Name__________________________

Algae

In spite of their numbers of species and economic contributions, seed plants are not the only ones occurring on earth. A wide variety of different types are found; many lack stem, root, and leaf. During the remainder of the semester you will have an opportunity to briefly survey some of these types, learn something about their appearance and structure, and study the reproductive life cycle of a few selected examples, to see if you can arrive at some insight concerning the evolution of the plant kingdom. You will observe the genetic (evolutionary) sequence, starting with what are considered to be more direct descendants of primitive organisms and working toward organisms that are considered more advanced (derived).

The group we consider today, is included under the general classification of "Algae", which, though practical, is a completely artificial taxon. Among species included in the algae there is a greater diversity of form, metabolism and reproductive detail than among the bryophytes and vascular plants taken together. You will look at only a few examples of this tremendous diversity.

I. Diversity of Form Among the Algal Divisions--A Gauntlet!

The table used in lecture compares the better-known algal divisions with respect to a variety of characteristics. One or more representatives of each of these divisions are available in the laboratory. Work as part of the class team to set up your assigned wet-mount(s) at your microscope. Add a drop of the culture liquid to the edge of the coverslip to be sure the mount can last through the "gauntlet". When all are ready, the instructor will start the gauntlet exercise. You will have a minute or so at each station. For each representative you should determine insofar as possible the:

  1. growth form
  2. presence or absence of nucleus and chloroplast(s)
  3. number and shape of chloroplast(s)
  4. number of nuclei per cell
  5. any particular wall characteristics
  6. visible pigmentation
  7. storage material (if possible to tell)
  8. anything that can be seen of reproduction
If a species is too actively motile to be easily seen, it can be slowed down by adding nickel sulfate to the mount. The divisions illustrated include (you write the headings!):
 
 
  
Cyanophytablue-green algaeAnabaena, Gloeocapsa, Oscillatoria
EuglenophytaeuglenoidsEuglena
Chrysophytayellow-greensVaucheria, diatoms
Chlorophytagreen algaeVolvox, Ulothrix, Oedogonium, Scenedesmus, Closterium, Cosmarium, Eudorina, Chlamydomonas, Coleochaete
Phaeophytabrown algaeSphacelaria
Rhodophytared algaeAcrochaetium


Make and label a quick sketch of the algae available to help distinguish them.
Drawing of Anabaena 
 
 
 
Drawing of Gleocapsa
Drawing of Tolypothrix 
 
 
 
Drawing of Euglena
Drawing of Vaucheria 
 
 
 
Drawing of Volvox
Drawing of Ulothrix 
 
 
 
Drawing of Oedogonium
Drawing of Scenedesmus 
 
 
 
Drawing of Closterium
Drawing of Cosmarium 
 
 
 
Drawing of Eudorina
Drawing of Chlamydomonas 
 
 
 
Drawing of Coleochaete
Drawing of Sphacelaria 
 
 
 
Drawing of Acrochaetium

To be sure you have seen some of the critical features, use your drawings and test your skills in identifying the algae found in the "Survey Mixture" using the following:

Dichotomous Key to the Survey Mixture

1. Cells single; not grouped in a colony or filament2
1. Cells grouped in a colony or end to end in a long filament3


2. Cells golden-brown, narrow, much longer than broad; from the top view the tips taper to rounded points while from the side view the tips are blunted and squared; frequently with fancy wall "etchings"Synedra--__________phyta
2. Cells bright green, long and slightly crescent shaped; round vacuoles at each tip may be filled with granules; a row of doughnut shaped bodies (pyrenoids) run from the tip to a central clear areaClosterium--__________phyta


3. Cells joined end-to-end in a long filament4
3. Cells as clusters, aggregates, or spheres but not as a filament6


4. Cells with obvious chloroplasts and distinct internal structures; cell at least 3 or 4 times as long as broad; cells bright green in color.5
4. Cells without obvious chloroplasts or distinct internal structures; interior of cell dense and granular; filament interrupted by large, thick walled structures (heterocysts); cells dark green or blue-green in color____________________--__________phyta


5. Filaments narrow; cell with the chloroplast covering only a small area leaving a large empty space; chloroplast concentrated in a cup-shaped band pressed against the cell wall____________________--__________phyta
5. Filaments fairly wide; cell nearly filled with chloroplast; large clear areas are not present in young cells; large, dark, rough spheres (female structures) larger than the vegetative cells and tiny periclinally-divided cells (male structures) may be interspersed along the filament____________________--__________phyta


