Form Metabolism and Reproduction

The cells of cyanobacteria often occur in chains or in hairlike filaments, which are sometimes branched. Several species occur in irregular, spherical, or platelike colonies, the individual cells being held together by the gelatinous sheaths they secrete (Fig. 17.11). The sheaths may be colorless or

312 Chapter 17

Fibrillar Sheath Cyanobacteria

Aphanocapsa Merismopedia

Figure 17.11 Representative cyanobacteria, x2500.

Aphanocapsa Merismopedia

Figure 17.11 Representative cyanobacteria, x2500.

pigmented with various shades of yellow, red, brown, green, blue, violet, or blue-black, which makes some colonies quite striking in appearance.

The cells themselves appear blue-green in color in about half of the approximately 1,500 known species. This color, which is due to the presence of green chlorophyll a and blue phycocyanin, is often masked by the presence of other pigments. Several yellow or orange carotenoid pigments similar to those of higher plants are usually present, and varying amounts of phycoerythrin (a red phycobilin pigment in a form unique to the cyanobacteria) may give the cells a distinct red color. The periodic appearances of large numbers of cyanobacteria with considerable amounts of phycoerythrin are believed to have given the Red Sea its name.

The organisms produce a nitrogenous food reserve called cyanophycin. The production of such food reserves is atypical for prokaryotic organisms. Cyanobacteria also produce and store carbohydrates and lipids. Flagella are unknown in the cyanobacteria, but some of these organisms are nevertheless capable of movement. Oscillatoria filaments (see Fig. 17.11),

Kingd om Bacteria, Kingdom Archaea, and Viruses 313

for example, seem to rotate on axes and move in a gliding fashion, apparently by the twisting of minute fibrils inside the cell walls while secreting mucilage that reduces friction. New cells are formed through fission, while new colonies or filaments may arise through fragmentation (breaking up) of older ones.

In the common genera Nostoc and Anabaena (see Fig. 17.11), which form chains of cells, fragmentation often occurs at special, larger, colorless, nitrogen-fixing cells called heterocysts, which are produced at intervals in the chains. Members of these two genera also may produce thick-walled cells called akinetes, which can resist freezing and other adverse conditions. When favorable conditions return, this survival feature enables the cells to germinate and become new chains or filaments. Some akinetes have been known to germinate after lying dormant for more than 80 years.

Cyanobacteria do not produce gametes or zygotes and do not undergo meiosis. Genetic recombination has, however, been reported—apparently taking place in similar fashion to that reported for other bacteria—but its occurrence evidently is rare.

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