Cyanobacteria, also known as blue-green algae, are a group of photosynthetic bacteria. They are the oldest organisms and have played an important role in shaping the Earth's atmosphere and influencing the evolution of life.
Cyanobacteria vary in size and shape. They range from single-celled organisms to filamentous or colonial forms. The unicellular cyanobacteria are usually spherical or rod-shaped, while filamentous forms exhibit long chains of cells. Some filamentous species may have specialized cells, such as heterocysts (for nitrogen fixation) or akinetes (for survival in harsh conditions).
Cyanobacteria are prokaryotic, meaning that they lack a nucleus and membrane-bound organelles. Their genetic materials, the circular DNAs, are located in a range of cytoplasm called nucleoids. Their cytoplasms also contain plasmids, which are smaller circular DNA molecules that can replicate themselves independently. Similar to Gram-negative bacteria, their supporting and protective cell walls contain lipopolysaccharides and peptidoglycans. The outer layers are a gelatinous sheath of polysaccharides and proteins that provide additional protection and hold the cyanobacterial cells together to form colonies.
Cyanobacteria are well known for their oxygenic photosynthesis similar to plants and algae. They possess some flattened concentric rings, called thylakoids whose membranes contain pigments and proteins required for photosynthesis, such as chlorophyll, carotene and phycocyanin ( phycoerythrin, phycocyanin and allophycocyanin). These pigments capture sunlight and convert it into chemical energy of organic matter, while releasing oxygen as a by-product. The abundance of chlorophyll and phycocyanin gives them a blue-green color, while some cyanobacteria have large amounts of phycoerythrin to appear red, such as Trichodesmium erythraeum. They proliferate to make the sea water red, hence the name of the Red Sea.
Cyanobacteria have the ability to fix nitrogen: nitrogenases convert nitrogen into nitrogen compounds. Since nitrogenases are inactivated in aerobic conditions, they only work in anoxic conditions. Therefore, some cyanobacteria also form specifical cells named heterocytes, which separate nitrogen fixation from photosynthesis. Their thicker cell walls allow nitrogen to diffuse into the cell, while inhibiting oxygen influx. They also discard photosynthetic pigments to stop oxygen production, and respiration consumes intracellular oxygen to make an anaerobic environment. The atmospheric nitrogen is converted into ammonia which can be transported to other cells via plasmodesmata and used to synthesize proteins and nucleic acids.
The air vesicles give buoyancy to the cyanobacteria. They can control position in the water to receive full sunlight or to avoid strong midday sunlight by adjusting the gas in the air vesicles.
Binary fission is the most common method of reproduction in cyanobacteria. It involves the division of a single cyanobacterial cell into two identical daughter cells. The process is similar to the binary fission described earlier for bacteria, where the genetic material is replicated, the cell elongates, and then divides into two daughter cells. This process allows for rapid population growth. In harsh environments, they can generate a type of cell called akinete which is an enveloped, thick-walled, non-motile, dormant cell. Such cells can exist for a long time until the right environment is available and they will grow again.