Theappeared early in the history of the , while conditions were not favorable to the life forms we know today. How did they live in these inhospitable times? How have they evolved and survived until today
Where do the algae come from?
There are between 1,500 and 2,500of cyanobacteria, they are very difficult to detect, so this figure is very approximate, and there is probably much more to discover.
They are classified inbecause they are always found in the water but they are not algae in the sense : we speak of cyanobionts therefore of blue plants!
From heterotrophic cells to photosynthetic bacteria
To know their history, one must go back very far …
The, 4 billion years ago, has an without and no , but rich in various : H2S, CH4, NH3, H2O, CO2 etc. Life, as we know it, is not possible in the very young ocean formed by as soon as the temperature of the atmosphere has dropped below 100 ° C.
The first cells are, practicing which is an anaerobic process. The are appearing there 3.5 billion years ago when the organic fermentable begin to decrease and the need to find a carbon source replacement.
The first photosyntheticconquer a new form of : . They use H2S ( ) as an donor . These living beings synthesize through the conversion of light energy into chemical energy (photoconversion). They have the energy needed to make organic in the cell by reducing CO 2 . Such cells still exist today.
The appearance and adaptation of cyanobacteria
A little later, around 2.5-3 billion years ago, cells choose water as an electron donor, these are. They live in a semi- because of the thick atmosphere and the thickness of the water. The is not enough, the light intensity is not strong enough to activate it properly; they therefore use a more sensitive (heteroprotein): , which gives them this blue hue. There may be other pigments in cyanobacteria in such a way that they absorb in all .
They thus reach the highest degree of energy autonomy, using only water and light to make their ATP. They are experiencing a great development and their high oxygen production by wateris the source of , but that’s another story!
This oxygen radically modifies the medium which, from reducer that it was 4 billion years ago, becomes oxidative 2.5 to 3 billion years ago, first in water only , then when the water is saturated with O 2 dissolved in the2.4 billion years ago.
There is little doubt as to the presence of a small amount (0.2 to 0.4% of the current content, low but stable pressure) of oxygen in the earlyatmosphere , which would suggest that the ocean was with an oxidized upper layer and an lower layer (Towe, 1990).
The amount of oxygen in the air increases from 1% 2.4 billion years ago to 21% today and a layer of ozone forms in the upper atmosphere, which protects life from, . A massive of cells accompanies this change, cells that do not support oxygen, very toxic for their structures, even cyanobacteria have to find a way to protect themselves, and they start to make : they are , we will come back to this.
It should be noted here that pure oxygen is always toxic to cellular structures and that oxygen does not “move” freely through the cell, it is carefully distributed in protected areas and evolution has selected for that, like the.
Unlike plants (which convertnitrogen into organic ) and animals (which convert organic nitrogen into another organic nitrogen), cyanobacteria (and a few others) are able to take nitrogen directly from the air, gaseous form, and transform it into organic nitrogen. They often live in association with other plants: , , beans… They thus put 100 million tons of mineral nitrogen available to plants on the planet.
Debates on the origin of these cyanobacteria
One of the current theories accepted by the current scientific community is that a primitive cell containing chlorophyll has incorporated or associated with another containing phycocyanin. Is it a predation with destruction of one of the two cells, the other keeping what is useful to him? Or the absorbed bacterium, tolerated by thebrings him the necessary pigment in exchange for nutrition? The result is a bacterium that has two pigments, one of which will make all the history of the plant kingdom and the other allows it to survive in this half-light of the on Earth.
The reproduction of cyanobacteria
Their only means of reproduction is, not to be confused with which only exists in .
Cyanobacteria do not have sexual reproduction (it seems) even if we know about certain exchanges of. This question is not clear yet. Some cyanobacteria (Nostoc for example) have large cells called heterocysts where a filament splits in two.