23.3 Groups of Protists
5 min read•june 14, 2024
Protists are a diverse group of eukaryotic organisms that don't fit neatly into plant, animal, or fungal categories. They're grouped into supergroups based on shared characteristics and evolutionary history. These supergroups include , , , , , and .
Each supergroup has unique features that set it apart. For example, Excavata have modified mitochondria, while Chromalveolata include species with secondary plastids. Rhizaria use threadlike , Archaeplastida have primary plastids, Amoebozoa use lobose pseudopodia, and Opisthokonta share characteristics with animals and fungi.
Protist Supergroups and Characteristics
Characteristics of protist supergroups
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- Excavata
- Comprise unicellular eukaryotes with modified mitochondria ( or ) that lack typical cristae
- Include many parasitic species (, ) that cause diseases in humans and other animals
- Possess a feeding groove in some species, which is used for ingesting bacteria and other particles
- Encompass diverse lineages such as (), (), and (Giardia)
- Chromalveolata
- Contain a wide range of photosynthetic and non-photosynthetic protists with complex evolutionary histories
- Include species with secondary plastids acquired from red algae through (, )
- Exhibit unique cellular structures such as (membranous sacs beneath the cell membrane in ciliates and ) or (filament for attachment in haptophytes)
- Comprise ecologically important groups like dinoflagellates (marine ), ciliates (), and apicomplexans ()
- Rhizaria
- Consist of unicellular protists that use threadlike pseudopodia () for feeding and locomotion
- Include species with intricate mineral skeletons or shells made of calcium carbonate () or silica ()
- Encompass both free-living and parasitic species, such as foraminiferans (marine plankton), radiolarians (marine plankton), and ()
- Archaeplastida
- Comprise photosynthetic protists with primary plastids derived from endosymbiotic cyanobacteria
- Include red algae (), green algae (, ), and land plants (mosses, ferns, flowering plants)
- Possess cell walls often composed of cellulose, a complex polysaccharide that provides structural support
- Represent a monophyletic group, meaning they all descend from a common ancestor with primary plastids
- Amoebozoa
- Consist of unicellular protists that use lobose pseudopodia (blunt, finger-like projections) for locomotion and feeding
- Include some species that form multicellular fruiting bodies () or slime molds () under certain conditions
- Encompass both free-living and parasitic species, such as (free-living), (parasite causing amoebic dysentery), and Dictyostelium (cellular slime mold)
- Opisthokonta
- Comprise animal-like protists and some fungi that share similar cellular and molecular characteristics
- Include species with a single posterior flagellum, such as (possible animal ancestors) and some zoosporic fungi ()
- Exhibit multicellular colonies or life cycles with both unicellular and multicellular stages in some species
- Encompass diverse groups like choanoflagellates, (amoebae), and (parasites of fish)
Evolutionary relationships among eukaryotes
- Protists represent a paraphyletic group, as they do not include all descendants of their last common ancestor (plants, animals, fungi)
- Different protist lineages gave rise to plants, animals, and fungi through independent evolutionary events
- Plants evolved from green algae within the Archaeplastida supergroup, acquiring primary plastids through endosymbiosis
- Animals evolved from animal-like protists (choanoflagellates) within the Opisthokonta supergroup, sharing a common ancestor with fungi
- Fungi also evolved from protists within the Opisthokonta supergroup, diverging from the animal lineage early in their evolutionary history
- Some protist lineages, such as red algae (Archaeplastida) and amoebozoans, do not have close relatives among plants, animals, or fungi, representing independent branches in the eukaryotic tree of life
- Molecular evidence (DNA sequences, gene comparisons) suggests that eukaryotes diverged into several major lineages early in their evolutionary history (over 1 billion years ago), giving rise to the diverse protist supergroups and eventually to plants, animals, and fungi
Distinguishing features of protists
- Excavata
- Possess mitochondria with reduced cristae (mitosomes) or modified into (enlarged DNA-containing region)
- Exhibit a feeding groove in some species (Trypanosoma), which is an invagination of the cell surface used for ingesting bacteria
- Display in some species, often arranged in a characteristic pattern (e.g., two flagella in Euglena, one directed anteriorly and the other posteriorly)
