Plants arose in water, and algae are examples of this stage in plant evolution. Life in water is much simpler than life on land, since the nutrient rich environment surrounds the organism.

Algae (s. alga) are photosynthetic, nonvascular aquatic plants. They may consist of a single cell (e.g., Chlamydomonas), of a cluster of cells (e.g., Gonium), or be a giant marine organism (e.g., Macrocystis—kelp). Algae are autotrophs or primary producers, part of the broad base of the aquatic food chain. They live in fresh water, brackish water, and marine environments. The photosynthetic portion is always at relatively shallow depth, within the zone of light penetration. Any large, flat multicellular form is referred to as a thallus. Gametes that are identical are called isogametes; if different, heterogametes (oogamy refers to a large, non-motile egg; in such cases sperm are small and motile).

Division Rhodophyta—red algae

About 4000 species of red algae are known. They inhabit the edges of the sea everywhere. A few species occur in fresh water. The kinds that you see are generally haploid; alternation of generations (diploid sporophyte and haploid gametophyte) is the norm. The reddish plastids are called rhodoplasts; they contain chlorophyll a and phycobiliproteins, which give them the red color; all lack chlorophylls b and c. Some red algae live at great depth, where red light wavelengths, which are of primary importance as energy for green and brown algae, fail to penetrate. Red algae are the source of agar, a gelatinous substance that is extracted from their cell walls; polysaccharides from these algae are used in the manufacture of ice cream and other food products.

Examples: Porphyridium (unicellular); Porphyra (multicellular membranous—used as food in Asia under the name of “nori”); Polysiphonia (branching form).

Division Phaeophyta—brown algae

About 1500 species of brown algae are known; nearly all are marine. Some are huge: The giant kelp may be as much as 100 meters long. Brown algae dominate the intertidal zone, especially rocky coastal sea shores in the temperate regions. Reproduction is mostly sexual. There is alternation of haploid gametophyte and diploid sporophyte generations. The gametophyte and sporophyte may look identical or be entirely different. Chloroplasts contain chlorophylls a and c, but never b. Fucoxanthin is responsible for the brown or olive-drab color of the thallus.

Examples: Ectocarpus; Laminaria and Macrocystis—kelps (these have a root-like holdfast, a stemlike stipe, and leaflike blades); Sargassum; Fucus—rockweed (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; iodine stain will help you find it. 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. The chloroplast is spiral with numerous pyrenoids. If iodine is available, prepare a stained slide of living Spirogyra. What is the function of the pyrenoids? The large central vacuole is within the spiral. 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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