Types of Blue-green Algae

There is some confusion regarding classification of algae. Some researchers have suggested that there may be as many as 7,500 species of blue-greens. This estimate may be high, however, given the tendency of alga to take on different forms in differing environments.

Gloeocupsa (figure 12-la) is a very simple blue-green that grows as single cells or in small clusters of cells. A beginner examining Gloeocupsa under a microscope may think that a nucleus is seen. This deceptive appearance is caused by the matrix that is secreted by and lies outside of the cell. Some cells show a double layer of matrix. When the cell attains a certain size, the protoplast divides, and each daughter cell produces a new matrix. There are twenty-three species of this genus in the United States. They may be found growing on wet rocks and the sides of aquarium tanks, and in ponds.

Figure 12-1 Several blue-green algae:( a) Gloeocapsa; (b) Nostoc, possessing a heterocyst and a hormogonium; (c) Anabaena, showing an akinete (a form of resting spore); (d) Chamaesiphon, with an exospore; (e) Oscillatoria, showing a breaking point and a hormogonium, and how the cell shape changes, (f) in the swaying movement.

Oscillatoria (figure 12-1e) is a common blue-green that grows in filaments. It may be found in lakes, ponds, and moist soil, where the filaments appear singly or in mats. There are approximately thirty species of Oscillatoria in North America. These blue-greens give water a putrescent odor. The formation of filaments is caused by the cells dividing transversely The cells of the filament are essentially alike, and each cell can be regarded as an individual. Oscillatoria is capable of an undulating movement, and this is the basis of the name. The movement is so slow that patience is required to detect it. Although several theories have been offered to explain this movement, the mechanism is not definitely known.
The drawing of Oscillatoria in figure 12-1e shows a dead cell where the filament can break, thus producing two shorter segments. The shorter pieces are called hormogonia, and this is one method of reproduction. Figure 12-1 also shows how the cells change shape during the swaying movement of the filament. The left side of the cell has a greater vertical dimension when the filament bends to the right (figure 12-lf); the water then shifts in the cell to produce a greater vertical dimension on the right side when the filament bends to the left (figure 12-lg).

Nostoc (figure 12-lb) forms in colonies as large as a plum. In such a colony, there are thousands of filaments embedded in a jellylike matrix. Although Nostoc does occur in soil, it is more commonly found floating in water. Both Nostoc and Anabaena (figure 12-lc) are capable of using atmospheric nitrogen to make compounds that can be used by higher plants. It is reasoned that the fertility of rice paddies is maintained by the presence of these algae. In this sense, they resemble the bacteria that grow in nodules on the roots of bean-family plants (and that can also manufacture useful compounds from atmospheric nitrogen).

Figure 12-lb shows empty cells lying intermittently along the filament. These are called heterocysts. The filament breaks at these points, producing hormogonia. Nostoc colonies may be found growing symbiotically with fungi, thus forming lichens.

The illustration of Anabaena (figure 12-lc) shows an akinete, a large, resistant cell containing a food reserve. Its thick cell wall allows it to withstand adverse conditions. When living conditions again become favorable, it can germinate and grow a new algal filament.

Some blue-greens produce endospores. Chamaesiphon (figure 12-ld) is an example.