{"id":1386,"date":"2017-01-18T23:56:32","date_gmt":"2017-01-18T23:56:32","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&p=1386"},"modified":"2017-07-05T16:56:53","modified_gmt":"2017-07-05T16:56:53","slug":"ecology-of-protists","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/chapter\/ecology-of-protists\/","title":{"raw":"Ecology of Protists","rendered":"Ecology of Protists"},"content":{"raw":"

Describe the role that protists play in the ecosystem<\/h2>\r\nProtists 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.\r\n
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Learning Objectives<\/h3>\r\n
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  • Identify protists that act as primary producers<\/li>\r\n \t
  • Provide examples of the protists' important roles in decomposition<\/li>\r\n \t
  • Describe important pathogenic species of protists<\/li>\r\n<\/ul>\r\n<\/div>\r\n

    Primary Producers<\/h2>\r\n[caption id=\"attachment_1392\" align=\"alignright\" width=\"300\"]\"This Figure\u00a01.\u00a0Coral polyps obtain nutrition through a symbiotic relationship with dinoflagellates.[\/caption]\r\n\r\nProtists 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\u00a01) 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.\r\n\r\nThe protists themselves and their products of photosynthesis are essential\u2014directly or indirectly\u2014to the survival of organisms ranging from bacteria to mammals (Figure\u00a02). As primary producers, protists feed a large proportion of the world\u2019s aquatic species. (On land, terrestrial plants serve as primary producers.) In fact, approximately one-quarter of the world\u2019s photosynthesis is conducted by protists, particularly dinoflagellates, diatoms, and multicellular algae.\r\n\r\n[caption id=\"attachment_1393\" align=\"aligncenter\" width=\"650\"]\"The Figure\u00a02.\u00a0Virtually all aquatic organisms depend directly or indirectly on protists for food.[\/caption]\r\n\r\nProtists 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.\r\n

    Protists as Decomposers<\/h2>\r\nVarious 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.\r\n\r\nThese 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 \u201ctied up\u201d in dead organisms.\r\n\r\nChytrids 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<\/em>, seems to be the cause of chytridiomycosis, a disease of frogs that is seriously affecting many wild frog populations around the world.\r\n\r\n[caption id=\"attachment_1797\" align=\"alignright\" width=\"350\"]\"Slime Figure\u00a03. Slime mold on lawn, U.S.A[\/caption]\r\n\r\nSlime 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.\r\n\r\nWater 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<\/em> causes the potato blight, which led to the Irish potato famine.\r\n\r\nLabyrinthulomycetes 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.\r\n

    Human Pathogens<\/h2>\r\nA 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.\r\n

    Plasmodium<\/em> Species<\/h3>\r\n[caption id=\"attachment_1395\" align=\"alignright\" width=\"300\"]\"The Figure\u00a04.\u00a0Red blood cells are shown to be infected with P. falciparum.<\/em>\u00a0In this light microscopic image, the ring-shaped P. falciparum<\/em> stains purple. (credit: modification of work by Michael Zahniser; scale-bar data from Matt Russell)[\/caption]\r\n\r\nMembers of the genus Plasmodium<\/em> 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\u00a04).\r\n\r\nOf the four Plasmodium<\/em> species known to infect humans, P. falciparum<\/em> 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.\r\n\r\nDuring the course of malaria, P. falciparum<\/em> can infect and destroy more than one-half of a human\u2019s 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<\/em> is transmitted to humans by the African malaria mosquito, Anopheles gambiae<\/em>. Techniques to kill, sterilize, or avoid exposure to this highly aggressive mosquito species are crucial to malaria control.\r\n
    \r\n\r\nThis movie depicts the pathogenesis of Plasmodium falciparum, the causative agent of malaria:\r\n\r\n