6. Colony small and composed of 3 to 24 cells; not motile; one cell thick.7
6. Colony large, round, hollow ball composed of 500 or more very small cells; motile, swimming with a rolling motion.___________________--__________phyta


7. Colony a flat plate of 6 to 24 polygonal cells; colony somewhat circular; cells may appear to be "piled up"Pediastrum--__________phyta
7. Colony usually of 4 spindle-shaped cells connected side by side; the tips of the cells often have long spines__________________-__________phyta

II. Observing the Results of Evolution in Algae

The Volvocales is a family of green algae which demonstrate an evolutionary trend from unicellular to colonial organization. Interestingly the morphology of each cell in the colonial species is remarkably similar to that of the free-living unicellular species. Use the key below to distinguish the members of this family in the Volvocales Mixture.
1. Cells grouped into a colony2
1. Cells not grouped into a colony; single, roundChlamydomonas


2. Colony one cell thick; flat or cup shaped7
2. Colony a round ball or sphere3


3. Colony composed of less than 100 cells4
3. Colony hollow round ball of more than 500 cells; new colonies can be seen forming inside the mature colonyVolvox


4. All cells in a colony are the same size; seldom more than 32 cells5
4. Cells in a colony of two different sizes; colony large, composed of up to 100 cells; a hollow sphereEudorina (Pleodorina) californica


5. Under high power (400x) the cells are round or spindle shaped, never triangular or wedge shaped; cells separated from each other and not tightly packed6
5. Under high power (400x) the cells are triangular or wedge-shaped; cells are very tightly packed and close togetherPandorina


6. Cells round in shape; 16 to 64 cells in a colonyEudorina elegans
6. Cells when viewed from the side are spindle-shaped; usually 4 or 8 cells in a colony; under low power the colony looks like a doughnut or crownStephanosphaera


7. Colony like a flattened horseshoe with several projections from the posterior; 16 to 32 cells in a colonyPlatydorina
7. Colony a loose square or rectangle; 2 to 16 cells in a colonyGonium

You should make diagrams of each species you find in the mixture and think about the evolutionarily relict (primitive) and the evolutionary advanced (derived) characteristics. Can you construct a phylogeny (an evolutionary sequence) from primitive to derived using the characteristics of species you have seen?

Phylogeny of the Volvocales from Primitive to Advanced


III. Sexual Reproduction in the Algae

There is more variation from the basic life cycle among algae than anywhere else among plants. While many species (the blue-green, euglenoids, and some greens) are not known to reproduce sexually, the majority do have some form of sexual reproduction, ranging all the way from the simplest type of isogamy in a green alga such as Chlamydomonas to the more complex oogamy of a red alga such as Polysiphonia. You do not have time to study the variety of life cycles but will look at least into one isogamous and one oogamous alga. Your instructor should have at least two of the following ready for your observation:

A. Chlamydomonas

At least eight hours before class, your instructor will have put two strains (+/-) of Chlamydomonas in slant culture under lights. After at least four hours above the compensation point, and four hours before class, the instructor will fill the tubes with a mating solution (aqueous medium). During this interval the sessile cells will become flagellated.

Place a drop of minus strain on a microslide and observe briefly without using a cover slip.
While you are at the microscope, add a drop of plus strain, and IMMEDIATELY observe.
You should find the cells clumping at once. Then mating pairs will begin to separate from the clumps and swim away as pairs. They attach at the anterior ends. You are observing isogamete fusion!
Add a cover slip, or put into a humid chamber to avoid evaporation. Plasmogamy should occur within a half-hour, and somewhat later the swimming zygospore will settle down, lose its four flagellae, and become a zygote.

B. Cosmarium

Three days before class, your instructor will have mixed cells of two mating strains of Cosmarium in a Petri dish bottom. This dish was held in a taped box inside of which an AlkaSelzerš tablet was dissolved. The box was placed in the light. The purpose was to have the algae above the compensation point and with sufficient carbon dioxide present to achieve mating stages. IT IS CRITICAL THAT THE ALGAE NOT COME IN CONTACT WITH THE ALKA-SELZER SOLUTION!

