1 LECTURE 11 Protists

BIO 120
Field Natural History
What is a Protist?
A. The Kingdom Protista is considered a catchall kingdom that contains all the eukaryotes that are not animals, plants
or fungi.
1. They are grouped into the same Kingdom not because they are closely related.
a. In fact, it is believed that they came from many different ancestors.
2. They vary greatly in their ecology, their anatomy and in their life history ( how they are born, how they make a
living, how they reproduce).
B. What are the characteristics of the Protista?
1. They are Eukaryotes. This is the only consistent, common feature among them.
2. Protists are about 1,000 times larger than prokaryotes.
a. This allows them to be more complex than prokaryotes. There is more room for organelles. The presence of
organelles allows for more efficient cell metabolism.
3. In general, they are unicellular, BUT they may grow in colonies or be multicellular.
4. They are very diverse: plant-like autotrophs, animal-like heterotrophs, and fungus-like heterotrophs (saprobes).
Some have more than one type of nutrition (both autotrophic and heterotrophic: what we call mixotrophic).
Protistan Diversity
A. The major types of protists. They are categorized as subkingdoms:
1. algae - photosynthesizing autotrophs
2. protozoa - ingestive heterotrophs – [protozoa = first animals]
3. fungus-like - absorptive heterotrophs (absorb nutrients from organic matter)
B. Example of basic protist, and the basic problem of classification.
1. Euglena straddles the distinction between what we call a plant and what we call an animal.
a. Euglena is a mixotroph. It can ingest food (bacteria) and photosynthesize.
2. Also has a flagellum so that it can propel itself and an eyespot so that it can orient to light.
3. Euglena is not entirely autotrophic – it needs minute quantities of vitamin B12 from the environment.
a. Some members of the phylum lack chloroplasts, and subsist entirely by eating bacteria.
C. Habitats of protists - similar to Monera (bacteria), they’re found pretty much anywhere there’s water, although they
don’t live in such a wide variety of chemical environments because their metabolism isn’t quite as diverse.
1. They are especially common in aquatic and also terrestrially where the soil is damp.
2. Also common as parasites in body fluids and tissues of other organisms.
D. Plant-like protists: subkingdom algae.
1. Algae are photosynthetic protists with a cellulose cell wall.
a. Because they are photosynthetic, it means that they have chlorophyll,
2. Algae are very important ecologically because they account for half of the global photosynthesis, which is the
main source of organic matter in ecosystems.
3. There are 4 phyla of algae. All are aquatic.
a. The brown algae – Phaeophyta –The brown algae are all multicellular. Clearly, this is a case where the
basis for classifying organisms as Protists because they are unicellular does not hold-up. One more example
of the Kingdom Protista being a catchall Kingdom.
b. The red algae – Rhodophyta – Only 200 species live in freshwater. The nearly 4,000 other species are
marine and are the seaweeds we see cast up onto the beach. Almost all are multicellular. A mucilaginous
component of the cell walls is used to make agar and carrageenan (used in ice cream!). Porphyra, or “nori”
is used in sushi.
c. The golden algae – Chrysophyta – The diatoms are included in this phylum. Golden algae are unicellular,
but often colonial. Mostly found in freshwater, they are able to form resistant cysts when ponds and lakes
dry up in the summer.
The green algae – Phylum Chlorophyta.
a. The green algae are of special interest because the ancestor of green plants was a member of this group.The
green algae have the most in common with green plants.
The same chlorophyll – chlorophyll a and b.
Cell walls made of cellulose.
They store surplus carbohydrate as starch.
Within the green algae are three different growth forms spanning the range from unicellular to colonial to
a. Unicellular algae such as Micrasterias (see figure 20.26b in your text)
b. Spirogyra is a filamentous green algae. The identical cells are joined together. Not quite colonial.
c. In between the unicellular and multicellular green algae is the intermediate form – a colonial form.
d. In colonies, individual cells retain a high degree of functional independence, but the cells are interconnected
physically. Volvox, is an example of a colonial green algae.
e. Volvox is an important example because it borders on true multicellularity. Scientists like examples like this
because it provides an intermediate link (evidence) between to stages of evolution.
f. True multicellular algae, such as Ulva (see figure 20.26d) have differentiated cells that have different
functions. Some form leaflike blades whereas others form rootlike holdfasts that anchor the alga to
Dinoflagellates (Pyrrhophyta), Diatoms (Chrysophyta) and Euglena (Euglenophyta) together with the unicellular
algae, and small invertebrate animals form the plankton. (from planktos, the Greek word for “wanderer”).
