Campbell Biology, Twelfth Edition - Chapter 28: Protists
Campbell Biology, Twelfth Edition - Chapter 28: Protists
Overview of Protists
Definition of Protists:
An informal term used to refer to all eukaryotes that are not classified as plants, animals, or fungi.
This group is not considered a kingdom, as some protists are more closely related to plants, fungi, or animals than to other protists.
Characteristics of Protists
Cellular Structure
Eukaryotic Features:
Protists possess a nucleus and membrane-enclosed organelles.
Organelles allow for compartmentalization of functions, increasing cellular complexity compared to prokaryotic cells.
The well-developed cytoskeleton in protists enables various cell shapes and flexible movement.
Diversity of Protists
Protists constitute a large portion of eukaryotic diversity.
Most protists are unicellular; however, some species are colonial or multicellular.
Organisms in protist lineages showcase significant structural and functional diversity, making them the most diverse group of eukaryotes.
Nutritional Diversity in Protists
Types of Nutrition:
Photoautotrophs: Organisms with chloroplasts that conduct photosynthesis.
Heterotrophs: Absorb organic molecules or ingest whole food particles.
Mixotrophs: Capable of photosynthesis and heterotrophic nutrition.
Reproductive Strategies
Protists exhibit various reproductive strategies:
Some reproduce only asexually, while others have both asexual and sexual phases in their life cycle.
All three basic types of sexual life cycles (animal, plant, fungal) are represented among protists.
Endosymbiotic Origin of Protists
Endosymbiosis
Definition: A symbiotic relationship where one organism lives inside the cells of another organism (the host).
Mitochondria and Plastids Evolution:
Evolved from bacteria that were engulfed by ancestors of early eukaryotes.
Mitochondria from an alpha proteobacterium evolved before plastids.
Molecular analysis shows both organelles evolved only once in the history of life.
Host Cell Features
The ancestral host cell possessed eukaryotic traits, including a cytoskeleton.
Proposed Taxon for Ancestral Host: Lokiarchaeotes, an archaean group is a candidate taxon for the lineage.
Plastid Evolution
Derived from the engulfment of a photosynthetic cyanobacterium by a heterotrophic eukaryote.
Red and green algae are descendants of plastid-bearing ancestors that arose from this process.
Secondary Endosymbiosis: Additional engulfment of red and green algae occurred multiple times in evolutionary history.
Example: chlorarachniophytes evolved when a heterotrophic eukaryote engulfed a green alga, retaining a vestigial nucleus (nucleomorph).
Supergroups of Eukaryotes
Current hypotheses categorize all eukaryotes into four supergroups:
Excavata
SAR (Stramenopiles, Alveolata, Rhizaria)
Archaeplastida
Unikonta
Excavata
Includes clades like diplomonads, parabasalids, and euglenozoans.
Example:
Giardia intestinalis: A diplomonad parasite causing intestinal infections in mammals.
SAR
Comprises three clades: Stramenopila, Alveolata, and Rhizaria.
Example:
Diatoms: Important photosynthetic stramenopiles.
Rhizarians include amoebas with threadlike pseudopodia, e.g., Globigerina.
Archaeplastida
Composed of red algae, green algae, and plants.
Example:
Volvox: A multicellular green algae.
Unikonta
Includes amoebas with lobe- or tube-shaped pseudopodia, animals, fungi, and protists related to animals or fungi.
Example:
Amoeba proteus: A tubulinid amoeba.
Excavata: Features
Characterized by unique cytoskeletal structures and an “excavated” feeding groove.
Composed of three monophyletic groups:
Diplomonads:
Lacking plastids and possessing reduced mitochondria (mitosomes).
Energy from anaerobic pathways. Example: Giardia intestinalis.
Parabasalids:
Possess hydrogenosomes for anaerobic energy.
Example: Trichomonas vaginalis - a sexually transmitted parasite affecting reproductive and urinary tracts.
Euglenozoans:
Diverse group, includes predators, autotrophs, and mixotrophs.
Characterized by a spiral or crystalline rod inside each flagellum.
SAR Supergroup: Definition
Monophyletic Group
Named from the first letters of its three major clades: Stramenopiles, Alveolates, and Rhizarians.
Stramenopiles
Important photosynthetic organisms with two types of flagella (hairy and smooth).
Examples include diatoms and brown algae, which play significant roles in aquatic ecosystems.
Diatoms:
Unicellular algae with a glass-like wall of silica, crucial for the ocean's phytoplankton.
Can withstand pressure and aid in carbon cycling.
Brown Algae
Largest and most complex multicellular algae with distinct structures (holdfasts, stipes, blades).
Major human commodities and roles in ecosystems.
Algin: Gel-forming substance used in food processing.
