Chapter 28

Protists

Concept 28.1: Eukaryotes and Protists

Protists are eukaryotes that are not plants, animals, or fungi.
Protists are not a kingdom because some protists are more closely related to plants, fungi, or animals than other protists.
Eukaryotic cells, including protists, have a nucleus and other membrane-enclosed organelles.
Organelles isolate functions, making eukaryotic cells more complex than prokaryotic cells.
Protists make up much of the diversity of eukaryotes.
Most protists are unicellular.

Structural and Functional Diversity

Protists are the most nutritionally diverse of all eukaryotes.
Photoautotrophs: contain chloroplasts.
Heterotrophs: absorb organic molecules or ingest larger food particles.
Mixotrophs: combine photosynthesis and heterotrophic nutrition.
Some protists reproduce asexually, while others have both asexual and sexual phases in their life cycle.
All three basic types of sexual life cycles (animal, plant, and fungal) are represented among protists.

Endosymbiosis in Eukaryotic Evolution

Protistan diversity has its origins in endosymbiosis.
Endosymbiosis: a relationship between two species in which one organism lives inside the cell or cells of the other organism (the host).
Mitochondria and plastids are derived from bacteria that were engulfed by ancestors of early eukaryotes.
Mitochondria evolved before plastids and arose from an alpha proteobacterium.
Molecular analysis indicates that mitochondria and plastids each evolved only once in the history of life.
The evolution of mitochondria gave rise to the eukaryotes.
Plastids arose later when a heterotrophic eukaryote engulfed a photosynthetic cyanobacterium.
Two lineages of photosynthetic protists, red and green algae, evolved from the plastid-bearing ancestor.

Unikonta

Includes amoebas with lobe- or tube-shaped pseudopodia, animals, fungi, and non-amoeba protists closely related to animals or fungi.
Amoeba proteus is a tubulinid amoeba.
Extensions are false feet or pseudopods.
They move slowly by cytoplasmic streaming.

Diplomonads and Parabasalids

Trichomonas vaginalis is a sexually transmitted parasite.
It travels along the human reproductive and urinary tracts and feeds on the vaginal lining in females.
$T. vaginalis$ infects about 140 million people per year worldwide.
Urinary tract infection (UTI) is a parasite

Euglenids

Euglenids have one or two flagella that emerge from a pocket at one end of the cell.
Some species are mixotrophs that switch between autotrophic and heterotrophic modes, depending on the environmental conditions.
Euglena move by whip-like action of the flagellum.
Lab Euglena were GREEN and mixotrophic.

Stramenopiles

Stramenopiles include some of the most important photosynthetic organisms on Earth.
Most have a “hairy” flagellum paired with a “smooth” flagellum.
Diatoms, oomycetes, and brown algae are three important groups of stramenopiles.

Diatoms

Diatoms are unicellular algae with a unique two-part, glass-like wall of silicon dioxide.
The wall withstands pressure up to 1.4 million protecting diatoms from the crushing jaws of predators
The fossilized remains of diatoms (glass and sharp plus trace amounts of minerals) are use in industrialize applications as a pesticide and insecticide
DIATOMACEOUS EARTH 2 kg/m.
Including about 100,000 species, diatoms compose much of the phytoplankton in the ocean and lakes.
Diatoms are so abundant and widespread that their photosynthetic activity affects global CO2 Levels
Phytoplankton produce nearly half of the earth’s oxygen and are critical to the food web in lakes and oceans.
After a diatom bloom, many dead individuals fall to the ocean floor, where decomposition is slow.
The breakdown and release of carbon stored in the diatoms on the ocean floor can take centuries.
Promoting diatom blooms by fertilizing the ocean with essential nutrients is a proposed approach to reduce atmospheric CO2 levels.

Brown Algae

Brown algae are the largest and most complex multicellular algae.
Carotenoids in the plastids produce the brown color.
Most are marine, including many species commonly called “seaweeds”.
Seaweeds and algae are technically NOT plants.
They lack a vascular system (xylem and phloem), roots, stems, and leaves commonly characteristic of plants.
Brown algal seaweeds have plantlike structures: the rootlike holdfast, which anchors the alga, and a stemlike stipe, which supports the leaflike blades.
Some have gas-filled, bubble-shaped floats to keep photosynthetic structures near the water surface.
Brown algae lack the true tissues and organs found in plants.
Brown algae are important commodities for humans.
Some species, such as Laminaria are eaten.
Algin, a gel-forming substance found in the cell wall, is used as a thickener in many processed foods.

