7.35: Diversity of Protists II

Alveolates are a group of organisms recognized by the presence of alveoli, which are cytoplasmic sacs located beneath the cell membrane. While their function remains uncertain, alveoli may help regulate water balance by controlling how much water enters and leaves the cell. In dinoflagellates, these structures may serve as armor plates. There are three major types of alveolates: ciliates, which move using cilia; dinoflagellates, which use flagella for movement; and apicomplexans, which are parasitic and infect humans and other animals.

Ciliates possess cilia at some point in their life cycle. These small, hair-like structures aid in movement and may cover the entire cell or appear in tufts or rows, depending on the species. One of the most well-known ciliates is Paramecium, which uses cilia for both locomotion and feeding. Paramecium consumes bacteria and other particles by sweeping them into a funnel-shaped oral groove. The material is then transported to the cell mouth, or gullet, where it is enclosed in a vacuole through phagocytosis. Digestive enzymes break down the contents for nutrients.

Ciliates are unique among protists because they have two types of nuclei: micronuclei and macronuclei. The macronucleus manages essential cellular functions like growth and feeding, while the micronucleus is involved in sexual reproduction. In Paramecium, sexual reproduction occurs when two cells partially merge and exchange micronuclei. The macronuclear genome of Paramecium contains approximately 40,000 genes, nearly twice as many as humans.

Many species of Paramecium and other protists serve as hosts for endosymbiotic bacteria, archaea, or eukaryotes, such as green algae. These symbiotic organisms often provide their host with essential nutrients, including vitamins or growth factors. Some ciliates, particularly those in the digestive tracts of termites, contain endosymbiotic archaea that convert hydrogen and carbon dioxide into methane. Additionally, some anaerobic ciliates are found in the rumen of ruminant animals, where they contribute to digestion and fermentation.

While symbiosis is common, some ciliates are parasitic. One example is Balantidium coli, an intestinal parasite primarily found in domestic animals but occasionally infecting humans, causing dysentery-like symptoms. This parasite forms cysts that allow disease transmission through contaminated food or water.

Dinoflagellates are diverse organisms found in both marine and freshwater environments. They acquired their ability to photosynthesize through secondary endosymbiosis. These organisms move with the help of two flagella of different lengths, positioned at different angles. The movement of these flagella causes dinoflagellates to spin, which is reflected in their name, derived from the Greek word for "whirling." Some dinoflagellates are free-living, while others live symbiotically with coral reef animals, providing them with food in exchange for shelter.

Certain dinoflagellates produce bioluminescence, emitting light when disturbed at night, creating a sparkling effect in coastal waters. However, some species are harmful. For instance, Gonyaulax forms dense blooms known as "red tides," which can lead to fish deaths and human poisoning when shellfish accumulate the organism's neurotoxins. This condition, called paralytic shellfish poisoning, can cause numbness, dizziness, and breathing difficulties, with severe cases leading to respiratory failure. Another harmful dinoflagellate, Pfiesteria piscicida, releases toxic spores that infect and kill fish. These toxins also cause lesions, allowing bacteria to invade the fish’s damaged tissue. Humans exposed to Pfiesteria may develop skin rashes and respiratory issues.

Apicomplexans are non-photosynthetic parasites that cause severe diseases in humans and animals. Examples include Plasmodium, responsible for malaria; Toxoplasma, which causes toxoplasmosis; and Eimeria, which leads to coccidiosis. Adult apicomplexans lack mobility and absorb nutrients directly across their cell membrane.

These organisms produce sporozoites, which are specialized structures that aid in host infection. The name "apicomplexan" comes from the presence of a unique organelle complex at one end of the sporozoite that helps the parasite invade host cells. Apicomplexans also contain apicoplasts, which are degenerate chloroplasts that no longer perform photosynthesis but still contribute to fatty acid, isoprenoid, and heme biosynthesis. It is believed that apicoplasts originated from red algae that were engulfed by an ancestral apicomplexan in a secondary endosymbiotic event. Over time, the chloroplast lost its ability to photosynthesize and took on other functions within the apicomplexan cell.

Apicomplexans infect both vertebrates and invertebrates, and some species require two hosts to complete their life cycle. For example, certain Plasmodium species, which cause malaria, alternate between mosquito and human hosts. Coccidia primarily infect birds but can also impact other animals. Malaria, which has been one of the deadliest diseases throughout human history, remains a major health threat worldwide.