Not only has cyanobacteria been an important element for
forming the earth's oxygen atmosphere, but it has also contributed to many
other attributes important to human life. Many Proterozoic oil deposits
are attributed to the activity of cyanobacteria. They are also important
providers of nitrogen fertilizer in the cultivation of rice and beans.
The other great contribution of the
cyanobacteria is the origin of plants. The chloroplast with which plants
make food for themselves is actually a cyanobacterium living within the
plant's cells. Sometime in the late Proterozoic, or in the early Cambrian,
cyanobacteria began to take up residence within certain eukaryote cells,
making food for the eukaryote host in return for a home. This event is
known as endosymbiosis, and is also the origin of the eukaryotic mitochondrion.
Because they are photosynthetic and aquatic, cyanobacteria are often called
"blue-green algae". This name is convenient for talking about organisms
in the water that make their own food, but does not reflect any relationship
between the cyanobacteria and other organisms called algae. Cyanobacteria
are relatives of the bacteria, not eukaryotes, and it is only the chloroplast
in eukaryotic algae to which the cyanobacteria are related.
