Algae
The algae are an extensive group of plants, which show a great diversity in their range of forms. Some taxonomists consider them to be a separate division in the plant kingdom, while others place them at a lower rank of classification, labeling them as a class within the division Thallophyceae.
All algae are primitive plants, and are taxonomically united by the presence of unicellular organs of reproduction. The algae are comprised of representatives from two different groups of living organism, the prokaryotes (1,450 algal species) and the eukaryotes (23,500 algal species). The thallus (plant body) of algae can show a wide diversity of forms, ranging from unicellular to multi-cellular, filamentous, flattened or ribbon-like. The smallest, diatoms (which range in size from 5 to 200 microns), are visible only through microscopic observation, but the largest, seaweeds, can be in excess of 150 feet (50m) long.
Algae are aquatic, either marine or freshwater, although some require only a little water to thrive. The classification of the algae is based on their structure, the presence of different photosynthetic pigments, the nature of the cell wall, and the arrangement and structure of the flagella. The more evolutionarily advanced forms of algae exhibit a relatively complex internal arrangement, although they can be readily distinguished from the so-called higher plants by their lack of vascular tissue.
There are fourteen main groups of algae. Diatoms, or Bacillariophyceae, have a wide economic importance. Many skin defoliants rely on diatoms to act as an abrasive. Diatoms are a group of microscopic algae that are enclosed by a silicaceous shell. The shell is composed of two parts, one overlapping the other in a lidded box arrangement. Diatoms reproduce by binary fission as well as sexually. They occur either singly or grouped into colonies, and are found abundantly in marine and freshwater plankton and benthos. With dinoflagellates and other algae, they are the primary producers in the food chain. Deposits of dead diatoms create both siliceous earths (more recent deposits) and oil reserves (older deposits).
The Charophyceae are more commonly known as the stoneworts. Instead of the silica of the diatoms, these organisms have cellulose cell walls encrusted with calcium carbonate. They are multi-cellular, filamentous, and are anchored to the bottom of fresh water ponds with rhizoids. They are unique among the algae because of their multi-cellular sex organs, called antheridia and oogonia. The Chlorophyceae, the so-called green algae, have the same photosynthetic pigments as higher plants. This is the largest and most diverse algal group. They also inhabit the widest range of environments. The aquatic species thrive in fresh or salt water for the aquatic species, while the "non-aquatic" prefer damp environments. The primitive Chlorophyceae are unicellular, single or colonial organisms, microscopic, motile or non-motile. The higher forms are multi-cellular, with either a filamentous or flattened thallus.
The Chrysophyceae are golden brown in color due to the carotenoid pigments associated with the chlorophyll. They differ from the other groups of algae in that their food storage product is oil rather than starch. The species encountered here are very diverse marine and fresh water algae with close evolutionary links to the protozoa. The Cryptophyceae are a small group of biflagellate unicellular organisms. They occur in both marine and fresh water habitats. The Cyanophyceae (blue-green algae) are a primitive group of prokaryotic organisms. They can be microscopic as unicellular or colonial organisms, or they can be multi-cellular as filamentous forms. They are widely abundant in all forms of water and at extremes of temperature. Their color is variable from black to blue, via red, green, and purple, due to differing amounts of chlorophyll types. Some members of this group are responsible for toxin production. When conditions are right, whole bodies of water can be poisoned due to a bloom of these organisms depositing large quantities of toxin into the water. The Euglenophyceae are unicellular, with one to three flagellae. They are often found in sewage-polluted water and can occur in a colorless form.
The Phaeophceae are the brown algae, the seaweeds. They possess several types of chlorophyll, but the brown-colored fucoxanthin hides the expression of the other types. They have a complex internal structure which, however, does not show vascularisation. They are virtually all marine benthic organisms and can vary in size from 0.2 inch (1mm) to 150 ft (50 m). One group of algae bears the common name "fire algae," or Pyrrophyceae, due to their yellow and red colorization. They are mostly unicellular and have interlocking armored plates surrounding their cell wall. They are a major component of marine and fresh water plankton and require a vitamin supply. The Rhodophceae are another group of seaweeds, the red seaweeds. They are very diverse in form but relatively small compared to the brown seaweeds. They are usually benthic marine organisms. They can be used as a source of agar and are edible. The final group of algae are the Xanthophyceae, the yellow green algae. They are a very diverse group united by their chlorophyll pigments. If it were not for the pigments, their diversity of form would fit easily within all the other groups of algae.
Three recently discovered groups of algae, Prymnesiophyceae, Eustigmatophyceae, and Prochlorophyceae, were described in 1992.
This is the complete article, containing 860 words
(approx. 3 pages at 300 words per page).