You should observe the cells in the chamber. If the instructor gives permission, you may open the box and remove cells for closer examination.
Look for large, dark, spiny-walled. spherical zygotes. Nearby you should find empty semicell walls from two cells that have combined.
What type of gametes are these?   isogametes   anisogametes   oogametes

C. Oedogonium

Three days before class, your instructor will have mixed filaments of two mating strains of Oedogonium in a Petri dish bottom. This dish was held in a taped box inside of which an Alka-Selzerš tablet was dissolved. The box was placed in the light. The purpose is to have the algae above the compensation point and with sufficient carbon dioxide present to achieve mating stages. IT IS CRITICAL THAT THE ALGAE NOT COME IN CONTACT WITH THE ALKA-SELZER SOLUTION!

Examine the filamentous green alga. All the vegetative cells are the same except for the holdfast cell. This species may grow attached to substrates or may be free-floating in ponds. Reduce the light in the microscope to observe unusual wall structures at one end of certain cells.

What do you see?___________________
You may also observe some swimming zoospores (vegetative reproductive swimming stages) or vegetative cells each containing a ready-to-release zoospore. If you find an empty vegetative cell,

what would you call it?______________________
You should observe the filaments looking for large oval oogonia each containing an egg. On the same filament (if homothallic) or on other filaments (if heterothallic) you should find very tiny antheridia. Sometimes the antheridial cells are completely empty...

why?_______________________________________________
What type of gametes are these?   isogametes   anisogametes   oogametes

If syngamy has occurred by motile sperm entering a pore in the wall of the oogonium, you may find an orange zygote inside the oogonium.
When the zygote germinates, it divides meiotically to form four haploid zoospores, each of which can develop into a new filament.

Which is the dominant generation?_____________________-phyte

In which generation(s) does asexual reproduction occur?____________________-phyte

What word would describe this life cycle?   sporic   gametic   zygotic

D. Ulothrix

This filamentous green alga usually is found in quiet or running fresh water where the plants are attached to rocks, pebbles or debris by means of a basal holdfast cell. All cells of the filament, except the holdfast cell, are essentially alike, may divide to add to the length of the filament, and may be active in asexual or sexual reproduction.

Examine the fresh material available.

What is the shape of the cell__________________________

Is there a nucleus? Yes   No Is there more than one nucleus? Yes   No

How many chloroplasts are there in each cell?______________________________
You will need to focus up and down through the entire thickness of the filament to determine the number! The scattered spots in the chloroplast are pyrenoids, areas associated with RuBP carboxylase.
Look for any cells in the filament in which the protoplast has divided mitotically into several (2-16) daughter protoplasts. If none are visible in the fresh material, obtain a prepared slide to find such a cell. Each of the individual protoplasts develops four terminal flagella and a prominent eyespot (a small, pigmented structure which may be sensitive to light) to become a zoospore. Zoospores are released from the parent cell (which can be called a zoosporangium) through a pore in the wall. After swimming around for a time, the zoospore settles down, loses its flagella, and grows into a new Ulothrix filament by repeated mitoses.

Should the new filaments be genetically
identical to the parent filament? Yes   No

What would you call this kind of reproduction?_____________________________

In another cell from the same or a different filament, find a cell in which the protoplast has divided into a larger number (16-64) smaller protoplasts. Each of these protoplasts appears similar to a zoospore except that it has only two flagella and is smaller. Each is a gamete, which on release from the gametangium swims around until it meets and fuses with a similar-appearing gamete.

What would you call these gametes?_____________________________

It has been shown that the gametes must come from different filaments...

the species is thus ________________-thallic.
The zygote resulting from the fusion of gametes forms a heavy wall and undergoes a period of dormancy. What advantage is there to the species in the production of a thick-walled dormant cell?

_____________________________________________________

When the zygote germinates, the nucleus undergoes meiosis to form four haploid swimming spores similar in appearance and behavior to the zoospores produced in a zoosporangium.

Which is the dominant generation?____________________-phyte

In which generation(s) does asexual reproduction occur?__________________-phyte

What word would describe this life cycle? sporic   gametic   zygotic

Use the life cycle diagrams to assist you in placing the structures you have observed into the perspective of a generalized life cycle.

Also, if tides and seasons are favorable, there may be some macroscopic marine algae on display. You will want to take some notes on these as well.


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