- Chromalveolata
- Contain secondary plastids with in photosynthetic species (dinoflagellates, diatoms), acquired through endosymbiosis of red algae
- Possess alveoli (membranous sacs) beneath the cell membrane in some species (ciliates, apicomplexans), which may function in cellular support or locomotion
- Exhibit a haptonema (filament for attachment) in some species (haptophytes), used for adhering to surfaces or capturing prey
- Display in some species (ciliates), often arranged in rows or whorls and used for locomotion and feeding
- Rhizaria
- Use threadlike pseudopodia (filopodia) for feeding and locomotion, which are thin, branching projections of the cell surface
- Produce mineral skeletons or shells in some species, composed of calcium carbonate (foraminiferans) or silica (radiolarians) and used for protection or buoyancy
- Exhibit (netlike pseudopodia) in some species (foraminiferans), which are branching and anastomosing projections used for capturing prey
- Archaeplastida
- Possess primary plastids with chlorophyll a and b, derived from endosymbiotic cyanobacteria and used for
- Display cellulose cell walls in many species (green algae, land plants), providing structural support and protection against osmotic stress
- Contain (light-harvesting structures) in red algae, which are protein complexes that capture light energy for photosynthesis
- Amoebozoa
- Use lobose pseudopodia for locomotion and feeding, which are blunt, finger-like projections of the cell surface
- Form fruiting bodies or slime molds in some species (Dictyostelium, Physarum), which are multicellular structures that produce spores for dispersal
- Produce in some species (Entamoeba), which are dormant, resistant stages that can survive harsh environmental conditions
- Opisthokonta
- Exhibit a single posterior flagellum in some species (choanoflagellates, zoosporic fungi), which is used for locomotion and feeding
- Possess collar cells with a flagellum surrounded by a collar of microvilli in choanoflagellates, which are used for capturing bacterial prey
- Display multicellular colonies or life cycles with both unicellular and multicellular stages in some species (choanoflagellates, ichthyosporeans)
Locomotion and Feeding Structures in Protists
- Pseudopodia: Temporary extensions of the cell membrane used for movement and feeding
- Lobose pseudopodia (found in Amoebozoa) are broad, blunt projections
- Filopodia (found in Rhizaria) are thin, thread-like projections
- Flagella: Long, whip-like structures used for locomotion and feeding
- Found in various protist groups, including Excavata and some Opisthokonta
- Often arranged in specific patterns characteristic of different protist lineages
- Cilia: Short, hair-like structures that beat in coordinated waves for movement and feeding
- Prominent in ciliates (Chromalveolata), where they are arranged in rows or whorls
- Also used to create water currents for feeding in some species
- Photosynthesis: Process of converting light energy into chemical energy, used by many protists for nutrition
- Occurs in plastids, which are organelles derived from endosymbiotic cyanobacteria
- Found in Archaeplastida and some Chromalveolata, enabling these protists to be primary producers in aquatic ecosystems
- : A diverse group of single-celled eukaryotes that are often motile and heterotrophic
- Include members from various protist supergroups, such as Amoebozoa, Excavata, and some Chromalveolata
- Exhibit a wide range of feeding and locomotion strategies, including the use of pseudopodia, flagella, and cilia
Key Terms to Review (53)
Alveoli: Alveoli are tiny air sacs located in the lungs that are crucial for gas exchange in mammals. They provide a large surface area for oxygen to diffuse into the blood and for carbon dioxide to diffuse out, making them essential components of the respiratory system.
Amoeba: An amoeba is a type of single-celled organism characterized by its irregular shape and ability to move using pseudopodia, which are temporary projections of cytoplasm. These organisms belong to the group of protists and are primarily found in water, soil, and as parasites in other organisms. Their unique movement and feeding strategies make them an essential subject of study within the diverse group of protists.
Amoebozoa: Amoebozoa is a diverse group of eukaryotic organisms, characterized by their ability to move and feed using pseudopodia, which are temporary projections of their cell membrane. These organisms are mostly unicellular and are found in various environments, playing crucial roles in ecosystems as decomposers and predators of bacteria and other small organisms.
