The kingdom Protista is the most diverse of the four kingdoms of eukaryotic organisms. Unlike members of the kingdoms Fungi, Animalia, and Plantae each with clearly distinguishing characteristics, protists range from microscopic organisms such as amoebas to large multicellular organisms such as the brown algae, kelp. In general, most protists are microscopic, unicellular organisms. Protists are divided into three groups: protozoans; slime molds; and algae. Protozoans are the animal-like protists, slime molds are the fungus-like protists, and algae are the plant-like protists. In today’s lab you will be concentrating on algae.

Like plants, algae photosynthesize and so produce their own food. Plants have multicellular sex organs that allow reproductive cells to be protected by several layers of nonreproductive cells. Algae, however, lack these multicellular sex organs. So instead of classifying algae by their reproductive strategies, algae are grouped by energy storage products, cell walls, and colors that are present because of the type and quantity of pigments. These pigments are substances that absorb light and are found in their plastids. The major groups of algae are the green algae, brown algae, golden-brown algae, and red algae.

In addition to being easily recognized by color, algae have unique cellular organizations. There are three categories of organization: unicellular; filamentous; and colonial. As the name implies, unicellular algae are single cell organisms that may or may not be motile. Filamentous algae are chains of cells attached end to end. The length of the filament varies and can be either branched or unbranched. Colonial algal species are made up of cells that are attached to each other but remain metabolically independent. These cells are held together by a gelatinous matrix or by thin strands of cytoplasm. The colonies can be in the shape of a sphere, a flat sheet or other three dimensional shapes.

Although not typical, there are multicellular algal species. These species are more complex than the colonial algae, with different kinds of cells for different functions.

Green algae are mostly found in freshwater and are the most diverse of the freshwater algae. They are considered to be ancestral to terrestrial plants with many of the same characteristics present: chlorophyll a; chlorophyll b; starch as storage; and cell walls made of cellulose.

One of the most common of the unicellular green algae is Chlamydomonas. It is rich in chlorophyll a and b and is found in soil, lakes and ditches. The cells are egg-shaped, and motile with two flagella. Each Chlamydomonas contains a large chloroplast and a pyrenoid which functions in production and storage of starch.

1. Using a compound microscope, observe a drop of water containing Chlamydomonas and watch the movement of the cells. Is it a smooth movement or is it shaky?

2. If the movement is too fast, add a drop of methylcellulose. Can you see both flagella?

3. Now examine the prepared slides of Chlamydomonas. Can you see any structures that you were unable to see with the live Chlamydomonas? The chloroplast occupies the majority of the cell.

Chlamydomonas normally reproduces asexually. However, during unfavorable environmental conditions, Chlamydomonas will undergo sexual reproduction. Specific cells, called vegetative cells, produce gametes as a result of mitosis. These gametes fuse to produce a diploid zygote. In most species of Chlamydomonas, the gametes of two strains have identical shape and appearance and are called isogametes. These isogametes are designated by + or -. If environmental conditions are favorable, the zygote will undergo meiosis to produce haploid spores. It is the spore that is the reproductive cell that is capable of developing into an adult without having to fuse with another cell.

1. On a microscope slide, place a drop of + Chlamydomonas gametes and a drop of - Chlamydomonas gametes side by side, but do not mix them. Do not use a coverslip.

2. While looking at the gametes using the low power objective, gently mix the two drops of isogametes.

3. Switch to the medium power objective and finally the high power objective to see the clumping of the gametes. Try to locate cells that have paired.

Why is it necessary for the zygotes of Chlamydomonas to have a resting stage and not undergo meiosis immediately?

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Spirogyra or watersilk, is found in running streams of cool freshwater. It has a slippery feel to it due to secretions of mucilage.

Spirogryra has nonflagellated isogametes. Sexual reproduction for this organism is referred to as conjugation, and occurs when filaments of opposite mating types like side by side. Each filament forms projections that grow toward each other; once they touch, the separating wall dissolves. This forms the conjugation tube. The cellular contents of the - strain go through the conjugation tube and fuse with the + strain. The zygote formed by this fusion develops a thick, resistant cell wall and is now referred to as the zygospore. The filament breaks down, releasing the zygospore, which then undergoes meiosis to form haploid cells, becoming new filaments.

Observe the prepared slide of conjugation in Spirogyra. Sketch your observations below and note the unique shape of the chloroplast (you may have to actually look for some that have not conjugated to see the chloroplast structure).

Are the filaments of Spirogyra branched?

What is the shape of the chloroplasts? Spiral

Now look at some prepared slides of Cladophora. Draw a few cells to show their general shape and branching pattern.

How is the shape and morphology of Cladophora different from Spirogyra?

