By Professor O. Roger Anderson
Lamont-Doherty Earth Observatory of Columbia University
Biology and Paleo Environment Division
1A Marine Biology
61 Route 9W - PO Box 1000
Palisades, NY 10964-8000
ora@ldeo.columbia.edu
What is a Protozoan?
Protozoa are single-celled organisms that have a true nucleus with chromosomes
(composed of DNA) enclosed by a membranous envelope (Fig. 1). They also have other
internal cellular structures known as organelles that perform the many physiological
functions needed to maintain the life of the organism. These organelles include
mitochondria that break down nutrient molecules during respiration to produce energy for
the cell. The mitochondria are the organelles where oxygen is consumed during
respiration. Carbon dioxide is produced as a waste product of this respiration. Other
organelles are digestive vacuoles that act like small stomachs within the cell to digest
food particles and molecules taken in by the cell. Most protozoa are very small (microns
in size) and require a microscope to see them. However, others can be in the millimeter
size range. You have probably heard previously about protozoa, most likely at school, or
by viewing television, or perhaps by reading. Amoebas are protozoa (Fig. 2). The
commonly observed ciliate, Paramecium (Fig. 3), also is a protozoan. Have you,
perhaps, heard of Euglena? Euglena (Fig. 4) is a beautiful green-pigmented protozoan
that has a tail-like flagellum that it uses to swim. More about this later when we explore
how protozoa move.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Look here for wonderful pictures of protozoa and learn about their biology:
http://microscope.mbl.edu/reflections/baypaul/microscope/general/page_01.htm
What are Protists?
Protozoa are classified within a much larger group of single-celled organisms that
includes the algae, other single-celled photosynthetic organisms, and some of the water
molds (slime molds). This larger group is called the protists (pronounced like “proteests”). They are placed in the kingdom Protista. The algae are photosynthetic singlecelled organisms. Algae cause the green-colored pond water, and sometimes the green
pond scum that we see typically in summer. These algae can be very small microscopic
spherical pigmented cells or even the long, thin filamentous algae that form thread-like
strands in ponds and streams. Therefore, the protists are a very large group of highly
diversified single-celled organisms. They are all included together because they are
single-celled (whether photosynthetic or not) and have a true nucleus and other internal
cellular organelles as described above.
A website that tells us all about protists and how to recognize them.
http://www.microbeworld.org/htm/aboutmicro/microbes/types/protista.htm
How are protozoa different from bacteria?
The complex cellular structure of protozoa and other protists make them different from
bacteria. The bacteria are also single-celled organisms, but they have no true nucleus.
The DNA is simply coiled up within the cell. There is no enclosing membranous
envelope surrounding the DNA. Furthermore, bacteria have none of the membraneenclosed organelles such as mitochondria and digestive vacuoles etc. that are found in
protozoa and the cells of all more complex organisms including plants and animals.
Evidence indicates that the bacteria evolved earlier than the protists. It is very likely that
some form of bacteria gave rise to the protists. Protists, likewise, were the evolutionary
ancestors that gave rise to higher organisms including plants, fungi and animals.
What are the major groups of protozoa?
There are three major groups of protozoa, exclusive of some parasitic protozoa. 1.
Amoeboid protozoa move by a gliding motion produced by lobe-like, pseudopodia
extending from the edge of the cell (for example, Fig. 2). Ciliates (Fig. 3) have numerous
hair-shaped cilia whose vibrating motion moves the ciliate through water. Flagellates
(for example, Fig. 4) have one or more whip-like flagella that they use to propel
themselves through the water.
Where do we find protozoa?
Protozoa can be found in most every habitat on Earth. They have been discovered in the
deepest parts of the oceans and on the highest peaks of the mountains. Ponds, streams,
rivers, swamps, most soil, and even the very acidic water caused by industrial pollution
contain some species of protozoa or other protists. It is easy to culture protozoa from the
natural environment. One way is to take a small clump of soil and place it in water (a
small jar will do) with a boiled rice grain or boiled wheat seed as a food source for
bacteria that the protozoa will eat. Wheat seeds are available from the food store or
some health food stores. Take a drop of the water (especially near the rice or wheat grain)
and examine it with a microscope each day for a week or more. You should begin to see
protozoa growing out from the soil. You may also take a small amount of pond sediment
and add it to water with a rice grain. Protozoa also should grow out from the pond
sediment sample.
