PHYLUM CHLOROPHYTA—THE GREEN ALGAE
Phylum Chlorophyta includes about 7,500 species of organisms commonly known as the green algae.They occur in a rich variety of forms and occur in very diverse and widespread habitats; they possess some of the most beautiful chloroplasts of all photosynthetic organisms. Some are unicellular and microscopic; in fact, the green alga Micromonas is only 1 μm in diameter - the smallest eukaryotic cell known. Most unicellular green algae are, however, considerably larger. Other green algae form threadlike filaments, platelike colonies, netlike tubes, or hollow spheres.
Some green algae are seaweeds, resembling lettuce leaves or long green ropes. Several unicellular species grow in greenish patches or streaks on the bark of trees, whereas others grow in large numbers on the fur of sloths and other jungle animals. The green fur camouflages the animals in their natural habitats. Still others thrive in snowbanks, live in flatworms and sponges, on rocks, or are found on the backs of turtles. They are the most common member in lichen “partnerships”. The greatest variety, however, is found in freshwater ponds, lakes, and streams. Ocean forms are also varied; there they are an important part of the plankton(free-floating, mostly microscopic organisms) and, therefore, of food chains. The chlorophylls (a and b) and other pigments of green algae are similar to those of higher plants. The green algae also are believed to have been the ancestors of higher plants and, like the higher plants, they store their food in the form of starch within the chloroplasts. With the exception of bryopsids, which are multinucleate, most green algae have a single nucleus in each cell. Most green algae reproduce both asexually and sexually. How they do so illustrates the forms of reproduction of most of the organisms discussed in the chapters to follow, and so we’ll examine several different representative green algae in some detail.
A small, actively moving little alga, Chlamydomonas is a common inhabitant of quiet freshwater pools. It has an ancient history among eukaryotic organisms, with fossil relatives occurring in rock formations reported to be nearly 1 billion years old. Chlamydomonas is unicellular, with a slightly oval cell surrounded by a complex multilayered wall that is partially composed of glycoproteins. A pair of whiplike flagella at one end pull the cell very rapidly through the water. The flagella are, however, difficult to see with ordinary light microscopes.
Date: 2016-01-03; view: 861