Volvox is made up of many cells that resemble Chlamydomonas. These cells are bound in a spherical matrix and possess two flagella attached to the surface of the colony. By synchronously beating the flagella, the colony spins through the water somewhat like the earth spinning on its axis. Because of its complexity, some biologists consider Volvox to be multicellular. In fact, some of its cells in the colony are functionally differentiated. Some cells can produce new colonies while eggs and sperm are produced by different cells in the colony.

Volvox reproduction occurs when motile sperm swim to and fuse with eggs, which are larger and nonmotile, thus forming a diploid zygote. This process is referred to as oogamy. The resulting diploid zygote enlarges and eventually forms into a thick-walled zygospore. As the parent colony breaks down, the zygospore is released. It then undergoes meiosis, producing haploid cells which then undergo mitosis and eventually a new colony is formed. Volvox is also capable of asexual reproduction; certain cells divide, bulge inward and produce new colonies called daughter colonies that are briefly retained within the original colony.

1. View Volvox by placing a drop of living Volvox culture on a depression slide.

2. Using low power magnification, observe the colonies, then search for flagella on the surface (they may be too small to actually see).

3. Look closely at the large, hollow sphere. Can you see any tiny spheres inside the large sphere? What are they?

The most complex algae you will view today are the brown algae; there are no unicellular or colonial species. Phaeophytes are almost exclusively marine, usually grow in cool water and are brownish in color because of a specialized pigment called fucoxanthin. They range in size from microscopic species to those that are over 50 m long (giant kelp). The thalli (multicellular, flattened bodies that are not organized into leaves, stems or roots) of some phaeophytes are similar to those of land plants. Two common genera of brown algae are Sargassum and Fucus.

Fucus, or rockweed, is usually attached to rocks in the intertidal zone. Attachments are made by structures on Fucus called a holdfast. The outer surface of Fucus is covered by a gelatinous sheath. The tips of Fucus branches, called conceptacles, are the reproductive structures and are called the oogonia (female) and antheridia (male) organs. Each of these organs is multicellular with the oogonia producing eggs and the antheridia producing sperm. This is an unusual feature among protists; most protists do not have multicellular reproductive organs.

Fucus is much more similar to animals in its life cycle than that of plants, fungi or many protists. The mature thallus is diploid, and the cells within the structure undergo meiosis, producing gametes. The multicellular haploid stage is not present in the lifecycle of Fucus.

1. Examine a specimen of Fucus. Identify the antheridia and oogonia. Make a sketch below.

 

 

 

 

 

 

 

 

2. Now examine prepared slides of antheridia and oogonia of Fucus. Make a sketch below.

How does the structure of the tissue surrounding the reproductive organs compare with that of green algae?

Green algae has no reproductive organelles but two cells that come together in nature to form a zygospore. In fucus the female parts have receptacles and the male parts antheridia which produce sperm cells.

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Red algae contain red phycobilins in their plastids, giving them their red coloration. these algae usually live in warm marine waters. The thalli of red algae differ from species to species and can be attached or free-floating, filamentous, or parenchymatous (fleshy).

Observe samples of Polysiphonia and make a sketch below.

Notice the branching and filamentous tissue. Gemetophytes of these organisms are unisexual; either male or female.

Now examine Porphyra and make a sketch below.

What are some differences between Polysiphonia and Porphyra?

The blade-like structures of Porphyra are made up of two layers of cells separated by colloidal material. As with brown algae, red algae are also of economic importance. Carrageenan, agar, and other products made from red algae.

Diatoms are usually golden-brown in color and contain chlorophylls a and c, and xanthophyll. Depending on environmental conditions, diatoms are usually plentiful and have a rapid reproductive rate. That, coupled with their photosynthetic ability, makes them particularly important as a principal component of the food chain in oceans.

Diatoms have highly structured cell walls made of silicon dioxide (glass). The walls are arranged in overlapping halves, like a petri dish or a shoebox with its top.

View a prepared slide of diatoms and draw what you see in the space below:

As diatoms die off, their cell wall remains behind. Diatomaceous earth consists of the siliceous skeletons of diatoms and is used in the manufacture of dynamite, pottery, abrasives, glazes and other commercial items. Diatomaceous earth results in the accumulation of the cell walls of diatoms that may be several hundred meters deep.

As a group, the Euglenoids are more like protozoa than algae. However, this group is included here because of the photosynthetic properties of certain Euglenoids. Euglenoids are mostly freshwater unicellular organisms. As with green algae, euglenoids contain chlorophylls a and b, but are noted for their cell walls which are made up primarily of proteins. This characteristic makes the cell more flexible. Euglenoids also have two flagella.

1. Observe both living cultures and prepared slides of Euglena. If necessary, use a drop of methylcellulose to slow down the movement of your specimen.

2. Pay particular attention to the colored eyespot near the base of the flagella. What do you think its function is?

3. Observe movement and changing shapes within your sample.

Depending on environmental conditions, Euglena can be autotrophic, heterotrophic or saprophytic (eats dead or decaying organic matter). This ability allows Euglena to thrive under most conditions.