Green Algae Profiles

Freshwater, Marine, and Diverse Taxonomic Resource Matrices

Green Algae make up the division Chlorophyta, which includes about 7,500 species of eukaryotic, unicellular organisms. Some of the unifying characteristics of this division include similar photosynthetic pigments that make up the chloroplast, which include Chlorophylls a and b, and α-, β-, and γ-carotenes; and some xanthophylls and primary carotenoids. Almost all green algae store their carbohydrates in the form of starch. For the most part, their cell walls are composed of polysaccharides, including cellulose. The most obvious organelle of green algae is their chloroplast, which is responsible for giving them their green color.

The "green algae" is a paraphyletic group because it excludes the Plantae. Like the plants, the green algae contain two forms of chlorophyll, which they use to capture light energy to fuel the manufacture of sugars, but unlike plants they are primarily aquatic. Because they are aquatic and manufacture their own food, these organisms are called "algae," along with certain members of the Chromista, the Rhodophyta, and photosynthetic bacteria, even though they do not share a close relationship with any of these groups.

The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales, the closest relatives of higher plants, full differentiation of tissues occurs. There are about 6,000 species of green algae. Many species live most of their lives as single-cells, other species form colonies or long filaments.

Most green algae are present in fresh water, but there are diverse green algae in tropical marine habitats. Like land plants, the greens store starch (amylose or amylopectin) and have chlorophyll a and b as well as secondary pigments: carotenes, lutein, zeaxanthin. (Some Chlorophyta also have siphonoxanthin.) The chloroplast endoplasmic reticulum is absent in green algae, and their cell walls are often composed of cellulose, hydroxyproline, glycosides, xylans, mannans or sometimes calcium carbonate.

Green algae may be unicellular, multicellular, colonial or coenocytic (composed of one large cell without cross-walls; the cell may be uninucleate or multinucleate). They have membrane-bound chloroplasts and nuclei. Most species are aquatic and are found commonly in freshwater and marine habitats; some are terrestrial, growing on soil, trees, or rocks. Some are symbiotic with fungi giving lichens. Others are symbiotic with animals.