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Algae are photosynthesizing eucaryotes that do not develop from
multicellular embryos. Eucaryotic algae include the green algae
(Chlorophyta), brown algae (Phaeophyta), red algae
(Rhodophyta), and several groups of microscopic algae. Some biologists
treat the algae as plants; others treat them as Protista. Green algae (Chlorophyta): Characteristic pigments, similar to those of higher plants, include chlorophylls a and b, xanthophylls, and α- and β-carotenes (or γ- and β- carotenes in one group). Starch is the main storage product, as in higher plants. Main cell wall constituents are cellulose and pectin. Chlorophyta are ecologically dominant in freshwater ecosystems, but many are also marine. Of all groups of algae, the Chlorophyta are most similar to higher plants. Euglenoids (Euglenophyta): One-celled; many pigments similar to green algae. No cell walls, except for some strips of protein. Can use photosynthesis or animal-like ingestion, depending on light levels. Brown algae (Phaeophyta): Largest of all algae; ecologically dominant in temperate and colder marine waters. Structurally most complex algae, with frequent division into a holdfast, a stalk or stipe, and a leaflike blade. Pigments include chlorophylls a and c, plus several unique xanthins. Many unique storage products and cell wall components. Dinoflagellates (Pyrrophyta): Small algae, mostly marine and planktonic. Pigments include chlorophylls a and c, plus some unique pigments. Two flagellae, arranged at right angles to one another. Mitosis unusual: no histones, no centrioles, no spindle fibers; nucleolus and nuclear envelope remain visible throughout mitosis. Golden-brown algae and diatoms (Chrysophyta): Diatoms are especially abundant in plankton and are the major primary producers in most marine ecosystems. Some pigments and storage products are unique; others show similarities to brown algae. Yellow-green algae (Xanthophyta): A group related to the Chrysophyta, but distinguished by pigments that include chlorophylls a and e. Red algae (Rhodophyta): Algae with chlorophylls a and d, plus other pigments resebmling those of Cyanobacteria. No "9 + 2" organelles (centrioles or flagellae); no motile cells, not even gametes. Unique storage products include "floridean starch" in cell walls. Ecologically dominant in tropical marine waters. |
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