Apicomplexans: Apicomplexans are a group of unicellular parasites belonging to the phylum Apicomplexa, characterized by their unique apical complex structure that aids in host cell invasion. These organisms are primarily known for their role as pathogens, particularly in humans and animals, and they exhibit a complex life cycle often involving multiple hosts.
Archaeplastida: Archaeplastida is a major group of eukaryotic organisms that includes red algae, green algae, and land plants. This group is characterized by the presence of plastids, which are organelles that originate from an endosymbiotic event with a cyanobacterium. Archaeplastida plays a vital role in photosynthesis and the ecology of various habitats, showcasing the evolutionary significance of this lineage in the diversification of photosynthetic organisms.
Caecilians: Caecilians are a group of limbless, serpentine amphibians primarily found in tropical regions. They are adapted to a burrowing lifestyle with elongated bodies and reduced or absent eyes.
Cercozoans: Cercozoans are a diverse group of protists, primarily characterized by their amoeboid form and the presence of filose or reticulose pseudopodia. This group includes various organisms that play significant roles in soil ecosystems, contributing to nutrient cycling and organic matter decomposition. Cercozoans are important for understanding the complexity and dynamics of microbial communities in various environments.
Chlamydomonas: Chlamydomonas is a genus of unicellular green algae that are found in various aquatic environments, characterized by their flagellated cells and photosynthetic capabilities. These organisms play a significant role in freshwater ecosystems as primary producers and are important for understanding the evolution of plant life.
Chlorophyll c: Chlorophyll c is a type of chlorophyll found primarily in certain groups of algae, including diatoms and brown algae. This pigment plays a crucial role in photosynthesis by allowing these organisms to capture light energy, especially in aquatic environments where light wavelengths can vary significantly. Chlorophyll c absorbs light most effectively in the blue and red regions of the spectrum, which helps support the photosynthetic processes of these organisms.
Choanoflagellates: Choanoflagellates are a group of unicellular and colonial protists characterized by their distinctive collar of microvilli surrounding a single flagellum. These organisms are significant as they are considered to be the closest living relatives of animals and play a crucial role in understanding the evolution of multicellularity. Their unique morphology and feeding mechanisms highlight important ecological functions in aquatic environments.
Chromalveolata: Chromalveolata is a major group of protists that includes diverse organisms such as brown algae, diatoms, and ciliates, characterized by unique cellular structures and pigmentation. This group plays a crucial role in aquatic ecosystems and consists of two primary lineages: Alveolates and Stramenopiles, which are distinguished by specific traits like the presence of alveoli or distinctive flagella. Understanding Chromalveolata is vital for grasping the ecological significance and evolutionary relationships among various protists.
Chytrids: Chytrids are a group of fungi classified within the phylum Chytridiomycota, known for their unique reproductive structures and aquatic habitats. They are characterized by having motile spores, called zoospores, that possess flagella, allowing them to swim in water. This group plays a crucial role in the decomposition of organic material in aquatic ecosystems and has implications for biodiversity, especially concerning amphibian populations impacted by chytridiomycosis.
Cilia: Cilia are hair-like structures that extend from the surface of many eukaryotic cells and play crucial roles in movement and sensory functions. These tiny organelles are made up of microtubules arranged in a specific pattern and are anchored to the cell by a basal body. Cilia can be found in various organisms, including protists, where they assist in locomotion and feeding, and in multicellular organisms, where they help move fluids across cell surfaces and participate in excretion processes.
Cysts: Cysts are specialized structures that many protists form to survive unfavorable environmental conditions. They are typically resistant, protective forms that can encapsulate the organism's cellular components, allowing it to endure harsh environments such as extreme temperatures or lack of nutrients. The ability to form cysts is a crucial adaptation for survival and plays a significant role in the life cycle of various protist groups.
Diatoms: Diatoms are a type of microscopic algae characterized by their unique silica-based cell walls, called frustules, which exhibit intricate and beautiful patterns. These organisms are crucial players in aquatic ecosystems, contributing to primary production and serving as a food source for various marine organisms. They are a major component of phytoplankton, significantly influencing the ecology of both freshwater and marine environments.