Can you think of some experiments to do with your culture jars of protists to see if light
or darkness causes a difference in the kind of protists that grow up in your culture? What
do you suppose happens to the kind of protists you see over a period of several weeks as
you observe your culture jars? Is there any difference in the kinds of protists you see
when you begin with a soil sample versus using a pond sediment sample to start your
cultures?
Some nice experiments on growing protozoa you can do at home or school.
http://www.nhm.org/microbes/013.html
Are there protozoa in seawater? Find out here:
http://www.microscopy-uk.org.uk/mag/artmar01/protoz.html
Looking at protozoa with the microscope:
http://www.funsci.com/
Want to build your own microscope? Here is an idea!
http://www.funsci.com/fun3_en/ucomp1/ucomp1.htm
How do protozoa move?
Protozoa have many different ways of moving when viewed in detail. But, we can
describe three main ways that they move.
We will examine three protozoa as examples.
How do amoebas move?
1. Some protozoa such as the amoebas move by amoeboid locomotion. Amoebas
move by a crawling motion. The amoeba cell extends lobe-like or finger-shaped
pseudopodia that project outward from the cell. The pseudopodia can attach to the
surface beneath the amoeba. By a flowing motion of the cytoplasm within the
cell and the pseudopodia, the whole cell glides forward on the surface.
Pseudopodia can have different shapes. Some are very blunt; others are very long
and tapered like a pencil.
WOW, here are some interesting amoebas and how they MOVE!
http://www.microscopy-uk.org.uk/mag/artmar01/provideo.html
http://www.microscopy-uk.org.uk/mag/wimsmall/sundr.html
http://www.microscopy-uk.org.uk/mag/artsep01/feed.html
How does Euglena move?
2. Euglena and many other related protozoa move by the whipping motion of a taillike structure called a flagellum. Protozoa that have flagella are categorized as
flagellates. The flagellum moves by a complex beating motion that sets up
streams of water around the flagellate. This propels the flagellate through the
water. In some respects, this is like the action of the tail of a fish. Other
flagellates, however, use their flagellum to set up water currents that draw them
forward. Their flagellum is acting somewhat like a “propeller” on a plane to
move them forward as the flagellum beats. Some flagellates have more than one
flagellum. There can be two or in some species even many – hundreds covering
the surface.
Some beautiful flagellates, including Euglena are found here:
http://www.microscopy-uk.org.uk/mag/wimsmall/flagdr.html
Video movies of flagellates swimming – remarkable views. [Scroll down
the page and click on “Movies.”]
http://bio.rutgers.edu/euglena/
How does Paramecium move?
3. Paramecium is a slipper-shaped (or oval-shaped) protozoan covered by short,
hair-like, beating structures called cilia. Protozoa that have cilia are called ciliates.
Ciliates are also different from other protozoa since they contain two kinds of
nuclei. There is a larger macronucleus and one or more much smaller micronuclei
(see the section below on reproduction to learn more about these two kinds of
nuclei). Cilia resemble flagella internally, but they are much shorter. Typically,
also, they are much more numerous and often give the ciliate a hairy appearance
when viewed closely with a microscope (Fig. 2). The cilia beat in a highly
organized pattern that ripples across the surface of the cell. This highly
coordinated beating motion allows the ciliate to swim very efficiently through the
water. When the cilia beat in one direction, the ciliate moves forward. Think of it
like oars on a boat that are used as paddles to make the boat move forward
through the water. When the cilia reverse the direction of their beating, however,
the ciliate moves backward. This is like paddling or stroking the oars in a forward
motion to make a boat move backward. Ciliates can move in many different
patterns and their swimming behavior is very interesting to watch. What do you
think -- can Paramecium and other ciliates change their swimming pattern to
avoid noxious or dangerous conditions, or to find food? How could you try to
investigate this?
