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Algae are small aquatic plants with simple bodies, but some attain large proportions. All algae are characterized by lacking tissues and organs of organ systems, and have few specialized cells. Half of the world's oxygen supply comes from these simple plants. These plants do not have true roots, stems, or leaves like the higher plants in phyla, nor do any of them have water-carrying vascular tissue.
A type of algae in one phylum is the blue green algae and it is classified with the bacteria. Blue-green algae were among the earliest forms of life on earth and will be among the last. They have no nuclei, and the chlorophyll in these primitive algae is not contained in chloroplasts. All other algae have cells with nuclei and chloroplasts. Their plasma membrane folds inward and studded with light-trapping pigments, some of which give each species its typical blue, red-green or almost black color. Unlike bacteria, blue-green algae have the same kind of chlorophyll pigments found in true algae and plants.
There are 1,500 known species of the blue-green algae that look as filaments because cells remain attached to one another after division. In many blue-green algae filaments, we see the first inklings of a division of labor: a process not seen among bacteria, but a feature of more complex organisms. These filaments consist of two cell types: one specialized for photosynthesis, the other for nitrogen fixation.
Blue-green algae are one major kind of prokaryote. Beside their photosynthetic activities, they are nitrogen fixers of no small importance. If they were not allowed to continue their activities, the biosphere would probably collapse for lack of enough usable nitrogen compounds for protein synthesis.
The blue-green algae live in saltwater, freshwater, and on land. They survive in near-boiling hot springs, on desert rocks, in permanent snowfields, and on extremely salty rocks. They can survive due to having simple requirements for raw materials and energy. Anywhere they live the environment will have some water, simple mineral salts, sunlight and carbon dioxide and these are all they require for their survival. Reproduction is by binary fission. The energy-rich nutrition is used at once for rapid growth and reproduction so they can compete well with other organisms.
Red algae (Rhodophyta) are plants that are close in form to ones that lived millions of years ago, in the shallow waters of the earth. There are over 4,000 species of red algae today. Body forms range from single cells, to filaments, to filaments massed tightly into sheets that may fan out a meter or so.
Red algae may have been derived from blue-green algae, as their structural similarities are alike. Both contain physcobilins (a class of light-trapping accessory pigments). Red algae range in color from green, to red, purple, and greenish-black. They have the accessory pigment need to adapt in deeper waters as chlorophyll functions more efficiently in light containing red wavelengths. The red wavelengths do not penetrate far below the water's surface so the available wavelengths are mostly blue-green. The accessory pigments in red algae absorb these wavelengths and pass on some of the energy to nearby chlorophylls engaged in photosynthesis.
The red algae reproduce by vegetative growth of fragments that break away from the parent body. There is some sexual reproduction known to occur in some species. In some red algae species, the zygote undergoes meiosis and leads to the formation of haploid spores. These spores produce gametes by mitosis. Both the spores and gametes must be dispersed by the random flow of currents; red algae generally have no motile cells. Spore dispersal as well as sexual fusion of gametes depends on the presence of free-flowing water.
The green algae includes about 7,000 species ranging from single motile cells to broad, sheet like forms such as Ulva (sea lettuce). The body plan is a series of straight or branched filaments, each a single cell thick.
Species are mostly freshwater organisms; and some even live on moist land. Their photosynthetic system includes chlorophylls a and b, carotenoids, and xanthophylls and is identical to that of complex land plants. Due to their pigments green algae will capture light wavelengths available in shallow waters.
Like the red and brown algae, the green algae show alternation of generations. In most species, the gametophyte is dominant; the diploid body is usually limited to the zygote. The green alga as the brown algae shows variation in the type of gametes produced. The simpler forms rely on isogamy: the gametes are identical in appearance. The complex forms rely on zoogamy: gametes will differ in size and motility, much like the motile male gametes (sperm) and larger, nonmotile female gametes (eggs) of animals. Eggs that develop while attached to a gametophytes receive protection from the parent plant. The motile sperm reach the eggs by swimming around in the surrounding water.
Brown algae include about 1,500 species. They obtain their olive-green or dark-brown color from an accessory pigment (one of the xanthophylls). These brown algae live on rocky coasts and anchor themselves to submerged rocks by structures called holdfasts. Some thrive in the open sea. The brown alga helps support unique marine animals.
Kelps are large and many-celled algae. Some are 45 meters long and have one end attached to the bottom. The kelps have long, tough stalks that grow up through the water. Among these stalks there are many flat, strap-shaped parts branching off. These carry on photosynthesis as leaves do in land plants, but they do not contain water carrying vascular tissue.
They usually locate to the bottom so they cannot grow in deep seas. Light for photosynthesis reaches only about 60 meters into the water. The Sargassum weed is one that floats on the surface of the water in warmer parts of the Atlantic Ocean. Otherwise, the plants of the open sea are the tiny algae, which drift in the upper levels of the water.
Another many-celled algae grow all over rocks along the shore and these are rockweeds. These are about 50 centimeters long and very tough. They can withstand the beating of the waves when the tides come. Rockweeds have exposure to sun and wind when the tide is out.
The sea covers three-fourths of the earth's surface and algae are the food makers of this vast area. Many small animals, such as shrimp and their relatives, eat simple algae. Fish eat the shrimp, and larger fish eat the smaller fish, in this way algae feeds the entire ocean community directly or indirectly.
There was a jelly-like material taken from algae called Irish moss that was once used as a desert. Texture is added to medicine and is used in ice cream and cosmetics. Iris moss can be found in some foods.
Algae like Chlorella could be food for cattle or humans. These one-celled plants grow very rapid under good conditions and be filtered out of the water and packed in sold cakes. Scientists are studying these as a new source of food.
They form growths in reservoirs and give water an unpleasant taste and odor. They can form a scum on the lining of swimming pools and can even be pests in aquariums as some forms turn the water green. Many algae even become a nuisance to all of us living organisms.
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