Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
|Examples: Ectocarpus; Laminaria and Macrocystiskelps (these have a root-like holdfast, a stemlike stipe, and leaflike blades); Sargassum; Fucusrockweed (found at the beach; tips of the thallus have bladders that you can pop).|
Division Chlorophyta—green algae
About 7000 species have been described. Many are marine (e.g., Ulva--sea lettuce, a membranous thallus); others are a major component of the freshwater phytoplankton. Together they have been estimated to fix over a billion tons of carbon per year. Chloroplasts are grass-green and contain chlorophylls a and b and carotenoid derivatives. Since the Chlorophyta and land plants share photosynthetic pigments and cell wall chemistry, it is presumed that some kind of green alga was the ancestor of the land plants. These plants have many different forms including motile unicellular and colonial, nonmotile unicellular, colonial, filamentous, membranous, and multinucleate. The microscopic examples you will see are all haploid, fresh water green algae.
Chlamydomonas is a motile unicell; each cell has two flagella. The chloroplast is cup-shaped. Starch is synthesized in a special region of the chloroplast called the pyrenoid. Recall that starch is a polysaccharide made of glucose sugars. Chlamydomonas belongs to a group called the Volvocales. Other examples are Gonium, which consists of groups of Chlamydomonas-like cells, and Pandorina in which the cells are more regularly arranged.
Volvox, itself, is a colonial alga, a hollow cluster of mostly identical cells that are held in place by a gelatinous matrix. The flagella of each cell cause the colony to roll through the water. You can see examples of asexual reproduction--cells within the colony divide and form hollow spheres called daughter colonies (left). In sexual reproduction a large egg and tiny sperm (heterogametes) are formed; fertilization results in thick-walled, spiny zygotes (right), also inside a parent colony.
Oedogonium is an example of a filamentous or thread-like green alga, and it does not swim. The chloroplast is netlike and has many pyrenoids. A holdfast attaches the filament to the substrate. Mitosis produces haploid zoospores (asexual reproduction). In sexual reproduction heterogametes meet and form a thick-walled zygote; when it germinates, meiosis produces haploid motile spores that settle down and a new haploid filament grows.
Spirogyra is also a filamentous green alga. Prepare a wet mount by picking out some strands with forceps and curling them on the slide; add a couple drops of water and put a cover slip over the alga. The choroplast spirals between the cell membrane and the central vacuole. Examine its form and note the pyrenoid bodies spaced along its length. Remove the cover slip, absorb the water with a paper towel, and replace the water with iodine solution. Have the pyrenoids changed color? What does this indicate? Would you have seen the same result if Spirogyra had been in the dark or in very cold water?
Asexual reproduction occurs if the filament breaks into pieces. Sexual reproduction is by conjugation. Two Spirogyra filaments lie side by side, and conjugation tubes bulge out from the cells in the opposite filaments; they meet and unite as continuous tubes. The contents of one cell move across into the other, and, when the nuclei fuse, a zygote is formed. In the following spring the zygote nucleus undergoes meiosis, three of the four nuclei die, and the new haploid cell germinates into a haploid filament.
Division Charophyta—Stoneworts and Brittleworts
These algae are often included in the division Chlorophyta, because they have the same photosynthetic pigments and store carbohydrate as starch. The body form and reproductive organs are different. Some botanists consider them to make a link between the green algae and land plants.
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