Chapter 28 Protists
Overview: A World in a Drop of Water
• In the past, taxonomists classified all protists in a single kingdom,
• However, it is now clear that Protista is in fact paraphyletic.
• Some protists are more closely related to plants, fungi, or animals
than they are to other protists.
• As a result, the kingdom Protista has been abandoned.
• Various lineages are recognized as kingdoms in their own right.
• Scientists still use the convenient term protist informally to refer to
eukaryotes that are not plants, animals, or fungi.
Concept 28.1 Protists are an extremely diverse assortment of
• Protists exhibit more structural and functional diversity than any other
group of organisms.
• Most protists are unicellular, although there are some colonial and
• At the cellular level, many protists are very complex.
• This is to be expected of a single cell that must carry out the
basic functions performed by all the specialized cells in a
• Protists are the most nutritionally diverse of all eukaryotes.
• Some are photoautotrophs, containing chloroplasts.
• Some are heterotrophs, absorbing organic molecules or
ingesting food particles.
• Some are mixotrophs, combining photosynthesis and
heterotrophic nutrition. • Protists can be divided into three groups, based on their roles in
• These groups are not monophyletic.
• Protists include photosynthetic algal protists, ingestive
protozoans, and absorptive protists.
• Protist habitats are also very diverse.
• The life cycles of protists vary greatly.
• Some are exclusively asexual, while most have life cycles
including meiosis and syngamy.
Endosymbiosis has a place in eukaryotic evolution.
• Much of protist diversity is the result of endosymbiosis, a process in
which unicellular organisms engulfed other cells that evolved into
organelles in the host cell.
• The earliest eukaryotes acquired mitochondria by engulfing alpha
• The early origin of mitochondria is supported by the fact that all
eukaryotes studied so far either have mitochondria or had them
in the past.
• Later in eukaryotic history, one lineage of heterotrophic eukaryotes
acquired an additional endosymbiont—a photosynthetic
cyanobacterium—that evolved into plastids.
• This lineage gave rise to red and green algae.
• This hypothesis is supported by the observation that the DNA of
plastids in red and green algae closely resembles the DNA of
• Plastids in these algae are surrounded by two membranes,
presumably derived from the cell membranes of host and
• On several occasions during eukaryotic evolution, red and green
algae underwent secondary endosymbiosis. • They were ingested in the food vacuole of a heterotrophic
eukaryote and became endosymbionts themselves.
• For example, algae known as chlorarachniophytes
evolved when a heterotrophic eukaryote engulfed a green
• This process likely occurred comparatively early in
evolutionary time, because the engulfed alga still carries
out photosynthesis with its plastids and contains a tiny,
vestigial nucleus called a nucleomorph.
Concept 28.2 Diplomads and parabasalids have modified
• Most diplomonads and parabasalids are found in anaerobic
• These protists lack plastids, and their mitochondria lack DNA, an
electron transport chain, and the enzymes needed for the citric acid
• In some species, the mitochondria are very small and produce
cofactors for enzymes involved in ATP production in the cytosol.
• Diplomonads have two equal-sized nuclei and multiple flagella.
• Giardia intestinalis is an infamous diplomonad parasite that lives in
the intestines of mammals.
• The most common method of acquiring Giardia is by drinking
water contaminated with feces containing the parasite in a
dormant cyst stage.
• The parabasalids include trichomonads.
• The best-known species, Trichomonas vaginalis, inhabits the vagina
of human females.
• If the normal acidity of the vagina is disturbed, T. vaginalis can
outcompete beneficial bacteria and infect the vaginal lining.
• The male urethra may also be infected but without symptoms. • The infection is sexually transmitted.
• Genetic studies of T. vaginalis suggest that the species became
pathogenic after some individuals acquired a particular gene
through horizontal gene transfer from other vaginal bacteria.
• The gene allows T. vaginalis to feed on epithelial cells.
Concept 28.3 Euglenozoans have flagella with a unique internal
• Euglenozoa is a diverse clade that includes predatory heterotrophs,
photosynthetic autotrophs, and pathogenic parasites.
• Members of this group are distinguished by the presence of a spiral
or crystalline rod inside their flagella.
• Most euglenozoans have disc-shaped mitochondrial cristae.
• The best-studied groups of euglenozoans are the kinetoplastids and
• The kinetoplastids have a single large mitochondrion associated with
a unique organelle, the kinetoplast.
• The kinetoplast houses extranuclear DNA.
• Kinetoplastids are symbiotic and include pathogenic parasites.
• For example, Trypanosoma causes African sleeping sickness,
a disease spread by the African tsetse fly, and Chagas’
disease, which is transmitted by bloodsucking bugs.
• Trypanosomes evade immune detection by switching surface
proteins from generation to generation, preventing the host from
• One-third of Trypanosoma’s genome codes for these surface
• Euglenids are characterized by an anterior pocket from which one or
two flagella emerge.
• They also have a unique glucose polymer, paramylon, as a
storage molecule. • Many species of the euglenid Euglena are autotrophic but can
become heterotrophic in the dark.
• Other euglenids can phagocytose prey.
Concept 28.4 Alveolates have sacs beneath the plasma membrane
• Members of the clade Alveolata have alveoli, small membrane-bound
cavities, under the plasma membrane.
• Their function is not known, but they may help stabilize the cell
surface or regulate water and ion content.
