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I know from school, that all live on the Earth need bacteria as low-level "machines" that break down/extract/convert/produce chemical elements and combinations, other high-level organisms needed. But it is a natural way.

But is it possible to have a world with plants (without mammals or microorganisms and without bacteria) that could exist in the long term. Saying the atmosphere of these world has already enough nitrogen, oxygen and CO2, and of course there is water.

What could break this artificially created world with such conditions (say the world created not from low-level living structures)? Could bacteria emerge in the world?

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This is the sort of question that should be considered from more than one perspective. Since this is speculation, take it as a given that there is a lot of 'what if' here.

I doubt most animals and plants can do entirely without bacteria - as you say most of the essential nutrients come from bacteria, who fix nitrogen. If only plants were left on earth, eventually the plants would use up all the nitrogen and they would have to find a way to fix more.

Can bacteria emerge from just a world of plants? I don't think viruses arise spontaneously, but since genomes often have viruses embedded in them, over the course of a billion years or so, its possible since bacteria and viruses continue to be impressed upon our genomes. Would it happen in time? Most would be skeptical whether that timing could work out.

In practice it would be hard to create a world like this. I would be interested to see whether you could sterilize the microorganisms off of seeds without killing the plant for instance. If you're asking about a small sterile environment with only plants, you could do it by adding the nutrients the plants need and giving them sunlight. Such self sustaining systems have been made with cyanobacteria and i'd be surprised if plants could not be included. But these are closed systems and judged by limited amounts of time, so whether this is an answer to your question is not clear. Here it looks like some water plants and fish have been done. If there was a plant that created CO₂ at an adequate rate its possible.

At one time there were no animals at all. Its thought that for a while there might have only been photosynthetic structures that lay in the water and soaked up the rays. Its not clear whether as some say there were colony formations of cyanobacteria as we see today, or whether there may have been larger pre-cellular or proto-cellular structures. But recovering the pre-cambrian configurations of genes from just plant genomes seems like a gambler's bet to me.

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    $\begingroup$ thank you for your arguments and broaden answer! the link about the closed ecosystems is interesting. I think the nitrogen will be the most problem. But also it could be possible that some cells will mutate/break and create free components which can transform further. May be then will the microorganisms appear? Hm...what will be the evolution if one take cyanobacteria to another planet with water?... $\endgroup$ – static Mar 7 '13 at 4:38
  • $\begingroup$ Thanks.. its hard to tell with things like this - its possible that plants might evolve to fix nitrogen, if there is enough of it around the world for them to survive long enough. If all the bacteria and animals were digested into the ecosystem its hard to say how long plants could go on - quite a long time I imagine. $\endgroup$ – shigeta Mar 7 '13 at 4:58
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Think of it this way: Life always goes uphill, from less developed to more adapted, better at surviving and reproducing.

Early life would be a basic mechanism, trivial in nature, that under the right conditions would kind of replicate itself (kind of, since the replica would be less similar to the original, than when comparing current parent-child similarity). This would basically be a cell. And a cell would remain a cell unless combining with other cells and specializing in functionality would have a certain benefit. Check out this educational video on abiogenesis by cdk007, based on Jack Szostak's work.

But, you know that evolution isn't the process of change of all individuals of a species in a single direction, towards another species. It's a process of forking. So, basic cells of species A could evolve into single cells of species B, but that would only mean the extinction of A if the new environment with the new organisms becomes hostile, if resources are scarce, if species A is less adapted than everyone else, so everyone else occupies A's niche.

Thus, when multicellular organisms evolve, there are A LOT of monocellular organisms already present and occupying important niches. All available sources of energy are being used. If something isn't used - it's a new niche for any accidental mutation that enables the organism to use that niche and prosper as a new species. So anyway you look at it, multicellular organisms won't start existing without single cell organisms surrounding them. So all multicellular organisms will have an environment that includes these "bacteria", so they will adapt to this environment. It's easier to learn to live with something rather than learning to fight with them and then occupy their niche, because the killing and changing to occupy the niche requires much more changes in the organism.

