21.3 The Ecology of Protists Lesson Overview 21.3 The Ecology of Protists

THINK ABOUT IT After a few days of rain, you notice a small spot of yellow slime at the base of a stand of tall grass. You mark its position. A few days later, you come back, and it has grown and moved away from the mark. Is it an animal? A fungus? A strange plant? The correct answer is none of the above. It’s a protist called a slime mold.

Diversity Organisms commonly called “algae” actually belong to many different groups. Some (the cyanobacteria) are prokaryotes, some (like green algae) belong to the plant kingdom, and some are protists. Photosynthetic protists include many phytoplankton species and the red and brown algae, as well as euglenas and dinoflagellates. These organisms share an autotrophic lifestyle, marked by the ability to use the energy from light to make a carbohydrate food source.

Diversity Not all photosynthetic protists are closely related to plants. In fact, the red algae are the most closely related to plants. Many other photosynthetic protists, however, are more closely related to nonphotosynthetic protists. In some cases certain species within a group have lost chloroplasts. In other cases endosymbiosis added a chloroplast to some species but not to their relatives.

Ecological Roles Photosynthetic protists play major ecological roles on Earth. The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible.

Feeding Fish and Whales Photosynthetic protists make up a large portion of phytoplankton, the small, free-floating photosynthetic organisms found near the surface of oceans and lakes. About half of Earth’s photosynthesis is carried out by phytoplankton.

Feeding Fish and Whales Phytoplankton provide a direct source of nourishment for organisms as diverse as shrimp and baleen whales indirect source of nourishment for humans When you eat tuna fish, you are eating fish that fed on smaller fish that fed on still smaller animals that fed on photosynthetic protists.

Supporting Coral Reefs Coral reefs, which are found in warm ocean waters throughout the world, provide food and shelter to large numbers of fish and other organisms. Protist algae known as zooxanthellae provide most of the coral's energy needs by photosynthesis. By nourishing coral animals, these algae help maintain the equilibrium of the coral ecosystem. Coralline red algae also help to provide calcium carbonate to stabilize growing coral reefs.

Providing Shelter largest known protist Giant Kelp- largest known protist a brown alga that can grow to more than 60 meters in length provide shelter for many marine species a source of food for sea urchins.

Recycling Wastes Many protists grow rapidly in regions where sewage is discharged, where they play a vital role in recycling waste materials. When the amount of organic waste is excessive, however, populations of protists like Euglena can grow to enormous numbers and create an algal bloom, which can disrupt ecosystem homeostasis.

Recycling Wastes An algal bloom in a pond or lake can deplete nutrients from the water, and the decomposition of the dead protists can rob water of its oxygen, causing fish and invertebrates to die. Algal blooms of marine protists called dinoflagellates create what is known as a red tide. The buildup of toxins produced by these protists can poison fish and shellfish.

Heterotrophic Protists How do heterotrophic protists obtain food?

Heterotrophic Protists How do heterotrophic protists obtain food? Engulf and digest their food Absorb molecules from the environment

Amoebas (Blendspace video #29) Amoebas can capture and digest their food, surrounding a cell or particle and then taking it inside themselves to form a food vacuole. A food vacuole is a small cavity in the cytoplasm that temporarily stores food. Once inside the cell, the material is digested and the nutrients are passed along to the rest of the cell. Indigestible waste materials remain inside the vacuole until the vacuole releases them outside the cell.

Ciliates (Blendspace Video #30) Paramecium and other ciliates use their cilia to sweep food particles into the gullet, an indentation in one side of the organism. The particles are trapped in the gullet and forced into food vacuoles that form at its base.

Ciliates The food vacuoles pinch off into the cytoplasm and fuse with lysosomes, which contain digestive enzymes. Waste materials are emptied into the environment when the food vacuole fuses with a region of the cell membrane called the anal pore.

Slime Molds A slime mold is a heterotrophic protist that thrives on decaying organic matter. Slime molds are found in places that are damp and rich in organic matter—on the floor of a forest or a backyard compost pile, for example. Slime molds play key roles in recycling nutrients in an ecosystem.

Slime Molds At one stage in their life cycle, slime molds exist as a collection of individual amoebalike cells.

Slime Molds Eventually these aggregate to form a large structure known as a plasmodium, which may continue to move.

Slime Molds The plasmodium eventually develops sporangia, in which meiosis produces haploid spores to continue the cycle.

Protists That Absorb Some protists survive by absorbing molecules that other organisms have released to the environment. Water molds, for example, grow on dead or decaying plants and animals, absorbing food molecules through their cellulose cell walls and cell membranes. This dead goldfish is covered with the common water mold Saprolegnia.

Symbiotic Protists—Mutualists and Parasites What types of symbiotic relationships involve protists?

Symbiotic Protists—Mutualists and Parasites What types of symbiotic relationships involve protists? Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit. Parasitic protists are responsible for some of the world’s most deadly diseases, including several kinds of debilitating intestinal diseases, African sleeping sickness, and malaria.

Symbiotic Protists—Mutualists and Parasites Many protists are involved in symbiotic relationships with other organisms. Symbiosis is a relationship in which two species live closely together. Many of these symbiotic relationships are mutualistic: Both organisms benefit. However, some are parasitic relationships, in which the protist benefits at the expense of its host.

Mutualists Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit. Example: red algae maintain a mutualistic relationship with the animals of the coral reef, which could not survive without the protists’ help.

Mutualists Trichonympha is another example of a mutualistic protist. Trichonympha is a flagellated protist that lives within the digestive system of termites and makes it possible for the insects to digest wood. Termites themselves do not have enzymes to break down the cellulose in wood. Trichonympha and other organisms in the termite’s gut manufacture an enzyme called cellulose that breaks the chemical bonds in cellulose, making it possible for termites to digest wood.

Parasites and Disease Parasitic protists are responsible for some of the world’s most deadly diseases, including several kinds of debilitating intestinal diseases, African sleeping sickness, and malaria.

Intestinal Diseases Water-borne protists are found in streams, lakes, and oceans. Water supplies contaminated by animal or human feces can spread protist parasites, causing serious and sometimes deadly outbreaks of intestinal disease.

Intestinal Diseases For example, the flagellated protist Giardia causes severe diarrhea and digestive-system problems. Even crystal-clear streams may be contaminated with Giardia, which produces tough cysts that can be killed only by boiling water thoroughly or by adding iodine to the water.

Intestinal Diseases Entamoeba causes a disease known as amebic dysentery. The amoebas live in the intestines, where they absorb food from the host. They also attack the wall of the intestine itself, destroying parts of it and causing severe bleeding.

Intestinal Diseases Cryptosporidium is resistant to the chlorine compounds often used to sanitize drinking water and therefore poses a special threat to public water systems. In 2008, an outbreak in Utah sickened more than 2,000 people.

African Sleeping Sickness The flagellated protists Trypanosoma cause African sleeping sickness. Trypanosomes are spread from person to person by the bite of the tsetse fly. Trypanosomes destroy blood cells and infect other tissues, including nerve cells. Severe damage to the nervous system causes some individuals to lose consciousness and lapse into a deep and sometimes fatal sleep. Control of the tsetse fly and the protist pathogens that it spreads is a major goal of health workers in Africa.

Malaria Malaria is one of the world’s most serious infectious diseases. Malaria is caused by Plasmodium, a spore-forming protist carried by the female Anopheles mosquito. Plasmodium requires two hosts to complete its life cycle: an Anopheles mosquito and a human.

Malaria

Malaria

Malaria

Malaria