Background for AOA Protist Lab
What are protists? Originally, they were a general collection of just about all of the microorganisms that
could not be classified as plants or animals. Some of these protists are heterotrophic and animal-like: they
feed off organic matter and other protists. Some of these protists are plant-like autotrophs that perform
photosynthesis. However, many of the autotrophs will turn to preying on others when there is not enough
sunlight, making them "mixotrophic", that is both auto- and heterotrophic. Plant-like organisms were
collectively called "algae", and the animal-like heterotrophs were called"protozoans".
Recently, molecular and genetic analysis has revealed that this collection of organisms is actually quite
diverse and not that closely related to each other. With the recent classification of eukaryotes into five
supergroups, it has been shown that the protists must be divided up into related groups and classified
such thatthere are protist representatives in ALL FIVE of the supergroups.
The notion of a separate group of "protists", then, was misguided. These organisms are actually several
groups from each of the eukaryotic supergroups, even though they are superficially similar in that they are
unicellular, mostly microscopic, and found in marine or freshwater aquatic environments.
In this lab we will look at protist representatives of each of the five supergroups:
 Excavata
 Rhizaria
 Chromalveolata
 Plantae
 Unikonta
Note: the division of eukaryotes into five supergroups is currently contested. Many scientists advocate
recognition of a sixth group; Opisthokonta consisting of animals, fungi and their protozoan relatives like
choanoflagellates which we will examine in this lab.
Before we explore the supergroups in more detail we will discuss how protists may help scientists
understand the evolution of eukaryotes.
Symbiosis
A commonly held theory of evolution states that the earliest eukaryotes acquired mitochondria by
engulfing proteobacteria. Similarly photosynthetic autotrophs are believed to have been unicellular
heterotrophic organisms that developed an endosymbiotic relationship with ingested or engulfed
photosynthetic cyanobacteria. Over time, the cell walls of the bacteria living inside these organisms
disappeared but the cellular respiratory and photosynthetic machinery remained and became internal
mitochondria or chloroplasts. Mitochondria and chloroplasts actually have their own genome and proteinmaking machinery, an indication that they derived from once-independent organisms.
Endosymbiosis is a natural and very common occurrence among the protists. We routinely find
photosynthetic algae inside heterotrophic protists, and you will see several examples in this lab. In fact, it
is actually difficult to find individuals of certain protist species that do NOT have algae symbiotes. The
host organism has access to the sugar products from the algae's photosynthetic process which in turn
makes use of metabolic wastes of the host.
Colonies: the Development of Multicellularity
Another way in which protists are useful for studying evolution is the development of multicellularity.
Scientists believe that truly multicellular organisms may have evolved from colonial organisms. Colonial
organisms are unicellular organisms that are capable of associating with each other. Some of the
organisms you will examine in this lab are capable of switching back and forth between living together in
colonies and living separately as individual unicellular organisms. Colonial organisms integrate very few
of their cellular activities. That is why the individual cells can survive on their own – outside the colony. In
true multicellular organisms like plants and animals the cells integrate many of their functions. Cells are
differentiated into various functional types and cannot survive on their own. A few of the algae you will
examine in this lab are truly multicellular and you will examine many different colonial protists.
Now let's examine the supergroups in more detail.
Excavata
Excavates are unicellular eukaryotes. Included in this supergroup are the most primitive living organisms,
many of which do not have mitochondria. But the group also contains more advanced organisms. The
majority possess two or more flagella, whip like cellular extensions, that are used both for locomotion and
moving food into a feeding groove.
Euglena
Euglena belongs to the phylum Euglenophyta. They are photosynthetic organisms with chloroplasts that
enable them to make their own food. Euglenids are oval-shaped and move quickly by rotation of
flagellum. While, Euglenids make their own food by photosynthesis, they are capable of eating prey if they
choose to. Euglenids do not have cell walls but their cell membrane has a layer of proteins and fibers
arranged in a spiral right underneath the membrane. This pellicle protects the Euglenids while providing
far more flexibility than a cell wall.
Rhizaria
This structurally diverse supergroup consists mostly of unicellular organisms that possess some form of
pseudopods; cellular projections that are extended and retracted to allow for locomotion and capturing
prey. The first multicellular member of this group, Guttulinopsis vulgaris, was identified in 2012. Many
rhizarians produce a hard shell that encases and protects them. Most protist fossils are fossilized
rhizarian shells.
