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Chapter 28. Protists

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Yun Doo Chung

on 23 September 2014

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Transcript of Chapter 28. Protists

Chapter 28. Protists
Protists are eukaryotes
Eukaryotic cells have organelles and are more complex than prokaryotic cells
Most protists are unicellular, but there are some colonial and multicellular species
Protists play key roles in ecological communities
Protists are found in diverse aquatic environments
Protists often play the role of symbiont or producer
Even a low-power microscope can reveal a great variety of organisms in a drop of pond water
Protist is the informal name of the group of mostly unicellular eukaryotes
Advances in eukaryotic systematics have caused the classification of protists to change significantly
Protists constitute a polyphyletic group, and Protista is no longer valid as a kingdom
Overview: Living Small
Which of these organisms are prokaryotes and which are eukaryotes?
Key Concepts
Most eukaryotes are single-celled organisms

Excavates include protists with modified mitochondria and protists with unique flagella

Chromalveolates may have originated by secondary endosymbiosis

Rhizarians are a diverse group of protists defined by DNA similarities

Red algae and green algae are the closest relatives of land plants

Unikonts include protists that are closely related to fungi and animals

Protists play key roles in ecological communities
Most eukaryotes are single-celled organisms
Protists, the most nutritionally diverse of all eukaryotes, include
Photoautotrophs, which contain chloroplasts
Heterotrophs, which absorb organic molecules or ingest larger food particles
Mixotrophs, which combine photosynthesis and heterotrophic nutrition
Endosymbiosis in Eukaryotic Evolution
There is now considerable evidence that much protist diversity has its origins in endosymbiosis
is the process in which a unicellular organism engulfs another cell, which becomes an endosymbiont and then organelle in the host cell
Mitochondria evolved by endosymbiosis of an aerobic prokaryote
Plastids evolved by endosymbiosis of a photosynthetic cyanobacterium
Diversity of plastids produced by endosymbiosis
Five Supergroups of Eukaryotes
It is no longer thought that amitochondriates (lacking mitochondria) are the oldest lineage of eukaryotes
Many have been shown to have mitochondria and have been reclassified
Our understanding of the relationships among protist groups continues to change rapidly
One hypothesis divides all eukaryotes (including protists) into five supergroups
Protistan Diversity
Excavates include protists with modified mitochondria and protists with unique flagella
The clade
Excavata (섭식구굴착류)
is characterized by its cytoskeleton
Some members have a feeding groove
This controversial group includes the diplomonads, parabasalids, and euglenozoans
Chromalveolates may have originated by secondary endosymbiosis
Some data suggest that the clade Chromalveolata is monophyletic and originated by a secondary endosymbiosis event
The proposed endosymbiont is a red alga
This clade is controversial and includes the alveolates and the stramenopiles
Red algae and green algae are the closest relatives of land plants
Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont
The photosynthetic descendants of this ancient protist evolved into red algae and green algae
Land plants are descended from the green algae
is the supergroup that includes red algae, green algae, and land plants
Unikonts include protists that are closely related to fungi and animals
The supergroup Unikonta includes animals, fungi, and some protists
This group includes two clades: the amoebozoans and the opisthokonts (animals, fungi, and related protists)
The root of the eukaryotic tree remains controversial
It is unclear whether unikonts separated from other eukaryotes relatively early or late
Structural and Functional Diversity in Protists
Protists exhibit more structural and functional diversity than any other group of eukaryotes
Single-celled protists can be very complex, as all biological functions are carried out by organelles in each individual cell
Diplomonads and Parabasalids
These two groups lack plastids, have modified mitochondria, and most live in anaerobic environments