Dictyostelium: Dictyostelium is a genus of soil-dwelling amoebae that are known for their unique life cycle and social behavior, particularly in forming multicellular structures. These protists can exist as single cells, but under certain conditions, they aggregate to form a multicellular organism that exhibits coordinated movement and development. This fascinating behavior makes Dictyostelium a model organism for studying cellular communication and differentiation.
Dinoflagellates: Dinoflagellates are a group of unicellular protists, characterized by their two flagella and a unique cellular structure, often including armored plates made of cellulose. These organisms are significant in both marine and freshwater ecosystems, playing vital roles in food webs and contributing to phenomena like red tides through harmful algal blooms.
Diplomonads: Diplomonads are a group of flagellated protists characterized by having two nuclei and multiple flagella. These unicellular organisms are typically found in anaerobic environments and are notable for their role as parasites in various hosts, including humans. Their unique cellular structure and adaptations allow them to thrive in low-oxygen habitats, which is an important feature for understanding their ecological impact and evolutionary significance.
Endosymbiosis: Endosymbiosis is a biological theory that explains how certain organelles within eukaryotic cells, such as mitochondria and chloroplasts, originated from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells. This process has significant implications for understanding the evolution of complex life forms and the relationships among different species.
Entamoeba: Entamoeba is a genus of parasitic amoebae that primarily inhabit the intestines of humans and other animals. Most notably, Entamoeba histolytica is responsible for amoebic dysentery, a severe intestinal infection that can lead to diarrhea and dysentery. This genus plays a significant role in the context of protists, as it highlights both the diversity of life forms and the impact of certain protists on human health.
Euglena: Euglena is a single-celled organism classified as a protist, known for its unique combination of plant-like and animal-like characteristics. It contains chloroplasts for photosynthesis, allowing it to produce its own food, while also having the ability to consume organic material when light is unavailable. This versatility makes Euglena an important representative of protists, showcasing the diverse adaptations within this group.
Euglenozoans: Euglenozoans are a diverse group of protists characterized by their unique flagella, which are often used for movement. This group includes organisms that are both photosynthetic and heterotrophic, making them ecologically significant in various environments. Their dual nutritional strategies, along with distinctive cellular features like a spiral or crystalline rod in their flagella, set them apart from other protists and highlight their evolutionary adaptations.
Excavata: Excavata is a diverse group of protists characterized by their unique feeding groove and distinct morphological features. This group includes various organisms that exhibit a range of lifestyles, from free-living to parasitic, and can be found in diverse environments. Excavata plays a significant role in understanding the evolutionary relationships among eukaryotes and contributes to the overall diversity of protists.
Filopodia: Filopodia are slender, finger-like projections that extend from the surface of certain cells, particularly in protists and other eukaryotic organisms. These structures are primarily composed of actin filaments and play crucial roles in sensing the environment, cell movement, and communication between cells. Filopodia allow cells to explore their surroundings and can be important for processes such as feeding and locomotion in various groups of protists.
Flagella: Flagella are long, whip-like structures that protrude from the surface of certain cells, primarily used for movement. These appendages are essential for many organisms, providing them with the ability to swim through liquids, and they play a crucial role in the biology of various protists and algae.
Foraminiferans: Foraminiferans are single-celled protists characterized by their intricate shells, known as tests, which are typically composed of calcium carbonate. These organisms play a significant role in marine ecosystems and are used in geological studies to understand past environmental conditions due to their fossil record.
Giardia: Giardia is a genus of microorganisms that are flagellated protists and are known for causing giardiasis, a common intestinal infection in humans and animals. These unicellular parasites are often found in contaminated water sources and can survive outside of a host for extended periods. Giardia has two main life stages: the cyst stage, which is resistant to environmental stress, and the trophozoite stage, which is the active form that inhabits the intestines of hosts.
Haptonema: A haptonema is a specialized, hair-like structure found in certain groups of protists, particularly among the haptophytes. It is used primarily for attachment and can also play a role in capturing food. The presence of haptonemas is significant as they highlight the unique adaptations of these protists in their aquatic environments.
