|The first plants, photosynthetic cyanobacteria, are still
abundant in a great variety of forms today and account for
about 20-30% of the current oxygen production in the
oceans. photo by Mary Cousins cc by SA 3.0
The ancestors of modern cyanobacteria invented photosynthesis over 3 billion years ago, or I should say they assembled it from processes obtained through horizontal gene transfer from at least three different ancient bacteria. (See "The first plants") Earlier bacteria that harvested sunlight created carbohydrate, but the cyanobacterial process also releases releases oxygen as a byproduct, and therein lies the most important part of the story.
|Cyanobacteria, such as this filamentous Oscillatoria were
classified as algae until their prokaryotic nature was discovered.
Photo by Wiedehopf20. CC SA 4.0 International
|Chroococcus is a cyanobacterium in which
cells divide within a gelatinous matrix.
Photo by Xvazquez CC by 3.0 unported.
All protists, animals, fungi, and higher plants are composed of eukaryotic cells. Bacteria and archaeans have the much smaller and structurally simpler cell structure we call prokaryotic. Though the name “eukaryote” emphasizes the presence of a true nucleus, these more advanced cells have additional complex organelles, internal membrane systems, and a sophisticated cytoskeleton that controls cell shape, cell and nuclear division, and the movement of organelles and materials within the cell. Such complex cells require a lot of energy for all this internal activity, and so could not have existed until oxygen was available.
When oxygen did finally become abundant in the seas, aerobic bacteria evolved, first to protect themselves, and then to harness the oxidative power of oxygen to break food materials more completely. What a boon that was! Anaerobic respiration can squeeze only 2 ATP molecules from a molecule of glucose, while aerobic respiration yields 38!
|The first eukaryotic cell evolved as a flexible, amoeba-like archaean
ingested aerobic bacteria, which evolved into mitochondria. Later,
cyanobacteria were captured by a primitive eukaryote, and became the
chloroplasts of the first true algae.
archaean, had already shed the rigid cell wall that surrounds most prokaryotes, and with its naked, flexible wall it could surround other cells and bring them inside for digestion. It was essentially a primitive amoeba with a rudimentary cytoskeleton.
Such a cell eventually met up with an aerobic bacterium, related to modern purple non-sulfur bacteria, and took it inside. Instead of digesting the bacterium, however, a truce developed between the two cells and a deal was struck. The host cell provided food to the captured aerobic bacterium, and the bacterium in turn absorbed oxygen, used it to break down the food, and paid rent to the host cell in the form of ATP molecules. That captured aerobic bacterium evolved into the energy–processing organelle we call the mitochondrion, and that symbiotic union was the first eukaryotic cell. Incidentally, but also of huge significance, cyanobacteria were also engulfed by some early eukaryotes and became the chloroplasts we find in algae and higher plants.
So what was the real reason for the 2 billion year delay? Iron. At the beginning, there was a huge amount of dissolved iron in the oceans as well as in the surface rocks. When exposed to oxygen, iron rusts, as anyone who has left tools outside too long knows. Technically speaking, this early iron was in its reduced state, and when exposed to oxygen it became oxidized. The oxidized form of iron, however, is not soluble in water, so it settled out, creating vast sedimentary deposits known as the banded iron formations. So all the oxygen produced by cyanobacteria was at first gobbled up by the huge amount of iron dissolved in the seas. Only after most of the dissolved iron in the oceans was depleted could oxygen start to accumulate in the environment, and that is what took 2 billion years - exactly the amount of time that some scientists propose that life was waiting for a fluke event to occur. On the contrary, it seems that the origin of eukaryotes happened as soon as it became possible.
So cyanobacteria were the first to feed the world through modern photosynthesis, they created a crisis that enabled and forced the evolution of aerobic bacteria and the first eukaryotes, and they became the chloroplasts for all eukaryotic algae and plants. Anything else? Actually, the cyanobacteria probably also invented aerobic respiration itself and passed it on to other bacteria through horizontal gene transfer.
Cyanobacteria had to have a means for protecting themselves from the oxygen they produced, and also for burning the fuel they created through photosynthesis. They most likely did this at first by running parts of the photosynthetic process backwards. Look at schematic diagrams of photosynthesis and aerobic respiration. Though details have changed over time, the two processes are roughly mirror images of each other. In modern cyanobacteria, photosynthesis and aerobic respiration take place in separate pathways, but these overlap and use some of the same protein complexes.(See Photosynthesis and Respiration in Cyanobacteria, by W. Vermaas)
If all that is not enough to call cyanobacteria superheroes of evolution, there is one more thing. Whether they invented the process themselves or acquired it from some ancient bacterium through lateral gene transfer, cyanobacteria were and still are major fixers of nitrogen. This all-important process converts atmospheric nitrogen, which is inert, into ammonia, which organisms can use to make proteins, nucleotides, and countless other essential molecules. Kudos to the cyanobacteria.
BTW - Much of this is discussed in more detail in my book, Plant Life (if you haven’t read my book yet, why not?!)
Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. Norton &^ Co
Gribbin, John. 2011. Alone in the Universe - Why our planet is unique. Wiley. NY.
Ward, Peter D and Donald Brownlee. 2000. Rare Earth - Why complex life is uncommon in the universe. Copernicus/ Springer-Verlag. NY.
Conway Morris, Simon. 2015. The runes of evolution: how the universe became self-aware. Templeton Press. West Conshohoken, PA.