites prey on terrestrial plants and include agents that cause massive destruction to food crops. The oomycete Plasmopara viticola parasitizes grape plants, which causes 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 nineteenth century. They are easily controlled once discovered, so careful monitoring of susceptible hosts is key because if left unaddressed, the organism can quickly spread and completely overwhelm the host species

Because the downy mildew pathogen does not overwinter in midwestern fields, crop rotations and tillage practices do not affect disease development. The pathogen tends to become established in late summer. Therefore, planting early season varieties may further reduce the threat posed by downy mildew. Fungicides can also be applied to control downy mildew. Broad spectrum protectant fungicides such as chlorothalonil, mancozeb, and fixed copper are somewhat effective in protecting against downy mildew infection.

Phytophthora infestans is an oomycete responsible for potato late blight. This disease causes potato stalks and stems to decay into black slime. Widespread potato blight caused by P. infestans led to the well-known Irish potato famine in the nineteenth century that claimed the lives of approximately one million people and resulted in the emigration of at least one 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.

Agents of Decomposition

The fungus-like protist saprobes are specialized to absorb nutrients from non-living 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.