Foraminifera
Foraminifera (often just called forams) are some of the rhizarians that produce hard shells, or tests. They
extend their pseudopods out of forams, or holes, in their shells. Most foram species are marine and they
are found in very diverse oceanic habitats. Some species are planktonic (floating and drifting on the water
surface) while others live at the very depths of the ocean.
Radiolarians
Radiolarians also form shells and are planktonic. They commonly form symbiotic relationships with algae
by trapping photosynthetic algae in their web of filaments or within their shells. These symbiotic partners
benefit each other; the Radiolarians use the photosynthetic products of the algae and the algae use the
Radiolarians waste products.
Chlorarachniophytes
Chlorarachniophytes are rhizarians that do no produce shells. They are mixotrophic like euglena. Their
photosynthetic ability is particularly significant because they are one of the few groups that underwent a
secondary endosymbiosis. The non-photosynthetic Chlorarachniophytes ancestor engulfed an eukaryotic
alga (not a prokaryote) and retained it as a photosynthetic organelle.
Chromalveolata
Chromalveolates are primarily single celled eukaryotes. Scientists believe, that like the chloroplasts of
Chlorarachniophytes, the plastids of Chromalveolates were acquired by secondary endosymbiosis with a
red alga. Some Chromalveolates later lost their plastids.
Diatoms
Diatoms are extremely abundant in both marine and fresh water environments. They are photosynthetic
and are responsible for 20% to 25% of the entire organic fixation of carbon on the planet! Diatoms are
mostly planktonic.
Diatoms are found as both unicellular organisms or as colonies. They form shells like the forams and
Radiolarians. Their shells are usually made up of two halves that fit together.
Diatoms do not have flagella. They move by secreting mucilage through specialized pores at the end of
their cells. The mucilage absorbs water causing it to swell and as it gets bigger it pushes the diatoms
forward.
Diatom shells are made of silica. When silica is scarce, diatoms fall to the depths of the ocean where they
lie in a kind of suspended animation until currents change and bring silica and the diatoms back up to the
upper layers of the ocean.
Paramecium
The paramecium is a medium sized, uncolored, oval shaped organism. Paramecia move by paddling with
the many cilia around their bodies. Cilia are cellular extensions, similar to flagella, but smaller. While
flagella tend to occur alone; cilia are found in large groups that beat together to create a motive force.
Besides locomotion, the cilia also function to wave food into the oral groove where it is then encapsulated
in a food vacuole. Excess water and waste is expelled via a star-shaped contractile vacuole. A trapdoor at
the opening of the vesicle opens and closes to contain the vesicle's contents until the pressure inside
forces it open.
The paramecium has a macronucleus and a micronucleus. The macronucleus is the largest dark object in
the paramecium. The paramecium reproduces by two means, either by binary fission (simple division) or
conjugation, a method of sexual reproduction where the two paramecia exchange chromosomal DNA.
Paramecia release dart-like objects calledtrichocysts - tiny pointed filaments. Most researchers believe
that the trichocysts are released in order to anchor the paramecium to keep it from being carried away by
water currents.
Stentor
One of the largest Chromalveolates is a trumpet-shaped organism called the Stentor. It can measure up
to 2 millimeters long when extended, making it just visible with the naked eye. Stentors are shaped like
long trumpets, with a ring of cilia around the end that creates a whirlpool around the stentor's open mouth,
drawing in prey.
Stentors are found in freshwater lakes and streams. They are usually grouped together in colonies fixed
to surfaces at the at the base, but they are capable of swimming freely, during which time they contract
into a oval, like this:
Not everything that a Stentor ingests becomes food. Some algae remain within the Stentor cell and they
develop a symbiotic relationship similar to the Radiolarians.
Dinoflagellates
Phylum Dinophyta consist of red and brown unicellular organisms called dinoflagellates.
There are about 2,000 marine dinoflagellate species, and there are some freshwater species.
Dinoflagellates are infamous for causing red tide in ocean waters, a thick carpet of the organisms that
prevent sunlight from getting through to underwater plants, rob them of oxygen and secrete poisonous
waste products as well. However dinoflagellates contribute a great deal to any ecosystem they are part
of. Roughly half of the dinoflagellate species are photosynthetic; after diatoms they are the second most
abundant marine photosynthetic organisms. The other half of the Dinoflagellate species is heterotrophs
and they mostly feed via phagocytosis.
