Fig.21.2 Blue-green algae

Blue-greens have no known sexual reproduction. Reproduction of unicellular forms is by cell division in which the cell wall folds in and “pinches” the cell into two cells. Colonial and filamentous forms fragment into separate pieces. Filamentous forms have various specialized cells: an akineteis a resistant cell filled with food reserves, which can germinate to form a new filament; a transparent heterocystfunctions in nitrogen-fixation, and frequently is located where the filament breaks. Some genera have exospores. These are cells that pinch off the filament.

Cyanobacteria that occur symbiotically in other organisms commonly lack a cell wall and appear to function essentially as chloroplasts. When a eukaryotic cell containing chloroplasts divides, the chloroplasts divide at the same time. Then cells of cyanobacteria occurring within the cells of other organisms divide in similar fashion, leading to speculation that chloroplasts originated as cyanobacteria or prochlorobacteria (discussed after the cyanobacteria), living within other cells. Fossils of photosynthetic organisms believed to be 3.5 billion years old and closely resembling present-day cyanobacteria have been found in Australia. It was not until half a billion years later, about 3 billion years ago, that these organisms began producing oxygen as a by-product of photosynthesis. The oxygen slowly began to accumulate, becoming substantial about 1 billion years ago. At the same time the oxygen was accumulating, other photosynthetic organisms appeared, and forms of aerobic respiration developed. Within the last half billion years, enough ozone, which is a by-product of oxygen, accumulated to become an effective shield against most of the harmful ultraviolet radiation coming from the sun. Photosynthetic organisms, which had been protected from the radiation by their watery environments, were then able to survive on land. Accordingly, cyanobacteria appear to have played a fundamental role in almost the entire history of living organisms.

Synechococcus

Structures of this unicellular spherical (coccoid) bluegreen include a mucilaginous sheath, cell wall, plasma membrane and protoplasm. Reproduction is by binary fission(splitting into two).

Fig. Synechococcus

Cylindrospermum

This filamentous blue-green has an akinete (resistant cell) and a heterocyst (spore-like cell) at the end of the trichome.

Date: 2016-01-03; view: 1161