Blue-green Algae (Division Cyanophyta)
The reference to “algae” in the name of these organisms indicates that the designation is used loosely for they are often considered to be more closely related to bacteria than to true algae.
The cell walls of blue-green algae differ from those of most bacteria in that they usually contain cellulose, but like those of bacteria, they also usually seem to contain muramic acid. Outside the cell wall proper, there is often a layer of a more or less firm, gelatinous material called a sheath, composed of pectic materials.
All blue-green algae possess photosynthetic pigments located in lamellar structures that appear to be flattened vesicles. These structures are similar to the chromatophores of the photosynthetic bacteria. Like the chromatophores, they are not contained within chloroplasts. The cells contain a number of pigments including chlorophyll, carotene, phycocyanin, and phycoerythrin.
The chlorophyll of blue-green algae is chlorophyll A, the pigment also found in higher plants, rather than bacterial chlorophyll. In addition to chlorophyll and various carotenoids, these organisms contain phycocyanin (blue pigment) or sometimes phycoerythrin (red pigment). It is the presence of phycocyanin with chlorophyll that gives these algae their characteristic blue-green color.
However, not all blue-green algae are blue-green. Black, brown, yellow, red, grass green and other colors also occur.
Some of these organisms are also able to fix free nitrogen in a fashion similar to bacteria. Reproduction in blue-green algae is by cellular fission as in bacteria. Reproduction of this major group is asexual.
Blue-green algae occur in numerous and varied habitats. They most frequently occur in masses, either floating or attached to some object in water. Some species are very common in or on soil, others are often found growing on the sides of damp rocks and on the bark of trees. They tend to be particularly abundant on wet cliffs and ledges. Many are mainly inhabitants of fresh waters.
Ponds or lakes containing a rich supply of organic matter, particularly nitrogenous compounds, often develop huge populations or “algal blooms” of blue-green algae which may make the water so green that objects only a few inches below the surface are entirely invisible.
These blooms are often seen in ponds and small lakes are sometimes formed by Anabaena, a filamentous form resembling a string of beads, or Coelosphaerium, a spherical mass of cells or by other blue-greens.
Although not necessarily indicators of pollution, blue-greens often thrive under such conditions; Lyngbya and Oscillatoriaa being notable examples. These types of algal blooms may give the water an objectionable odor, clog filters of water supplies, and even be toxic to some livestock. The pigment phycocyanin so common in many of these forms is water soluble and release of it into lake water through algal decomposition may color lake water.
A few species of blue-greens live in habitats that rank among the most inhospitable known. For instance, the hot springs that occur in various parts of the world. These species can grow well at temperatures as high as 85° C.
The occurrence of blue-green algae in environments so bleak that no organisms can inhabit them is probable due to the fact that the nitrogen fixing species have the simplest nutritional requirements of any known organisms. In addition to N2 and CO2, which are readily available from the atmosphere, they need only water, light, and minerals to reproduce and grow.