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.

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.

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.

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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