Most dinoflagellates have a tough armor-plated shell. In addition, they are characterized by hollow horns
that assist in floatation. Dinoflagellates have two flagella; each of which is located in a grove in the
surface of the shell. The transverse flagellum is a wide and flat and is wrapped around the cell. When it
beats it produces a ripple of waves that move along the flagella and propel the dinoflagellate. The
longitudinal flagellum is similar to the flagellum you will observe in euglena. It extends outward from the
cell and only produces one or two waves with each beat.
Plantae
Plantae includes the plants you are familiar with as well as a wide variety of protists. Members of this
supergroup all possess a plastid (chloroplast) derived from primary endosymbiosis.
As we mentioned earlier several members of this supergroup are multicellular. Some algae grow into
multicellular, plant-like structures. They have structures that resemble stems and leaves, and a base called a "holdfast" - that anchors the organism to the ocean or lake floor. But seaweeds are not true plants
because they lack a specialized vascular system (the internal conducting system for fluids and nutrients),
roots, stems, leaves, and enclosed reproductive structures like flowers and cones. In fact, because all the
parts of a seaweed are in contact with the water, they are able to take up fluids, nutrients, and gases
directly from the water, and do not need an internal conducting system.
Chlorophytes
Chlorophytes, or green algae, are large group of freshwater algae, with some 7,000 species. They are
unicellular or colonial. Most colonial chlorophytes are filamentous or form long threadlike colonies.
Charophytes, such as spirogyra, are the closest algae to true plants. They are the only algae whose
sperm are flagellated. The chloroplasts ofgreen algaecontain chlorophyll a and b.
Spirogyra
Spirogyrais an advanced colonial organism and some biologists believe it should be classified as
multicellular. It forms long, unbranched filaments. Those filaments can tangle together to form thick mats.
Volvox
Volvox of is a truly remarkable organism that forms spherical colonies of individual volvox cells that are
interconnected by very fine filaments of cytoplasm. The amazing thing is that this ball of cells is able to
rotate and swim toward a light source by coordinating the beating flagella of all the cells.
Volvox reproduce both sexually and asexually. In both cases, a daughter colony of clumped cells grows
inside the hollow sphere of the colony, until it is ready to break out on its own. At that time, the clump of
cells must turn itself inside-out in order to orient their flagella properly.
Rhodophyta
Red algaebelong to the phylum Rhodophyta. They are all multicellular. They have additional
photosynthetic pigments, called phycobiliproteins, that give them their red color, and they store
photosynthetic products in a different form of starch than green algae. The phycobiliproteins allow red
algae to grow in greater depths than green algae. As sunlight hits the water at an angle (morning and
evening and especially in the winter) it is refracted into a spectrum of colors like in a prism. The longer
blue and purple wavelengths penetrate deeper into the water than the shorter red, yellow and green.
Phycobiliproteins absorb blue and purple light; allowing the red algae to grow in greater depths.
Phaeophyta
Brown algaebelong to the phylum Phaeophyta. They are all multicellular. They have additional
photosynthetic pigments, called fucoxanthin, that give them their brown color, and they do not store
photosynthetic products as starch.
Unikonta
Unikonta is the supergroup that contains Animalia and Fungi, and the protists that are structurally and
genetically most similar to them.
Amoebozoa
The Amoebozoa are a varied group. They include the slime moldsthat are a popular lab organism for
studying various biological processes (see our Something Practical box at the end of the Procedures).
Other members are theEntamoeba parasites found in humans and other animals (which cause travelers'
diarrhea; also known as Amoebic dysentery) and theLobose Amoeba. TheLobose Amoeba include two
classes of amoeba that extend just one or a few flat pseudopods; unlike the amoeba of the other
supergroups that have a large array of filaments protruding like a star. The amoeba you will examine in
this lab is the best known example of this group. The pseudopods they extend are bulges of cytoplasm
pushed out by flexible actin filaments right underneath the cell membrane. The process by which
pseudopods are extended is similar to how our muscles contract. Amoebas use pseudopods for
locomotion and feeding. They move forward by extending pseudopods and pulling the rest of the cell after
the pseudopods. They also engulf their food with their pseudopods.
Choanoflagellates
Choanoflagellatesare unique in having just one flagellum that is surrounded by a collar of microvilli (tiny
fingers). The microvilli aid in the digestion of food stuffs encountered by these unicellular organisms.
The choanoflagellates provide another hint at the evolution of multicellular animals. They are found as
both individual cells, but also as colonies, and if you look at the choanocyte cells of sponges (simple
animals) you will see that they are just an orderly arrangement of cells that look and function identically to
the choanoflagellate protists.