Protists: Chapter 28

• Chapter 28 focuses on Protists
• More information and resources can be found by following the link and password

Hypermastigote

• A protist that lives in the gut of termites and cockroaches
• Aids in the digestion of wood for its hosts

3 Domains and Protists

• Protists are informally referred to as the "Junk Drawer" group, due to their diverse and sometimes unclear relationships

Domain Eukarya Characteristics

• Have a membrane-enclosed nucleus
• Contain mitochondria
• Contain chloroplasts
• Possess an endomembrane system
• Have a cytoskeleton
• Flagella and cilia have a 9 + 2 arrangement of microtubules
• Genome consists of multiple chromosomes
• Can be unicellular or multicellular

Protist Cell Type:

• All protists are eukaryotic

Protist Nutrition:

• Some protists are photosynthetic, like algae
• Some are heterotrophic, like Amoeba
• Certain protists, such as euglenoids and dinoflagellates, can switch between photosynthetic and heterotrophic modes, a phenomenon called mixotrophy

Protist Size:

• Most are unicellular, like Euglena
• Some are multicellular, like brown algae

Protist Habitat:

• Found in a variety of habitats
• Most require water and are found in moist environments
• Some are parasitic, either endoparasitic or ectoparasitic

Protist Movement:

• Most are motile
• Many have cilia or flagella at some point in their life cycles
• Eukaryotic cilia and flagella share the same 9 + 2 arrangement of microtubules, differing from the ultrastructure of prokaryotic flagella

Protist Reproduction:

• Reproduction is highly varied among protists
• Asexual reproduction occurs through mitosis
• Many also reproduce sexually
• Protists exhibit all three basic types of sexual life cycles
• Resistant cells called cysts are produced to endure harsh conditions

Protist Phylogeny:

• Protists are polyphyletic, meaning their evolutionary origins are diverse
• Found in many phylogenetic groups of Eukarya
• Protists are divided into three categories based on ecological relationships that include animal-like, plant-like (algae), and fungus-like protists
• These groupings are not formal taxonomic classifications
• Supergroups are used instead to reflect broader evolutionary relationships

Domain Eukarya Origin:

• Evolved from prokaryotes
• Involves the evolution of the endomembrane system
• Evolution of endosymbionts into chloroplasts and mitochondria

Endosymbiotic Hypothesis:

• Explains the origin of certain eukaryotic organelles through endosymbiosis, where one cell engulfs another, leading to a symbiotic relationship

Plastid Origin:

• Plastids, organelles used in photosynthesis, originated through endosymbiosis

Protist Phylogeny and Protist Classification

• "Protists" are represented in yellow in the Eukaryote Phylogeny diagrams

Supergroup Excavata

• It is a monophyletic group
• Based on cytoskeleton structure
• Some members have an "excavated" feeding groove on one side of their body
• Others are classified based on the structure of their flagella
• Examples include Trypanosoma and Giardia; an example mixotroph is Euglena

Diplomonadida & Parabasalids

• Lack plastids
• Possess modified mitochondria, possibly lacking electron transport chains
• Typically anaerobic and parasitic
• Examples include Giardia intestinalis and Trichomonas vaginalis

Giardia intestinalis

• A diplomonad representative that causes "beaver-fever"
• Diarrhea results from the infection of human (and other mammal) intestines
• Infection is caused by contaminated water with cysts
• Boiling water kills the parasite

Euglenozoa Clade

• Can be predatory heterotrophs, photosynthetic autotrophs, mixotrophs, or parasites
• Characterized by a rod with a spiral or crystalline structure inside their flagella
• Representative organisms include Trypanosoma sp. and Euglena sp.

Trypanosoma sp.

• Infects humans
• Causes African sleeping sickness (neurological disease, usually fatal if untreated) and Chagas' disease (leads to congestive heart failure)
• Known for evading the immune response by changing their surface protein structures

Euglena sp.

