Cyanobacteria

Cyanobacteria have diverse shapes and sizes. Some are long; others are like rods or spheres. The cell walls usually have a thick outer covering or sheath. The cells of cyanobacteria filaments have interconnecting cytoplasm, and some even have specialized functions.

Cyanobacteria are photosynthetic. They contain chlorophyll a. the pigment found in plants, rather than the chlorophyll found in bacteria. Cyanobacteria have xanthophylls and carotenes, plus additional accessory pigments called phycobilins. The chlorophyll and other pigments are not enclosed in chloroplasts. Instead they are located on sheets of membrane found in the cytoplasm. Like plants, and unlike photosynthetic bacteria, cyanobacteria use water as a raw material of photosynthesis. They also produce oxygen as a byproduct of the process of photosynthesis.

Not all cyanobacteria are blue-green in color. The presences of the accessory pigments cause these organisms to have wide range of colors. Various species of cyanobacteria are bright green, golden yellow, blue-black, violet, and many other colors.

Some cyanobacteria carry out nitrogen fixation. For example, in Asia nitrogen fixation by cyanobacteria in rice paddies enables farmers to grow rice on the same land year without adding fertilizers.

Cyanobacteria algae reproduce by binary fission. Colonies of algae also reproduce through fragmentation; a process where colony breaks into pieces and each piece forms a new organism or colony. Some cyanobacteria can also produce resistant spores that survive in harsh conditions.

The rate at which cyanobacteria grow depends on the chemical content of the water in which they live. Dumping phosphates and certain other chemicals into lake water can result in an uncontrolled growth of cyanobacteria and algae called an algal bloom. The water takes on the color of the algae living in it. For example, the waters of the Red Sea are not red, but occasionally an algal bloom will cause the water to have a red tint. The bacteria responsible for the bloom are a species of cyanobacteria that has a very high content of red phycobilin. The decay of overabundant algae reproduces the amount of oxygen in water. This process in turn a cause fish to die and makes the treatment of the water more difficult. For this reason, the use of phosphate in detergents has been reduced in recent years so that fewer chemicals are added to water.

Blue-green algae are one-celled living things. Their cells are small and do not have a nucleus. Like the cells of bacteria, the cells of blue-green algae have three main parts: the cell wall, the cell membrane and the cytoplasm. Like bacteria they reproduce by fission. Blue-green algae cells often grow side by side and form chains. These chains are called anabaena. But still each cell in the chain is a single living thing. The cells don’t work together. Opposite to bacteria they have chlorophyll in cells. Having it they can make their own food.

Algae also make it possible for animals to exist on land. As algae carry out photosynthesis, they release oxygen into the atmosphere. Algae are so plentiful that they produce 90% of the world’s atmospheric oxygen.

The blue-green algae are prokaryotes and are organized much like the other prokaryotes, the bacteria. They are photosynthetic, but, unlike any photosynthetic bacteria, they contain chlorophyll a, which is also found in all photosynthetic eukaryotes. They have several kinds of accessory pigments, including xanthophylls, which are yellow carotenoid, and several other carotenoids. The cells of blue-green algae may also contain one or two pigments known as phycobilins. Chlorophyll and the accessory pigments are not enclosed in chloroplasts, as they are in plant cells, but are scattered in a membrane system distributed in the peripheral portion of the cell. Photosynthesis takes place in chlorophyll-containing membranes scattered throughout the cell and the nucleus is a single molecule of DNA.

The blue-green algae have a cell wall that doesn’t contain cellulose, but is made up of the same sorts of polysaccharides linked with polypeptides that occur in the bacteria.

The cells lack cilia, flagella or any other type of locomotive organelles, yet but some filamentous blue-green algae are capable of gliding motion. Reproduction is by simple fragmentation of cell division. Like some of the bacteria, many species are able to form thick-walled spores in which they can lie dormant during periods unfavorable to growth.

Cells of the blue-green algae have an outer mucilaginous sheath, or coating. The outer sheath is often deeply pigmented, particularly in species that spread up onto the land, and their colors include a light golden yellow, brown, red, emerald green, blue, violet, and blue-black. In addition, the carotenoids and phycobilins modify the color of the cells in which they occur. Thus, despite their name, only about half of the blue-green algae are actually blue-green in color. Indeed, the Red Sea was named of the dense concentration of red-pigmented blue-green algae that float on its surface.

Individual blue-green algae are microscopic, but they often grow in large masses as much as 1 meter or more in length. Some blue-green algae are unicellular, others are filamentous, a few form branched filaments and a very few form plates or colonies.

Almost all species are photosynthetic. Many are also capable of nitrogen fixation. The photosynthetic, nitrogen-fixing blue-green algae have the simplest nutritional requirements of any living things, needing only N and CO; which are always present in the atmosphere, a few minerals and water.

The ecological importance of the blue-green algae appears to be less than that of the nitrogen-fixing bacteria, at least for agriculture. However, in South-east Asia, rice can be growth on the same land for years without the addition of fertilizers because of the rich growth of nitrogen-fixing blue-green algae in the rice paddies.

Because of their nutritional independence, the blue-green algae are able to colonize bare areas of rock and soil. A dramatic example of such colonization was seen on the island of Krakatoa in Indonesia, which was denuded of all visible plant life by its cataclysmic volcanic explosion of 1883. Filamentous blue-green algae were the first living things to appear on the pumice and volcanic ash; within a few years they had formed a dark-green gelatinous growth. The layer of blue-green algae eventually became thick enough to provide a substrate for the growth of higher plants. It is very probably that the blue-green algae were similarly the first colonizers of land in the course of biological evolution.

Date: 2014-12-22; view: 788