Say I have three unicellular organisms: a eukariote, a bacterium and an archaeon. If I cut off nutrition from them at the same time, how long will it take for them to die? What will their death look like, and do they actually need to die? If so, why?
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This is very dependent on the organism within each of the groups you mention. While for the most part, archea are the extremophiles and have the ability to withstand many extreme conditions, nutrient limitation survival greatly varies. I think you could easily find organisms in each group that could withstand nutrient limitation well.
A good example would be yeast, which is a single celled organism you may have actually worked with. If you take laboratory yeast, in their diploid state, and starve them, they will undergo meiosis, and in their spore form, live for a very very long time without nutrients, protected by their ascus (which is actual their dead mother cell).
Yeast can also completely desiccate and still live. When you pick up an active yeast packet for baking, those yeast have not only been starved, but have been completely sucked dry of their water. Yet when you add water back, they live (though a great number of them will die).
I think in general, most single celled organisms have a certain tolerance for low nutrient conditions. This does happen to them a lot. All it means is they slow their cell cycle down, stop dividing, and just undergo a caution period. There is all sorts of signaling in the cells that sense the nutrients, and slow growth based on their conditions.
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$\begingroup$ Thank you. But why will they (some of them?) eventually die? $\endgroup$– ymarCommented Aug 28, 2012 at 1:14
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$\begingroup$ Why will they die? That's kind of difficult to answer. There are some hypotheses that unicellular organisms to a certain extent have multicellular characteristics. And this includes the ability to altruistically die for the benefit of the rest of the colony or species. By dying they would free up the limited resources, as well as allow their own proteins and cellular components to be taken up. Others believe it is just random, and some cells live a bit longer making them able to take advantage of what I said above, making the death in no way planned or organized. $\endgroup$ Commented Aug 28, 2012 at 12:25
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$\begingroup$ I'm planning to ask about cell death later. But I would like to know what it is that actually kills those cells that don't commit suicide. Is it just random damage, or is there something else? And is there a criterion that allows to pronounce a cell dead as we can do with humans? $\endgroup$– ymarCommented Aug 28, 2012 at 16:01