|MadSci Network: Zoology|
I guess I would argue that all organisms have some sort of activity/inactivity cycle, but it wouldn't necessarily be comparable to what you're familiar with in humans or raccoons or even plants (which have a light/dark cycle because they conduct photosynthesis, obtaining energy from the sun during the day and transforming it into sugars at night). In microorganisms, there could be activity and inactivity based on photosynthesis (just as in plants), the cell cycle, or spore production. The cell cycle is the process all cells go through, and includes a period of growth, when the cell is basically enlarging itself and replicating its DNA, and a period of mitosis or cell division, during which the cell splits into two "daughter" cells. So microorganisms could appear to have a cycle of activity (mitosis) and relative inactivity. The other aspect of microorganisms that you could consider an active-inactive cycle is when they produce spores. A lot of fungi and bacteria reproduce by spores, which are sort of like the seeds produced by flowering plants, in the sense that they can remain dormant for a while, and a new organism grows from them. But spores can remain inert for up to hundreds of years while environmental conditions are tough. Then, after a nice rain shower, or some other required environmental change occurs, the spore becomes active and grows into a new organism. So this could look like an activity-inactivity cycle, with the spore being the inactive phase and the adult organism the active phase. This is similar to your and my activity cycle in that it is maintained by environmental factors, but these are factors much less predictable than whether the sun will come up tomorrow. To answer the other part of your question about the life span of microorganisms, I can either play biologist or philosopher. Some people argue that microorganism are immortal, because they simply make exact copies of themselves over and over. Barring mutations (mistakes in the DNA replication), a bacterium that you put in a test tube today could copy itself endlesslesly and produce identical "offspring" for centuries, thus appearing to be immortal. I would argue, however, that the bacterium's life span lasts from one mitosis to another; in other words, from the time its "parent" cell divided to produce it, until the time it divides to produce its own daughter cells. That life span could be just a few minutes.
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