Here is a website that tells all about ciliates.
http://www.microscopy-uk.org.uk/mag/wimsmall/cilidr.html
How do protozoa get food for nutrition
Protozoa typically obtain their nutrition by engulfing food particles and small nutrient
molecules from the environment. Hence, we say that they have a life process similar
to animals. That is, most can obtain their food by taking up food particles from the
environment, rather than making their own food by photosynthesis. These protozoa
do not have the green pigment, chlorophyll, as found in photosynthetic organisms.
Therefore, we sometimes call them the colorless protozoa. However, some species of
protozoa that are closely related to the colorless ones, are able to produce chlorophyll
and can do photosynthesis. These are typically flagellates that contain chloroplasts.
Since they are capable of photosynthesis, we call them the photosynthetic flagellates.
Or, sometimes we use the scientific term phytoflagellates. The prefix “phyto” comes
from the Greek word for plants and signifies that these colored flagellates can make
food by photosynthesis similar to plants. In some cases, these phytoflagellates, being
related to colorless flagellates, can return to getting food like animals. That is, they
can either do photosynthesis, or when external food sources are abundant, they can
engulf food particles and digest them in an animal-like fashion. Protozoa consume a
variety of prey. Many smaller protozoa feed on bacteria. Some use algae as food.
Others that are large enough even consume other protozoa if they are able to engulf
them as food and take them into food vacuoles within their own cells as explained
below.
How do amoebas get food?
Amoebas and other amoeboid organisms have many different ways of using their
pseudopodia to obtain food. But in a typical case, here is what happens. When an
amoeba approaches a food particle, it extends its pseudopodia forward on either side
of the food particle and then by flowing around the food particle, the pseudopodia
engulf it within a food vacuole. The food vacuole containing the food particle
receives digestive enzymes from the cell cytoplasm and the food is digested - being
broken down into small nutrient molecules that can pass out of the vacuole into the
cytoplasm of the amoeboid organism. Once in the cytoplasm, these nutrient
molecules are used to produce energy for the cell through metabolism. Some are used
as “molecular building blocks” to create new cytoplasmic structures of the cell.
How do flagellates get food?
As we mentioned above some flagellates can do photosynthesis, using light to
synthesize organic nutrient molecules. However, some flagellates can engulf food
particles by capturing them on the surface of the cell in a cup-like depression. The
food particle is then engulfed into the cell cytoplasm in a vacuole. The surface of the
cell with the attached food particles gradually is pulled inside and sinks inward until it
fully pinches off as a membrane-enclosed vacuole containing the food. The food is
digested when the food vacuole receives digestive enzymes from the flagellate
cytoplasm – much as we described for amoebas. In some cases, flagellates take up
very small food molecules by absorption across their surface membrane. That is, the
useful food molecules simply diffuse across the membrane into the cell -- if the
molecules are sufficiently small. Otherwise, the food cup as described above must
engulf the food particle.
How do ciliates get food?
Again, there are many differences among species, but let’s look at how Paramecium
gets its food. It is a good example of how a ciliate feeds. The Paramecium has an
oral groove on one side. This is a shallow depression that is lined with beating cilia.
The beating action of the cilia causes a stream of water to move along the oral groove
carrying food particles with it. At one end of the oral groove is a specialized, funnel
shaped region (oral cavity) where the food particles are concentrated and eventually
swept into a food vacuole at the base (narrowed end) of the funnel. The food vacuole
becomes filled with food particles and then pinches off into the cytoplasm of the
ciliate. As with the amoebas and flagellates, once the food vacuole is carried into the
cytoplasm of the ciliate, it receives digestive enzymes. The food is digested into
smaller molecules to be used as nutrients by the cell. You may want to investigate
how other ciliates get food. Try to find a book on protozoa in your library or use the
computer to search the Worldwide Web for the topic of “Feeding in ciliates.”