Hydrogenosomes: Hydrogenosomes are organelles found in certain anaerobic protists and fungi that function in energy metabolism, producing hydrogen gas as a byproduct. These structures are analogous to mitochondria but serve a distinct role by facilitating ATP production through anaerobic processes, making them crucial for organisms that thrive in oxygen-free environments.
Ichthyosporeans: Ichthyosporeans are a group of unicellular organisms within the broader category of protists, specifically classified under the phylum Ichthyosporea. These organisms are primarily known for their parasitic and saprobic lifestyles, often affecting fish and other aquatic organisms. They have distinct life cycles that may include both vegetative and reproductive stages, showcasing their unique adaptations to aquatic environments.
Kinetoplasts: Kinetoplasts are specialized structures found within the mitochondria of certain protists, particularly in the group known as kinetoplastids. These unique organelles contain a network of circular DNA molecules, which are involved in the cell's energy production and are essential for the organism's survival. Kinetoplasts play a critical role in the biology of these protists, as they help in the replication of mitochondrial DNA and are crucial for their cellular functions.
Mitosomes: Mitosomes are small, membrane-bound organelles found in certain protists that are believed to be vestigial remnants of mitochondria. They play a role in cellular metabolism, particularly in energy production and iron-sulfur cluster assembly, despite lacking the typical functions associated with fully developed mitochondria. Mitosomes are an example of how protists have adapted to their environments and may highlight the evolutionary transitions among eukaryotic organisms.
Nucleariids: Nucleariids are a group of unicellular, amoeba-like protists that are characterized by their unique feeding habits and genetic traits. They belong to the larger group of unikonts and are closely related to fungi, suggesting a shared evolutionary ancestry. Nucleariids play an important role in the ecosystem, particularly in nutrient cycling and as a food source for other microorganisms.
Opisthokonta: Opisthokonta is a major group within the domain Eukarya that includes all animals, fungi, and several protists. This group is characterized by the presence of a single posterior flagellum in their motile cells, which distinguishes them from other eukaryotic groups. Opisthokonta plays a crucial role in understanding the evolutionary relationships among these diverse organisms.
Parabasalids: Parabasalids are a group of anaerobic protists that belong to the supergroup Excavata, characterized by their unique hydrogenosomes and lack of mitochondria. They are primarily known for their role in the digestion of cellulose and their symbiotic relationships with various organisms, including some that live in the intestines of herbivorous animals. These protists are notable for their flagella, which aid in movement and feeding.
Paramecium: Paramecium is a single-celled organism belonging to the protist group, characterized by its slipper-like shape and covered with hair-like structures called cilia. These cilia help Paramecium move through water and feed on bacteria and other small particles. As a well-studied model organism in biology, Paramecium showcases important features of protists, including its unique cellular structure and its methods for maintaining osmotic balance.
Photosynthesis: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water while releasing oxygen as a byproduct. This process is fundamental to life on Earth as it provides the primary energy source for nearly all living organisms and contributes to the cycling of carbon and oxygen in ecosystems.
Phycobilisomes: Phycobilisomes are complex, light-harvesting protein-pigment aggregates found in certain algae and cyanobacteria. These structures play a crucial role in photosynthesis by capturing light energy and funneling it to the photosynthetic reaction centers. They contain phycobilins, which are pigments that absorb light in the blue and green spectrum, allowing organisms to utilize a broader range of light wavelengths for photosynthesis.
Physarum: Physarum is a genus of slime molds that are known for their unique life cycle and ability to exhibit fascinating behaviors, including movement and feeding. These organisms belong to the group of protists and are often studied for their complex network formation and problem-solving abilities, making them a captivating example of how simple organisms can demonstrate advanced traits typically associated with higher organisms.
Plankton: Plankton are small, often microscopic organisms that drift in aquatic environments. They are crucial to marine ecosystems, serving as the primary producers and a food source for many marine animals.