Have modified mitochondria called mitosomes
Derive energy from anaerobic biochemical pathways
Have two equal-sized nuclei and multiple flagella
Are often parasites, for example, Giardia intestinalis (also known as Giardia lamblia)
Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically
Include Trichomonas vaginalis, the pathogen that infects human female vagina.
is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and parasites
The main feature distinguishing them as a clade is a spiral or crystalline rod of unknown function inside their flagella
This clade includes the kinetoplastids and euglenids
Euglenozoan flagellum
have a single mitochondrion with an organized mass of DNA called a kinetoplast
They include free-living consumers of prokaryotes in freshwater, marine, and moist terrestrial ecosystems
This group includes Trypanosoma, which causes sleeping sickness in humans
Another pathogenic trypanosome causes Chagas’ disease
Members of the clade Alveolata have membrane-bounded sacs (alveoli) just under the plasma membrane
The function of the alveoli is unknown
The alveolates include
The clade
includes important phototrophs as well as several clades of heterotrophs
Most have a “hairy” flagellum paired with a “smooth” flagellum
Stramenopiles include diatoms, golden algae, brown algae, and oomycetes
are unicellular algae with a unique two-part, glass-like wall of hydrated silica
Diatoms usually reproduce asexually, and occasionally sexually
Diatoms are a major component of phytoplankton and are highly diverse
Fossilized diatom walls compose much of the sediments known as diatomaceous earth
After a diatom population has bloomed, many dead individuals fall to the ocean floor undecomposed
This removes carbon dioxide from the atmosphere and “pumps” it to the ocean floor
Golden algae
are named for their color, which results from their yellow and brown carotenoids
The cells of golden algae are typically biflagellated, with both flagella near one end
All golden algae are photosynthetic, and some are mixotrophs
Most are unicellular, but some are colonial

Red Algae
Red algae
are reddish in color due to an accessory pigment called phycoerythrin, which masks the green of chlorophyll
The color varies from greenish-red in shallow water to dark red or almost black in deep water
Red algae are usually multicellular; the largest are seaweeds
Red algae are the most abundant large algae in coastal waters of the tropics
Green algae
are named for their grass-green chloroplasts
Plants are descended from the green algae
Green algae are a paraphyletic group
The two main groups are chlorophytes and charophyceans
Charophytes are most closely related to land plants
are amoeba that have lobe- or tube-shaped, rather than threadlike, pseudopodia
They include slime molds, gymnamoebas, and entamoebas
Slime molds, or mycetozoans, were once thought to be fungi
Molecular systematics places slime molds in the clade Amoebozoa
Gymnamoebas are common unicellular amoebozoans in soil as well as freshwater and marine environments
Most gymnamoebas are heterotrophic and actively seek and consume bacteria and other protists
are parasites of vertebrates and some invertebrates
Entamoeba histolytica causes amebic dysentery, the third-leading cause of human death due to eukaryotic parasites
Symbiotic Protists
Some protist symbionts benefit their hosts
Dinoflagellates nourish coral polyps that build reefs
Wood-digesting protists digest cellulose in the gut of termites
Photosynthetic Protists
Many protists are important
that obtain energy from the sun
In aquatic environments, photosynthetic protists and prokaryotes are the main producers
In aquatic environments, photosynthetic protists are limited by nutrients
These populations can explode when limiting nutrients are added
Some protists reproduce asexually, while others reproduce sexually, or by the sexual processes of meiosis and fertilization
The plastid-bearing lineage of protists evolved into red and green algae
The DNA of plastid genes in red algae and green algae closely resemble the DNA of cyanobacteria
On several occasions during eukaryotic evolution, red and green algae underwent secondary endosymbiosis, in which they were ingested by a heterotrophic eukaryote
Trypanosoma, the kinetoplastid that causes sleeping sickness
Trypanosomes evade immune responses by switching surface proteins
A cell produces millions of copies of a single protein
The new generation produces millions of copies of a different protein
These frequent changes prevent the host from developing immunity
Euglena movement
Euglenids have one or two flagella that emerge from a pocket at one end of the cell
Some species can be both autotrophic and heterotrophic
Green Algae
Most chlorophytes live in fresh water, although many are marine
Other chlorophytes live in damp soil, as symbionts in lichens, or in snow
Multicellular chlorphytes
Larger size and greater complexity evolved in chlorophytes by

The formation of colonies from individual cells (e.g., Volvox)
The formation of true multicellular bodies by cell division and differentiation (e.g., Ulva)
The repeated division of nuclei with no cytoplasmic division (e.g., Caulerpa)
Many species of
plasmodial slime molds
are brightly pigmented, usually yellow or orange
At one point in the life cycle, plasmodial slime molds form a mass called a plasmodium (not to be confused with malarial Plasmodium)
The plasmodium is not multicellular
It is undivided by plasma membranes and contains many diploid nuclei
It extends pseudopodia through decomposing material, engulfing food by phagocytosis
Some protists are parasitic
Plasmodium causes malaria
Pfiesteria shumwayae is a dinoflagellate that causes fish kills
Phytophthora ramorum causes sudden oak death
Biomass of photosynthetic protists has declined as sea surface temperature has increased
If sea surface temperature continues to warm due to global warming, this could have large effects on
Marine ecosystems
Fishery yields
The global carbon cycle
Marine protists in a warmer world