• Alveolata includes flagellated protists (dinoflagellates), parasites
(apicomplexans), and ciliates.
• Dinoflagellates are abundant components of marine and
• Dinoflagellates and other phytoplankton form the foundation of
most marine and many freshwater food chains.
• Other species of dinoflagellates are heterotrophic.
• Most dinoflagellates are unicellular, but some are colonial.
• Each dinoflagellate species has a characteristic shape, often
reinforced by internal plates of cellulose.
• Two flagella sit in perpendicular grooves in the “armor” and produce a
• Dinoflagellate blooms, characterized by explosive population growth,
can cause “red tides” in coastal waters.
• The blooms are brownish red or pinkish orange because of the
presence of carotenoids in dinoflagellate plasmids.
• Toxins produced by some red-tide organisms have produced
massive invertebrate and fish kills.
• These toxins can be deadly to humans as well.
• Some dinoflagellates form mutualistic symbioses with coral polyps,
the animals that build coral reefs. • Photosynthetic products from the dinoflagellates provide the
main food resource for reef communities.
• All apicomplexans are parasites of animals, and some cause serious
• The parasites disseminate as tiny infectious cells (sporozoites)
with a complex of organelles specialized for penetrating host
cells and tissues at the apex of the sporozoite cell.
• Apicomplexans have a nonphotosynthetic plasmid called the
apicoplast, which carries out vital functions including the
synthesis of fatty acids.
• Most apicomplexans have intricate life cycles with both sexual
and asexual stages and often require two or more different host
species for completion.
• Plasmodium, the parasite that causes malaria, spends part of its life
in mosquitoes and part in humans.
• The incidence of malaria was greatly diminished in the 1960s by the
use of insecticides against the Anopheles mosquitoes, which spread
the disease, and by drugs that killed the parasites in humans.
• However, resistant varieties of Anopheles and Plasmodium
have caused a malarial resurgence.
• About 300 million people are infected with malaria in the
tropics, and up to 2 million die each year.
• The search for malarial vaccines has had little success because
Plasmodium is evasive.
• It spends most of its time inside human liver and blood cells,
and continually changes its surface proteins, thereby changing
its “face” to the human immune system.
• The need for new treatments for malaria led to a major effort to
sequence Plasmodium’s genome.
• By 2003, researchers had identified the expression of most of
the parasite’s genes at specific points in its life cycle. • This research could help scientists identify potential new targets
• Identification of a gene that may confer resistance to
chloroquine, an antimalarial drug, may lead to ways to block
drug resistance in Plasmodium.
• Ciliates are a diverse group of protists, named for their use of cilia to
move and feed.
• The cilia may cover the cell surface or be clustered into rows or
• Some ciliates scurry about on leglike structures constructed
from many cilia.
• A submembrane system of microtubules coordinates ciliary
• The cilia are associated with a submembrane system of microtubules
that may coordinate movement.
• Ciliates have two types of nuclei, one or more large macronuclei and
• Each macronucleus has dozens of copies of the ciliate’s
• The genes are not organized into chromosomes but are
packaged into small units with duplicates of a few genes.
• Macronuclear genes control the everyday functions of the
cell such as feeding, waste removal, and water balance.
• Ciliates generally reproduce asexually by binary fission of the
macronucleus, rather than mitotic division.
• The sexual shuffling of genes occurs during conjugation, during which
two individuals exchange haploid micronuclei.
• In ciliates, reproduction and conjugation are separate processes.
• In a real sense, ciliates have “sex without reproduction.” Conjugation provides an opportunity for ciliates to eliminate transposons
and other types of “selfish” DNA that can replicate within a genome.
• During conjugation, foreign genetic elements are excised when
micronuclei develop from macronuclei.
• Up to 15% of a ciliate’s genome may be removed every time it
Concept 28.5 Stramenopiles have “hairy” and smooth flagella
• The clade Stramenopila includes both heterotrophic and
• The name of this group is derived from the presence of
numerous fine, hairlike projections on the flagella.
• In most cases, a “hairy” flagellum is paired with a smooth
• In most stramenopile groups, the only flagellated stages are
motile reproductive cells.
• The heterotrophic stramenopiles, the oomycetes, include water
molds, white rusts, and downy mildews.
• Many oomycetes have multinucleate filaments that resemble
• However, there are many differences between oomycetes and
• Oomycetes have cell walls made of cellulose, while fungal
walls are made of chitin.
• The diploid condition, reduced in fungi, is dominant in
oomycete life cycles.
• Oomycetes have flagellated cells, while almost all fungi
• Molecular systematics has confirmed that oomycetes are not
closely related to fungi. • Their superficial similarity is a case of convergent
• In both groups, the high surface-to-volume ratio of
filamentous hyphae enhances nutrient uptake.
• Although oomycetes descended from photosynthetic ancestors, they
no longer have plastids.
• Instead, they acquire nutrients as decomposers or parasites.
• Water molds are important decomposers, mainly in fresh water.
• They form cottony masses on dead algae and animals.
• White rusts and downy mildews are parasites of terrestrial plants.
• They are dispersed by windblown spores, and form flagellated
zoospores at another point in their life cycles.
• One species of downy mildew threatened French vineyards in
• Another species causes late potato blight, which contributed to
the Irish famine in the 19th century.
• Late blight continues to cause crop losses today.
• Researchers are working to develop resistant potatoes by