Plasmodiophora: Plasmodiophora is a genus of parasitic protists belonging to the phylum Plasmodiophoromycota, primarily known for causing diseases in plants, especially important agricultural crops. These unicellular organisms are characterized by their ability to form plasmodia, which are multi-nucleated masses that arise during their life cycle. They play a significant role in the context of protists, particularly in how they interact with plant hosts and contribute to plant diseases.
Plasmodium: Plasmodium is a genus of single-celled parasites that are known for causing malaria in humans and other animals. These organisms have complex life cycles that involve both vertebrate and insect hosts, showcasing key features of protists. As important pathogens, they exhibit unique adaptations that facilitate their survival and reproduction within different environments.
Porphyra: Porphyra is a genus of red algae, commonly found in coastal waters around the world, that is often harvested for food and other uses. It is a key member of the group of protists known as Rhodophyta, characterized by its distinctive coloration due to the presence of phycoerythrin and phycocyanin pigments. This genus includes species like Porphyra umbilicalis, which is popular in culinary applications, particularly in Asian cuisine.
Protozoans: Protozoans are a diverse group of single-celled eukaryotic organisms that can exhibit both animal-like and plant-like characteristics. They are primarily classified under the kingdom Protista and are essential components of many ecosystems, acting as both predators and prey. Protozoans vary widely in shape, size, and habitat, and they can be found in various environments, from freshwater to marine and even in soil.
Pseudopodia: Pseudopodia are temporary, foot-like extensions of a cell's cytoplasm used for movement and feeding. These extensions play a crucial role in the locomotion of certain eukaryotic cells, allowing them to navigate their environment and engulf food particles. Pseudopodia are particularly significant in the study of protists, where they exemplify the diversity and adaptability of eukaryotic life forms.
Radiolarians: Radiolarians are a type of single-celled protists characterized by their intricate silica-based skeletal structures, known as tests. These microscopic organisms are predominantly found in oceanic environments and play a crucial role in marine ecosystems by contributing to the marine food web and the biogeochemical cycles of carbon and silica. Their unique morphology and the presence of pseudopodia help in capturing prey and facilitate buoyancy in water.
Raphe: A raphe is a longitudinal groove found on the surface of certain protists, especially diatoms. It plays a critical role in their locomotion and attachment to surfaces.
Reticulopodia: Reticulopodia are specialized, net-like extensions of the cytoplasm found in certain protists, specifically within the group known as foraminifera. These extensions play a crucial role in capturing food particles and facilitating movement, as they can expand and retract to form a complex mesh that interacts with the environment. Reticulopodia are an adaptation that enhances the organism's ability to thrive in various aquatic environments, showcasing the diversity of feeding strategies in protists.
Rhizaria: Rhizaria is a diverse group of protists that are primarily characterized by their hair-like extensions called 'filose' pseudopodia. This group includes important organisms like foraminifera and radiolarians, which play significant roles in marine ecosystems and sediment formation. Rhizaria are known for their intricate shell structures, contributing to the biological and geological processes within aquatic environments.
Tests: Tests are hard, protective shells produced by some protists. They serve as a defense mechanism and aid in buoyancy for the organism.
Trichomonas: Trichomonas refers to a genus of parasitic protozoa, most notably Trichomonas vaginalis, which is known to infect the human urogenital tract. This organism is characterized by its flagella and is primarily transmitted through sexual contact. Understanding Trichomonas is essential for studying protists and their role in human health, as well as the mechanisms of transmission and disease caused by this parasite.
Trypanosoma: Trypanosoma is a genus of protozoan parasites that belong to the group of protists known as flagellates. These single-celled organisms are responsible for significant diseases in humans and animals, most notably sleeping sickness and Chagas disease. Understanding Trypanosoma is crucial for grasping the diversity and impact of parasitic protists within their ecosystems and host organisms.
Ulva: Ulva, commonly known as sea lettuce, is a genus of green algae that typically grows in shallow marine environments and intertidal zones. This multicellular algae is characterized by its broad, flat, and leafy appearance, which can often be seen attached to rocks or floating in the ocean. As a member of the protist kingdom, Ulva plays an essential role in coastal ecosystems by providing food and habitat for various marine organisms.