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The primary classification of algae is based upon cellular, not organismal properties: the chemical nature of the wall, if present; the organic reserve materials produced by the Cell; the nature of the photosynthetic pigments; and the nature and arrangement of the flagella borne by motile Cells. In terms of these characters, the algae are arranged in a series of divisions, summarized in Table 26.1.
 
The primary classification of algae is based upon cellular, not organismal properties: the chemical nature of the wall, if present; the organic reserve materials produced by the Cell; the nature of the photosynthetic pigments; and the nature and arrangement of the flagella borne by motile Cells. In terms of these characters, the algae are arranged in a series of divisions, summarized in Table 26.1.
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== The Major Groups of Algae ==
 
== The Major Groups of Algae ==
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For comparison, a model Human Cell is 10 microns [or100,000 Atoms]  in diameter.
 
For comparison, a model Human Cell is 10 microns [or100,000 Atoms]  in diameter.
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== The Photosynthetic Flagellates==
 
== The Photosynthetic Flagellates==
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Many fossil diatoms are known, because the siliceous skeleton of the wall (Figure 26.6) is practically indestructible, and as diatoms are one of the major groups of algae in the Oceans, large fossil deposits of diatom walls have accumulated in many areas. These deposits, known as ''diatomaceous Earth'', have industrial uses as abrasives and filtering agents.
 
Many fossil diatoms are known, because the siliceous skeleton of the wall (Figure 26.6) is practically indestructible, and as diatoms are one of the major groups of algae in the Oceans, large fossil deposits of diatom walls have accumulated in many areas. These deposits, known as ''diatomaceous Earth'', have industrial uses as abrasives and filtering agents.
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== The Natural Distribution of Algae ==
 
== The Natural Distribution of Algae ==
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The marine algae play a very important role in the cycles of matter on Earth, since their total mass (and consequently their gross photosynthetic activity) is equal to that of all land plants and is probably much greater. This role is by no means evident, because the most conspicuous of marine algae, the seaweeds, occupy a very limited area of the Oceans, being attached to rocks in the intertidal zone and the shallow coastal waters of the continental shelves. The great bulk of marine algae are uniCellular floating (''planktonic'') organisms, predominantly diatoms and dinoflagellates, distributed through the surface waters of the Oceans. Although they sometimes become abundant enough to impart a definite brown or red color to local areas of the sea, their density is usually so low that there is no gross sign of their presence. It is the enormous total volume of the Earth's Oceans which makes them the most abundant of all photosynthetic organisms.
 
The marine algae play a very important role in the cycles of matter on Earth, since their total mass (and consequently their gross photosynthetic activity) is equal to that of all land plants and is probably much greater. This role is by no means evident, because the most conspicuous of marine algae, the seaweeds, occupy a very limited area of the Oceans, being attached to rocks in the intertidal zone and the shallow coastal waters of the continental shelves. The great bulk of marine algae are uniCellular floating (''planktonic'') organisms, predominantly diatoms and dinoflagellates, distributed through the surface waters of the Oceans. Although they sometimes become abundant enough to impart a definite brown or red color to local areas of the sea, their density is usually so low that there is no gross sign of their presence. It is the enormous total volume of the Earth's Oceans which makes them the most abundant of all photosynthetic organisms.
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== The Nutritional Versatility of Algae==
 
== The Nutritional Versatility of Algae==
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Many algae that perform normal photosynthesis in the light, using CO2 as the carbon source, can grow well in the dark at the expense of a variety of organic compounds; such forms can thus shift from photosynthetic to respiratory metabolism, the shift being determined primarily by the presence or absence of light. Algae completely enclosed by Cell walls are osmotrophic and dependent on dissolved organic substrates as energy sources for dark growth. However, a considerable number of unicellular algae that lack a Cell wall, or are not completely enclosed by it, can phagocytize bacteria or other smaller microorganisms and thus employ a phagotrophic mode of nutrition as well. It is not correct, accordingly, to regard the algae as an ''exclusively'' photosynthetic group; on the contrary, many of their unicellular members possess and can use the nutritional  capacities characteristic of the two major subgroups of nonphotosynthetic eukaryotic protists, the protozoa and fungi.
 
Many algae that perform normal photosynthesis in the light, using CO2 as the carbon source, can grow well in the dark at the expense of a variety of organic compounds; such forms can thus shift from photosynthetic to respiratory metabolism, the shift being determined primarily by the presence or absence of light. Algae completely enclosed by Cell walls are osmotrophic and dependent on dissolved organic substrates as energy sources for dark growth. However, a considerable number of unicellular algae that lack a Cell wall, or are not completely enclosed by it, can phagocytize bacteria or other smaller microorganisms and thus employ a phagotrophic mode of nutrition as well. It is not correct, accordingly, to regard the algae as an ''exclusively'' photosynthetic group; on the contrary, many of their unicellular members possess and can use the nutritional  capacities characteristic of the two major subgroups of nonphotosynthetic eukaryotic protists, the protozoa and fungi.
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== The Leucophytic Algae==   
 
== The Leucophytic Algae==   
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