• Some are mixotrophic, performing photosynthesis when sunlight is available and switching to heterotrophic feeding at night; others are phagocytic

Supergroup SAR Clade

• Very diverse group
• Classification is based on DNA sequence data
• Thought to potentially originate from secondary endosymbiosis of red algae in the Alveolates & Stramenopiles
• Includes Stramenopiles, Alveolates, and Rhizarians
• An example includes Paramecium, a ciliate

Stramenopiles Clade

• Named for "straw hair," owing to numerous, fine, hairlike projections on their flagella
• Store food energy as laminarin, a glucose polymer
• Includes diverse photosynthetic autotrophs and heterotrophs
• Chloroplast derived from an ancestral red alga

Stramenopiles: Diatoms

• Found in freshwater and marine environments
• Their cell wall is made of silica in two parts, like a box and lid
• Able to glide via cytoplasmic streaming
• Their photosynthetic activity can affect carbon dioxide (CO2) levels: high abundances absorb CO2, then the diatoms die and sink to the ocean bottom (carbon sink) to possibly encourage blooms

Stramenopiles: Golden Algae

• Contain yellow and brown carotenoids
• Have biflagellated cells
• Found as freshwater and marine plankton
• Can be photosynthetic or mixotrophic
• Some are colonial, but most are unicellular
• Able to form cysts with degrading environmental conditions

Stramenopiles: Brown Algae

• Large, multicellular
• Mostly marine
• Most common along temperate coasts
• Some possess specialized structures like holdfasts, stemlike stipes supporting leaflike blades, and gas-filled floats
• Brown Algae go through alteration of generations during their life cycle

Alveolata Clade

• Have membrane-bound sacs (alveoli) under the plasma membrane with unknown function
• Abundant in multiple habitats and include both photosynthetic and heterotrophic individuals

Alveolata: Dinoflagellates

• Have a pair of flagella in perpendicular grooves
• This causes them to spin as they swim
• Marine or freshwater, some are parasitic forms
• Mostly unicellular
• Some species are bioluminescent
• Some cause disease, such as paralytic shellfish poisoning

Alveolata: Apicomplexans

• Also called sporozoans
• Parasitic
• Have complex lifecycles, often requiring two or more different host species

Apicomplexans - Plasmodium sp.

• Representative species that causes malaria
• Spread by mosquitoes
• Involves a complex lifecycle

Ciliates

• Use cilia for locomotion and feeding
• Mainly in freshwater
• Mostly predators of bacteria or small protists

Rhizarian Clade

• Characterized by threadlike pseudopodia
• Based on DNA similarity
• Includes foraminiferans, radiolarians, and cercozoans

Rhizarian: Foraminifera (“forams”)

• Have porous shells, called tests, of calcium carbonate
• Fossil forams are used for aging sedimentary deposits

Rhizarian Clade Info

• Many species in this group are amoebas that move and feed by pseudopodia and cytoplasmic streaming
• Dispersed across many eukaryotic taxa, so is not a monophyletic group

Rhizarian: Radiolarians

• Have porous shells made of silica
• Their pseudopodia are reinforced by bundles of microtubules

Rhizarian: Cercozoans

• Contains mostly amoeboid and flagellated protists that feed with threadlike pseudopodia
• Mostly heterotrophs
• At least one species is autotrophic, likely acquiring its plastid directly from a cyanobacterium

Supergroup Archaeplastida

• A monophyletic group from common ancestor that engulfed cyanobacteria
• Secondary endosymbiosis led to this group
• Includes red algae, green algae, and land plants

Red Algae

• Mainly multicellular
• All are marine
• Have phycoerythrin as an accessory pigment that absorbs blue and green light
• Examples include nori (Porphyra)

Archaeplastida: Green Algae

• An ancestor to all Plants
• Mostly freshwater, some marine, terrestrial
• Exhibit unicellular, colonial, and multicellular forms
• Have cell walls made of cellulose
• Can be free-living or symbiotic life-styles

Supergroup Unikonta

• Extremely diverse group with two clades: amoebozoans and opisthokonts
• Relationship between the two is supported by myosin proteins and some studies of multiple genes/genomes

Amoebozoans Clade

• Characterized by lobe- or tube-shaped pseudopodia
• Use pseudopodia to move and to feed
• Includes amoebas and slime molds
• Representative organism: Amoeba proteus

Amoebozoans: Plasmodial Slime Molds

• Single mass of cytoplasm undivided by plasma membranes
• Contains many nuclei
• Shows no cytokinesis following mitosis

Opisthokonts Clade

• Includes animals, fungi, and many groups of protists
• Nucleariids and choanoflagellates, can be found in the Fungi and Animal diversity chapters, respectively

Key Protist Facts

• All protists are eukaryotic