Alternation of Generations

A variety of life cycles have evolved among the multicellular algae.
Some have alternation of generations, in which both haploid and diploid stages are multicellular.
The concept of alternation of generations is important for understanding the life-cycles of protista, fungi, and plants.
The diploid generation is called a sporophyte because it produces spores.
Haploid spores develop into multicellular haploid gametophytes that produce haploid gametes.
Fertilization of gametes results in a diploid zygote, which develops into a new sporophyte.
Once the gametes are fertilized a SPOROPHYTE is formed.
2 stages: Sporophyte And Gametophyte Sporophyes produce Spores, Gametophytes Produces sperm and eggs (gametes)

Oomycetes

Oomycetes include water molds, white rusts, and downy mildews.
They were misidentified as fungi due to their multinucleate filaments that resemble fungal hyphae.
One key difference is that oomycetes cell walls are composed of cellulose, rather than chitin.
Based on molecular analysis, oomycetes and fungi are not closely related.

Dinoflagellates

Dinoflagellates are abundant components of marine and freshwater phytoplankton.
They have two flagella housed in the grooves of armor-like cellulose plates that surround the cell.
Beating of the spiral flagella causes dinoflagellates to spin as they move through the water
Dinoflagellate blooms cause “red tides” where the water appears brownish red or pink due to the carotenoids present in their plastids.
Red tides are toxic and can cause massive kills of invertebrates and fishes.
Ocean warming caused by climate change is facilitating more frequent red tides.

Apicomplexans

Most life cycles include both sexual and asexual stages, and require two or more different hosts.
Plasmodium, the parasite causing malaria, lives in both mosquitoes and humans
Malaria is Caused by Plasmodium Plasmodium in liver cell, haploids penetrate red blood cells
Plasmodium evades the host immune system by living inside cells and continually changing its surface proteins.
Approximately 220 million people in the tropics are infected and 450,000 die each year from malaria.
The first licensed malarial vaccine was recently approved and routine use began in Africa in 2019
Current malarial vaccine: RMosquirix and new one developed in 2023
The first malaria vaccine approved by the World Health Organization (WHO) in 2021. It is recommended for children in areas with moderate to high malaria transmission

Ciliates

Ciliates are named for their use of cilia to move around and feed on bacteria or other protists.
The cilia may completely cover the cell surface or be clustered in a few rows or tufts
Lab PARAMECIUM Twirling around due To beating cilia

Rhizarians

Many species of rhizarians are amoebas
Amoebas are protists that move and feed using pseudopodia, extensions of the cell surface
Rhizarian amoebas differ from amoebas in other clades by having threadlike pseudopodia
Three clades included in the rhizarians are radiolarians, forams, and cercozoans

Red and Green Algae

Plastids arose when a heterotrophic protist acquired a cyanobacterial endosymbiont
The photosynthetic descendants of this ancient protist evolved into red algae and green algae
Plants are descended from the green algae
Archaeplastida is the supergroup that includes red algae, green algae, and plants

Red Algae

An accessory pigment called phycoerythrin masks the green of chlorophyll giving red algae its color
Color varies from greenish-red in shallow water to dark red or almost black in deep water
Most are multicellular; the largest are seaweeds
Red algae photosynthesis even though they are red
Reproduction is sexual in red algae and life cycles often include alternation of generations
Red algae are common in coastal waters of tropical oceans
Some species are consumed by humans, such as Porphyra (“nori”) that is used to wrap sushi

Green Algae

Green algae are named for their green chloroplasts, which are structurally and chemically similar to those found in plants
Most chlorophytes live in fresh water, although there are many marine and some terrestrial species
Various unicellular species are free-living while others live symbiotically with other eukaryotes
Some live in environments exposed to intense visible and ultraviolet radiation
Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages
Nearly all species have biflagellated gametes with cup-shaped chloroplasts
Alternation of generations has evolved in some chlorophytes, including Ulva
Two lifecycles- asexual and sexual reproducution

Protists in Ecological Communities

Protists are found in diverse aquatic and moist terrestrial environments
Protists play two key roles in their habitats: that of symbiont and that of producer
2 major roles of protists

Symbiotic Protists

Some protist symbionts benefit their hosts
Some dinoflagellates live within the polyps and nourish reef-building corals
Some protists inhabit the guts of termites and aid with the digestion of wood
Some protist symbionts are parasites
Plasmodium causes malaria in humans
Phytophthora ramorum causes sudden oak death

Photosynthetic Protists

Many protists are producers, organisms using energy from light (or inorganic compounds) to convert to organic compounds
In aquatic communities, the main producers are photosynthetic protists and prokaryotes
All other organisms are consumers that directly or indirectly depend on producers for food
Consumers “eat” producers The whale eating small fish would be a Secondary Consumer.
Photosynthetic protists are limited by nutrients; populations explode when nutrients are added
Population booms can have major ecological consequences, such as the formation of marine “dead zones”
Growth and biomass of photosynthetic protists and prokaryotes have declined with increasing sea surface temperature
Phytoplankton communities rely on upwelling of cold, nutrient-rich water from the below
Warm surface water acts as a barrier to upwelling