(source) www.youtube.com/watch?v=jl0TzaWUQWk
Euglena, a euglenid commonly found in pond water
have two flagella and each cell is reinforced by cellulose plates
They are abundant components of both marine and freshwater phytoplankton
They are a diverse group of aquatic phototrophs, mixotrophs, and heterotrophs
Toxic “red tides” are caused by dinoflagellate blooms
dinoflagellate swiming
(source) www.youtube.com/watch?v=B2_H4vikJyk
are parasites of animals, and some cause serious human diseases
They spread through their host as infectious cells called sporozoites
One end, the apex, contains a complex of organelles specialized for penetrating host cells and tissues
Most have sexual and asexual stages that require two or more different host species for completion
The apicomplexan Plasmodium is the parasite that causes malaria
Plasmodium requires both mosquitoes and humans to complete its life cycle
Approximately 900,000 people die each year from malaria
Efforts are ongoing to develop vaccines that target this pathogen
The two-host life cycle of plasmodium, a apicomplexan that causes malaria
, a large varied group of protists, are named for their use of cilia to move and feed
They have large macronuclei and small micronuclei
Genetic variation results from conjugation, in which two individuals exchange haploid micronuclei
Conjugation is a sexual process, and is separate from reproduction, which generally occurs by binary fission
Structure and function in the ciliate paramecium caudatum
Paramecium movement
Moving Diatoms
Various Diatoms
Golden Algae
Dinobryon, a colonial golden alga found in fresh water
Brown Algae
Brown algae
are the largest and most complex algae
All are multicellular, and most are marine
Brown algae include many species commonly called “seaweeds”
Brown algae have the most complex multicellular anatomy of all algae
Giant seaweeds called kelps live in deep parts of the ocean
The algal body is plantlike but lacks true roots, stems, and leaves and is called a thallus
The rootlike holdfast anchors the stemlike stipe, which in turn supports the leaflike blades
Seeweeds: adapted to life at the ocen's margins
Alternation of Generations
A variety of life cycles have evolved among the multicellular algae
The most complex life cycles include an
alternation of generations
, the alternation of multicellular haploid and diploid forms
Heteromorphic generations are structurally different, while isomorphic generations look similar
The diploid sporophyte produces haploid flagellated spores called zoospores
The zoospores develop into haploid male and female gametophytes, which produce gametes
Fertilization of gamates results in a diploid zygote, which grows into a new sporophyte
The life cycle of the brown alga Laminaria: an example of alternation of generation
include water molds, white rusts, and downy mildews
They were once considered fungi based on morphological studies
Most oomycetes are decomposers or parasites
They have filaments (hyphae) that facilitate nutrient uptake
Their ecological impact can be great, as in potato blight caused by Phytophthora infestans
The life cycle of a water mold
Rhizarians are a diverse group of protists defined by DNA similarities
DNA evidence supports Rhizaria as a monophyletic clade
Amoebas move and feed by pseudopodia; some but not all belong to the clade Rhizaria
Rhizarians include radiolarians, forams, and cercozoans
Marine protists called
have tests fused into one delicate piece, usually made of silica
Radiolarians use their pseudopodia to engulf microorganisms through phagocytosis
The pseudopodia of radiolarians radiate from the central body
, or
, are named for porous, generally multichambered shells, called tests
Pseudopodia extend through the pores in the test
Foram tests in marine sediments form an extensive fossil record
Many forams have endosymbiotic algae
include most amoeboid and flagellated protists with threadlike pseudopodia
They are common in marine, freshwater, and soil ecosystems
Most are heteroptrophs, including parasites and predators
Paulinella chromatophora is an autotroph with a unique photosynthetic structure
This structure evolved from a different cyanobacterium than the plastids of other photosynthetic eukaryotes
The life cycle of chlamydomonas, a unicellular chlorophyte
Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages
What is the root of the eukaryotic tree?
Slime molds
Plasmodial slime molds
The life cycle of a plasmodial slime mold
Cellular slime molds
form multicellular aggregates in which cells are separated by their membranes
Cells feed individually, but can aggregate to form a fruiting body
Dictyostelium discoideum is an experimental model for studying the evolution of multicellularity
Cellular slime molds
The life cycle of Dictyostelium, a cellular slime mold
include animals, fungi, and several groups of protists
A symbiotic protist (parabasalid)
Protists: key producers in aquatic
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