For example, here is an interesting website with videos!
http://www.micrographia.com/specbiol/protis/cili/cili0100.htm
This nice website shows “Trumpet-shaped” ciliates feeding.
http://www.micrographia.com/specbiol/protis/cili/hete0100.htm
What are the ecological relationships among the protists?
Ecology is the study of how organisms interact with one another and with their
environment. There are many complex patterns of interaction among the protists that
live in the same environment. These are called biotic factors since the interactions are
among biological organisms. The interactions of organisms with their environment
are grouped under the heading of abiotic factors. Protists have certain requirements
for survival in each environment where they live. These abiotic factors include how
much light is required, especially for photosynthetic protists. The range of
temperatures suitable for survival, and the temperature that is optimal for growth, are
abiotic factors. The pH of the environment (how acid or alkaline) is also important.
Available sources of organic and inorganic nutrients are also critical for the nutrition
of the protists. There are many more abiotic factors that could be discussed and you
may think of some yourself that we have not mentioned.
Protists also have complex patterns of interaction among themselves. Some protists
compete for the same space and resources. This competition may lead to one or more
especially well-adapted species flourishing. These may become the major occurring
species and dominate in numbers. In other cases, there is a better balance among the
number of species in a particular habitat. An elaborate food web occurs among the
protists. Some protists prey on bacteria; others consume smaller photosynthetic
protists including algae. Some protozoa consume blue-green bacteria (cyanobacteria)
as food. Larger protozoa may consume smaller ones. Thus, we see that among the
protists there are complex patterns of predator and prey relationships forming a food
chain much as we see in the larger environment surrounding us. For example, in our
environment, photosynthetic plants produce food by capturing sunlight as an energy
source. Rabbits eat grass as food. The rabbit can be consumed by a variety of
predators such as the fox, hawks, and other larger carnivores. A similar situation
occurs among the protists. Green algae produce food by photosynthesis much like the
larger photosynthetic plants. Small ciliates can consume microalgae very much as the
rabbit consumes grass. In turn these small ciliates may be eaten by larger ciliates or
in some cases by larger amoeba that engulf them as prey. Therefore, protists exhibit
similar food chains within their very small world as we see around us in our larger
world.
Protists also exhibit another interesting pattern of food relationships. This pattern is
not so much a chain of events as it is a circular pattern. We call this the microbial
loop (Fig. 5). The pattern of flow of nutrients and energy for life is in a circle or loop.
Green algae and other photosynthetic protists utilize sunlight to produce food
molecules. Some of these molecules diffuse out of the photosynthetic protists into the
surrounding environment. Also, when algae are consumed by other protists, some of
the nutrients the algae contain are lost to the environment during feeding. Bacteria
can use these dissolved nutrients to grow and flourish. The bacteria, in turn, serve as
a food source for protozoa that consume bacteria. When the bacteria-consuming
protozoa digest the bacteria, the protozoa release inorganic nutrients into the water.
These nutrients (such as nitrogen compounds, phosphorous compounds, etc.) are
used in turn by the photosynthetic protists to support growth and reproduction. The
loop is complete. We started with the photosynthetic protists. Eventually through a
process of predator/prey relationships nutrients are released into the environment to
once again support the growth of the photosynthetic protists. Overall, this complex
pattern of interactions keeps food and energy moving from one protist to another, so
they have sufficient resources to survive and reproduce.
How do protozoa reproduce?
Asexual reproduction
Some protozoa, as with many single-celled organisms, reproduce asexually by cell
division. In asexual reproduction, a cell produces two identical nuclei from the parent
nucleus by mitosis. Then the two nuclei migrate to opposite sides of the cell and the
cytoplasm begins to pinch in half, gradually splitting into two new cells. Thus,
asexual reproduction produces two genetically identical daughter cells. Both
daughter cells have identical DNA in their nuclei. This results from the fact that the
mother nucleus reproduced itself in preparation for splitting of the cell. This type of
asexual reproduction is known as binary fission. Binary refers to the production of
two daughter cells by the splitting or fission of the mother cell into two new cells.