These single celled organisms are found floating in marine or fresh water near the surface. If the cell is capable
of photosynthesis, then they may be referred to as phytoplankton.
Dinoflagellates (Pyrrhophyta). Most are unicellular. Characterized by two flagella, usually at right angles to
each other.
a. Responsible for large blooms. One species with a red pigment turns water red (red tide).
b. They also release powerful nerve toxins that kill any fish that eat them.
c. Toxins concentrated by filter feeders, i.e. clams, oysters, where toxins are concentrated. The poison does
not effect the shellfish, but they are poisonous if consumed by animals, including people
d. Pfiesteria piscicida is a toxic dinoflagellate that has been implicated as the primary causative agent of major
fish kills along coastal areas between Maryland and North Carolina.
E. Animal like protists - Subkingdom Protozoa.
1. What distinguishes the 4 phyla of protozoa from the plant-like and fungus-like protists includes:
a. They lack cell walls.
b. They ingest food particles – either they eat bacteria, dead organic matter, absorb organic molecules, or the
contents of cells (that makes them parasites.)
c. They move about freely.
2. They phyla are largely divided by how the protists move about.
3. Phylum (Zoo)Mastigophora:
a. Are characterized by flagella (zooflagellates) and so they are very motile.
b. Heterotrophic. Obtain food in a variety of ways. Some are free living, some symbiotic, some are parasitic.
4. Many of the members of this phylum are feared because more than half of the people on earth will contract some
type of protozoan infection in their lifetime. Among them:
a. Four species of Plasmodium that cause malaria.
b. Trypanosoma causes deadly encephalitis – “sleeping sickness”. It causes an inflammation of the brain after
being introduced by the bite of an infected insect.
c. Giardia lambia – the cause of ‘hikers diarrhea” The cysts are shed in the feces of muskrats and beavers.
Comes from drinking contaminated stream water.
d. Leishmania –transmitted by sand flies, it causes lesions on the skin, which can become secondarily infected
and cause disfigurement. Was a problem for soldiers during the Desert Storm campaign.
5. The phylum Rhizopoda are characterized by having no permanent locomotor apparatus. All are heterotrophs.
a. Included in this group are the Amoebas, which move about and feed by the formation of pseudopodia (“false
feet”). They engulf their prey by extending a pseudopod around it in a process called phagocytosis.
BIO 120
Field Natural History
b. Phylum Actinopoda (Radiolarians) and phylum Foramnifera - like amoebas, but encased in hard shells of
silica or calcium carbonate. The pseudopodia protrude through the holes in the shells to gather food
particles but not for movement.
c. The phyla Rhizopoda, Actinopoda and Foramnifera belong to the group of amoeboid protozoans called
The phylum Ciliophora are the ciliates. Most complex of the animal-like protists. They propel themselves with
many small hairs that move in an organized, rhythmic pattern.
a. The hairs are also involved in the propulsion of the organism.
b. The cilia have a fixed cell shape and two nuclei per cell.
c. The ciliates are so different from the other eukaryotes that they even use the genetic code differently!
F. Fungus-like protists: Characterized as either slime molds or water molds. (They are not molds like Fungi. They’re
more similar to amoebae and algae than Fungi. The fact that some look like Fungi is due to convergence.)
1. They are heterotrophic and they are either saprophytic (they feed on dead organic matter) or parasitic (they
feed on living cells).
2. Interesting example: is the cellular slime molds (phylum Acrasiomycota) . They are free-living, amoeba-like
cells in soil. When food is scarce, thousands of amoeba cells come together to form a slug, which migrates to a
new habitat. Once there, the colony differentiates. Some cells form the base, others the stalk and others form a
swollen tip that develops spores. Each spore, when released, becomes a new amoeba.
3. Water molds (phylum Oomycota) - responsible for the late blight in potatoes which caused the Irish potato
famine of 1845-1847.
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