Life Cycle: Alternation of Generations
Involves alternating multicellular haploid and diploid stages, predominantly in multicellular algae.
Sporophyte: The diploid generation that produces spores.
Gametophyte: Multicellular haploid that creates gametes that fuse to form the diploid zygote.
Distinction between heteromorphic (different structures) and isomorphic (similar structures) generations.
Oomycetes
Water molds resembling fungi but with cellulose cell walls.
Acquire nutrients through parasitism; include notable pathogens affecting crops (e.g., Phytophthora infestans causing potato late blight).
Alveolates
Characterized by membranous sacs (alveoli) beneath the plasma membrane.
Includes dinoflagellates, apicomplexans (malaria-causing Plasmodium), and ciliates (e.g., Paramecium).
Ecological Importance of Protists
Play crucial roles as:
Symbionts: e.g., dinoflagellates sustaining coral reefs.
Producers: Photosynthetic organisms that form the base of food webs.
Impact on Food Webs
Photosynthetic protists significantly affect carbon levels and marine ecosystems,
Contributes to ocean health and maintaining fishing yields.
Global warming effects on species distribution and nutrient availability.
Overview of Protists
Definition of Protists:
An informal term used to refer to all eukaryotes that are not classified as plants, animals, or fungi.
This group is not considered a kingdom because some protists are more closely related to plants, fungi, or animals than to other protists.
Protists can be found in a variety of environments, including freshwater, marine, and moist terrestrial habitats, showcasing the adaptability of these organisms.
Characteristics of Protists
Cellular Structure
Eukaryotic Features:
Protists possess a nucleus and membrane-enclosed organelles, distinguishing them from prokaryotic organisms such as bacteria.
Organelles allow for compartmentalization of functions, facilitating more complex cellular processes compared to prokaryotic cells.
The well-developed cytoskeleton in protists not only enables diverse cell shapes but also supports flexible movement, crucial for their survival in varying environments.
Diversity of Protists
Protists constitute a large portion of eukaryotic diversity, serving as a bridge in the evolutionary history from unicellular to multicellular organisms.
Most protists are unicellular; however, notable exceptions include colonial or multicellular species such as the brown algae and certain slime molds.
Organisms in protist lineages showcase significant structural and functional diversity, making them the most diverse group of eukaryotes, with adaptations for various ecological niches.
Nutritional Diversity in Protists
Types of Nutrition:
Photoautotrophs: Organisms such as phytoplankton that have chloroplasts capable of conducting photosynthesis, playing a crucial role in carbon fixation and oxygen production.
Heterotrophs: These protists absorb organic molecules or ingest whole food particles, including parasitic forms that rely on host organisms for nutrients.
Mixotrophs: Capable of utilizing both photosynthesis and heterotrophic nutrition, providing adaptability in nutrient-scarce environments, as seen in certain euglenoids.
Reproductive Strategies
Protists exhibit various reproductive strategies, adapting to environmental conditions:
Some reproduce only asexually through processes like binary fission, while others have complex life cycles that involve both asexual and sexual phases.
All three basic types of sexual life cycles (animal, plant, fungal) are represented among protists, showcasing their evolutionary versatility.
Endosymbiotic Origin of Protists
Endosymbiosis
Definition: A symbiotic relationship where one organism lives inside the cells of another organism (the host), shaping eukaryotic evolution significantly.
Mitochondria and Plastids Evolution:
Evolved from bacteria that were engulfed by ancestors of early eukaryotes, leading to the development of the complex cellular structures present in modern protists.
Mitochondria are derived from an alpha proteobacterium that was engulfed first, whereas plastids originated from cyanobacteria.
Molecular analysis shows both organelles evolved only once in the history of life, which emphasizes the significance of endosymbiosis in cellular evolution.
Host Cell Features
The ancestral host cell possessed hallmark eukaryotic traits, including a cytoskeleton, enabling intricate cellular functions.
Proposed Taxon for Ancestral Host: Lokiarchaeotes, a candidate taxon among archaea, is believed to be a close relation to the ancestor of the engulfing cells, offering insights into the evolutionary transitions.
Plastid Evolution
Plastids in protists are derived from the engulfment of a photosynthetic cyanobacterium by a heterotrophic eukaryote, marking a pivotal moment in the development of photosynthetic life.
Red and green algae are descendants of plastid-bearing ancestors originating from this primary endosymbiotic event.
Secondary Endosymbiosis: Involving additional engulfments of red and green algae multiple times throughout evolutionary history.
Example: Chlorarachniophytes evolved when a heterotrophic eukaryote engulfed a green alga, retaining a vestigial nucleus (nucleomorph), evidencing the intricate evolutionary interactions.