This website shows binary fission – with an interesting difference.
http://www.microscopy-uk.org.uk/mag/art98/celldiv.html
Sexual reproduction
Some species of protozoa reproduce sexually. Let’s look at how some flagellates
reproduce by cell fusion (Fig. 6). In this process two cells attach to one another at
their surfaces. Then, in some species these two parent cells may completely fuse with
each other. Once the two cells are fused into one larger cell, the two nuclei combine
to form a new nucleus. This nucleus now has chromosomes from each of the two
parent cells. We say the nucleus is diploid, since it has twice the number of
chromosomes found in each of the parent cells. The cell containing this nucleus is
called a zygote. The zygote can subsequently divide to produce offspring. Each of
these offspring has a nucleus with the number of chromosomes found in the original
parent cells. That is the nuclei are haploid. These haploid cells can complete the life
cycle by mating and fusing as described above.
Some ciliates reproduce sexually, but the parent cells do not fuse with one another.
This form of sexual reproduction is called conjugation. The reproducing ciliates
become attached at their surface (Fig. 7). Some small pores develop at the point of
attachment. These pores are large enough to allow nuclei to pass from one ciliate to
the other. The paired ciliates exchange reproductive nuclei that are haploid. The
reproductive nucleus from one parent passes through the pores connecting the two
parent cells. When this nucleus enters the other cell, it fuses with a reproductive
nucleus in that parent cell. This produces the zygote nucleus with chromosomes from
each parent. The zygote nucleus is diploid. Later, the diploid nucleus will undergo a
process of division that makes it haploid. Then, the ciliate with haploid nuclei divides
asexually repeatedly giving rise to many more offspring. These can later pair with a
compatible individual and undergo sexual reproduction as described above.
Paramecium reproduces by mating as shown at this website – click on
Paramecium or choose others:
http://protist.i.hosei.ac.jp/PDB/Images/Subjects/Sexual_reproduction/mr01.html
What are parasitic Protozoa?
Most of the protozoa we have described above are found living in the natural
environment and are not harmful as disease producing protozoa. Indeed, many of
them are very beneficial since they help to make our environment more productive.
They also improve the quality of water by removing bacteria and other particles that
they engulf as food. We call these protozoa “Free-living protozoa” since they are
able to survive in the open environment.
Some protozoa, however, have become adapted to invade and live in the cells and
tissues of other organisms. These are the parasitic protozoa. They are parasites since
they typically cannot live in the open environment on their own in the active stages of
their life cycle, but must invade another organism’s body to obtain protection and
nourishment. Some of these parasitic protozoa are serious disease-producing
organisms. They can cause diseases in animals and humans. Protozoan diseases are
known worldwide. However, the tropical countries, where the climate is warm, often
have a larger number of different human protozoan parasites than more northern
countries. Each of the three types of protozoa (amoebas, flagellates, and ciliates)
includes some species that are parasitic in animals and/or humans. However, it is
important to remember that most species belonging to these three groups are freeliving and overall are beneficial. Other parasitic protozoa occur in different
taxonomic groups. These are sometimes non-motile during part of their life cycle, at
least. The apicomplexa, coccidia, and gregarines are examples of groups of parasitic
protozoa.
Parasitic protozoa include the intestinal amoeba parasite (Entamoeba histolytica) that
causes diarrhea and serious intestinal infections. The malaria parasite (e.g.,
Plasmodium spp.) is also a protozoan that attacks our liver and red blood cells. Some
pathogenic protozoa can attack the central nervous system such as the trypanosomes
(flagellates) that cause African sleeping sickness. Some protozoa cause serious
diseases of the respiratory tract and lungs. Others simply live in our intestinal tract
and can cause very uncomfortable responses, but they are not usually deadly
pathogens.