Life on earth is balanced, such as each species occupies its important place in the circulation of substances. This happened not just by chance, but it's the only probable outcome from the process of evolution by natural selection. Having a world that's totally rid of any trace of primitive life forms is next to impossible. On earth we still have species of archaea that are similar to species present on earth billions of years ago. Read this article on Wikipedia about Nanoarchaeum equitans to get a grasp on the diversity of life.

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  • $\begingroup$ interesting thoghts, and thank you for the links. but this point argues, why such a world could not start existing in a natural way, why it could not appear "by itself", something like our world (as I understood). but what if you have, say, isolated (physically: with all the known forces) giant spehre, where only simple plants exist: would this world survive as is, would it be destroyed by itself, or would it evolution and some day there would appear other types of organisms, due to the spontaneous mutations of plants DNAs and natural selection? $\endgroup$ – static Oct 19 '13 at 14:04
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    $\begingroup$ "Life always goes uphill, from less developed to more adapted, better at surviving and reproducing.". That just simply isn't true. $\endgroup$ – Amory Oct 20 '13 at 7:06
  • $\begingroup$ Amory, these are not my words. It was specifically explained, as a solid rule, by Richard Dawkins. And to explain why this happens - any mutation that reduces the likelihood of that individual to reproduce, will not propagate through many generations. That's why all changes go towards better adaptation to the environment. $\endgroup$ – AlexanderMP Oct 23 '13 at 11:54
  • $\begingroup$ static, answering your question - I don't think they would. The food chain will have missing links, so it will not be a complete biosphere. The plants might not survive long enough to reproduce, let alone become subjects to evolution. $\endgroup$ – AlexanderMP Oct 23 '13 at 11:56
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One of the most important roles played by bacteria is the fixation of nitrogen. Biological nitrogen fixation is carried out primarily by bacteria. There are no known plants that express nitrogenase type enzyme. So plants cannot fix nitrogen.

Without bacteria in the ecology... the ecosystem is in trouble from nitrogen starvation.

That said, denitrification, the removal of fixed nitrogen and conversion back to nitrogen gas is also catalyzed by bacteria. So no bacteria, no lost of nitrogen by biological means.

Nitrogen can be fixed by lightning discharge. About 5%-8% of all nitrogen is fixed this way http://www.biology-pages.info/N/NitrogenCycle.html. However lightning does tend to cause forest fires... which results in the lost of nitrogen.

So where does that leave us? We will have to guess how much nitrogen is lost from forest fires (and burial by sedimentation) vs gain from lightning discharge.

That is is not at all easy to guess. In a world of high rainfall (lots of rain forest)... you can have plenty of lightning and no fires. But a world of swamps would have also plenty of organic matter (and nitrogen) being lost through sedimentation. A drier world would have more fires.

If anything... I would guess that such a world is unstable in the long term, certainly on a geological time scale. Eventually the environment will change to one where nitrogen is lost, and fixation by lightning is reduced... and the ecology will collapse from nitrogen starvation.

Could bacteria emerge in the world?

My guess is no. A second abiogenesis event is unlikely to occur when life is already present. The plant alive on your world would prevent any new form of life re-emerging. Plants are not quiet neighbors minding their own business. Plants secrete toxin to poison other plant. Limit growth. (Walnuts are a great example) Plant root do absorb simple sugars and amino acids. Example. http://link.springer.com/article/10.1007/BF02202082 So the raw material for new life will not accumulate.

That said... there is no reason why plant life cannot simplify itself to a single cell... such extremes have happened to parasitic barnacles of the Sacculina genus.. which look less like a crustacean (ie crab) than barnacles normally do and more like fungus. https://en.wikipedia.org/wiki/Sacculina So we could have single cell plants.. descended from higher plants.

What is uncertain is if plants could re-evolve an enzyme to fix nitrogen. Nitrogenase (nif) appears to have evolved once in methanogens and by lateral gene transfer was moved to many other bacteria. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3733012/#B7

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