Supergroups of Eukaryotes
Current hypotheses categorize all eukaryotes into four supergroups based on genetic and morphological data:
Excavata
SAR (Stramenopiles, Alveolata, Rhizaria)
Archaeplastida
Unikonta
Excavata
This supergroup includes diverse clades like diplomonads, parabasalids, and euglenozoans, characterized by unique genetic features and modes of living.
Example:
Giardia intestinalis: A diplomonad that acts as a parasite causing intestinal infections in mammals, illustrating the impact of protists on human health.
SAR
The SAR supergroup comprises three clades: Stramenopila, Alveolata, and Rhizaria, collectively contributing to ecological balance and carbon cycling in aquatic environments.
Example:
Diatoms: Important photosynthetic stramenopiles with silica walls, critical for ocean's food web and carbon cycling.
Rhizarians include amoebas with threadlike pseudopodia, such as Globigerina, which play a role in sediment productivity in marine environments.
Archaeplastida
This supergroup is composed of red algae, green algae, and land plants, showcasing the evolutionary lineage that led to terrestrial vegetation.
Example:
Volvox: A multicellular green alga significant for studying multicellularity and colony structures in protists.
Unikonta
Unikonta includes amoebas with lobe- or tube-shaped pseudopodia, alongside animals, fungi, and protists closely related to these groups.
Example:
Amoeba proteus: A tubulinid amoeba that exemplifies the complexity of unicellular eukaryotes and their movement mechanisms.
Excavata: Features
Characterized by unique cytoskeletal structures and an “excavated” feeding groove that influences their mode of nutrition.
Composed of three monophyletic groups, each exhibiting distinct characteristics:
Diplomonads:
Lacking plastids and possessing reduced mitochondria (mitosomes), with energy derived from anaerobic pathways. An important example is Giardia intestinalis.
Parabasalids:
Known for possessing hydrogenosomes for anaerobic energy production.
Example: Trichomonas vaginalis, a sexually transmitted parasite affecting reproductive and urinary tracts in humans, highlighting the public health significance of these organisms.
Euglenozoans:
This diverse group includes predators, autotrophs, and mixotrophs, showcasing a wide array of nutritional strategies.
They are characterized by a spiral or crystalline rod within each flagellum, which aids in effective movement.
SAR Supergroup: Definition
Monophyletic Group: Named from the first letters of its three major clades: Stramenopiles, Alveolates, and Rhizarians, representing a significant evolutionary branch of eukaryotes.
Stramenopiles
Important photosynthetic organisms characterized by the presence of two types of flagella (one hairy and one smooth).
Examples include diatoms and brown algae, both of which play crucial roles in aquatic ecosystems as primary producers.
Diatoms:
These unicellular algae possess a glass-like wall of silica, contributing significantly to the ocean's phytoplankton and carbon cycling.
They can withstand environmental pressures and contribute to marine food webs.
Brown Algae
Brown algae represent the largest and most complex multicellular algae, with distinct structural adaptations including holdfasts, stipes, and blades.
They serve as major commodities for human use, providing resources such as algin, a gel-forming substance widely utilized in food processing, and they play significant roles in ecosystem dynamics.
Life Cycle: Alternation of Generations
This life cycle involves alternating between multicellular haploid and diploid stages, predominantly observed in multicellular algae, indicating a complex reproductive strategy.
Sporophyte: The diploid generation that produces spores for dispersal and colonization.
Gametophyte: The multicellular haploid stage that produces gametes, which fuse to form the diploid zygote, showcasing genetic variation.
There is a distinction between heteromorphic (showing different structures) and isomorphic (exhibiting similar structures) generations, revealing diversity in life cycle strategies among protists.
Oomycetes
Water molds resemble fungi morphologically but possess cellulose in their cell walls, marking a divergence in evolutionary history.
They acquire nutrients primarily through parasitism, including notable pathogens affecting crops (e.g., Phytophthora infestans causing potato late blight), impacting agriculture and food security.
Alveolates
Alveolates are characterized by membranous sacs (alveoli) positioned beneath their plasma membrane, which may play a role in the regulation of buoyancy and storage.
This group includes diverse organisms such as dinoflagellates, apicomplexans (e.g., malaria-causing Plasmodium), and ciliates (e.g., Paramecium), each contributing to ecological dynamics and human health.
Ecological Importance of Protists
Protists play crucial roles as:
Symbionts: For instance, dinoflagellates sustain coral reefs through mutualistic relationships, highlighting their importance in marine ecosystems.
Producers: Photosynthetic organisms that form the base of food webs in aquatic systems, serving as essential food sources for a myriad of organisms.
Impact on Food Webs
Photosynthetic protists significantly influence carbon levels and marine ecosystems, contributing to ocean health and maintaining fishing yields which are vital for human economies.
They are also affected by global warming, which impacts species distribution and nutrient availability in aquatic environments, emphasizing the need for conservation efforts.