Interested to know more about free-living and parasitic protozoa? Here
is a nice website:
http://www.modares.ac.ir/elearning/Dalimi/Proto/Lectures/week3.htm
FIGURE LEGENDS FOR SOP.prot.doc
Fig. 1 A diagram of an amoeba cell showing the internal structures or organelles that
are surrounded by membranes, including the nucleus, water-containing vacuole, and
digestive vacuoles.
Fig. 2 A light microscopic view of a living amoeba (Amoeba proteus). Lobe -like
pseudopodia extend outward from the surface of the amoeba toward the right-hand
side. The amoeba is moving toward the right.
Fig. 3 A paramecium (ciliate) observed with the scanning electron microscope. The
surface of the paramecium is covered by many fine, hair-like cilia. A funnel-like
depression in the surface serves as a mouth, or more properly called an oral cavity.
Fig. 4 A flagellate (Euglena) diagram. Note the flagellum extending from a pocketlike depression at the anterior end of the cell. Chloroplasts and a nucleus are visible
within the Euglena cell. The cell contains spirally arranged strips of protein on the
surface that serve as protection. These spiral strips can also move against one another
to make the cell wriggle and change shape.
Fig. 5 A diagram showing the “Microbial loop.” This diagram illustrates the
complex circular pattern of the flow of nutrients among protists and bacteria. The
photosynthetic protists, such as the algae, produce food by photosynthesis. Some of
the organic molecules produced by photosynthesis are lost from the cell into the
environment. These include carbohydrates (sugars). The bacteria in the same locale
use the organic molecules as a food source and flourish. Ciliates and other protozoa
feed on the bacteria. As a result of this feeding, and also during excretion of waste
molecules by the protozoa, nutrients are released again into the environment. The
photosynthetic protists absorb these nutrients (nitrogen compounds, phosphorous
compounds, etc.) to support photosynthesis. Therefore, they can continue to
reproduce and flourish. Thus, the circle or loop is complete.
Fig. 6 Sexual reproduction by cell fusion. The flagellate, Chlamydomonas, illustrates
sexual reproduction by cell fusion. Each Chlamydomonas cell acts as a gamete. When
two compatible cells are ready to reproduce they join at their tips where the flagella
are attached. The joined cells lose their flagella while connected to one another.
Then, the two cells merge or fuse with one another. The two nuclei inside the
cytoplasm of the merged cells are at first separate. However, the nuclei also fuse very
soon to form a zygote nucleus. Subsequently, the nuclei undergo repeated division as
the daughter cells divide to produce a new generation of offspring. Notice that each
offspring is haploid (with half the number of chromosomes) we label this 1N. When
the zygote is formed from the fusion of two parent nuclei it is diploid (with twice the
number of chromosomes) and is labeled 2N.
Fig. 7 Sexual reproduction in ciliates is by a process known as conjugation. A
compatible pair begin to swim along side one another (A) and then join along one
surface (B). Notice that each cell has a larger nucleus (called the macronucleus) and
some smaller nuclei (micronuclei). The micronuclei serve as the reproductive nuclei
or gamete nuclei. After the parent cells have joined, the micronuclei in some species
divide to form several daughter nuclei (C). These are now haploid (1 N, containing
only half the number of chromosomes). Only TWO of these nuclei will survive to
become the gamete nuclei. One gamete nucleus from each of the parent cells
migrates across the contact surface into the other parent cell. This is a special process
of cross-fertilization. Each parent fertilizes the other parent by donating a gamete
nucleus to the other. Notice the arrows showing the path of the gamete nuclei as each
crosses over into the other cell. The gamete nucleus that migrates into the other
parent cell fuses with the nucleus of that parent cell (D). This produces a zygote
nucleus containing chromosomes from both of the parents (2N). After crossfertilization (conjugation), the two parent cells separate and swim away (E). The
nuclei proliferate to produce new micronuclei and a larger macronucleus (F). The
cells can divide repeatedly by asexual reproduction, thus creating a new population of
the offspring with genetic traits acquired from both parents. Overall, this is similar to
heredity in animals where, during sexual reproduction, the egg comes from one parent
and is fertilized by sperm from the other parent. This produces offspring with genetic
characteristics from both parents.