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Are there (unicellular) Earth lifeforms that most of their life fly high in the atmosphere without contact with surface? For instance, in clouds, etc? If so, at what max altitude have they been detected?

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    $\begingroup$ From @jakebeal's keyword "aeroplankton" you could go to scholar.google.ca/… $\endgroup$
    – Ben Bolker
    Sep 5 at 22:33
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    $\begingroup$ Are you only interested in unicellular examples, or do you mention that just as a suggestion? If you’re interested in multicellular examples too, then common swifts (and some related species) seem to qualify. They nest and roost at the surface during their brief breeding season, but barely if ever during the rest of the year: “Our data show that swifts are airborne for >99% of the time during their 10-month non-breeding period; some individuals never settled […]”— from doi.org/10.1016/j.cub.2016.09.014 $\endgroup$
    – PLL
    Sep 7 at 9:05
  • $\begingroup$ @PLL but what do they drink and eat? They need to land. $\endgroup$
    – Anixx
    Sep 7 at 9:23
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    $\begingroup$ @Anixx: Unless you’ve seen other research contradicting the paper I linked (which all sources I’ve found seem to agree with), it seems clear that they don’t need to land during the non-breeding months. They feed on insects in flight; I don’t know whether they get their moisture requirements from their food, or clouds, or dipping into ponds/rivers while in flight (some birds do that, but I’ve not heard of swifts doing so), or something else. EDIT: quick searching suggests they get moisture mostly from rain/clouds aloft, and sometimes from skimming surface water. $\endgroup$
    – PLL
    Sep 7 at 9:41
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    $\begingroup$ In response to answer the max altitude part of the question: space.com/microbes-fungi-space-station-identified.html although I couldn't say for certain that this is the maximum they have been detected. $\endgroup$
    – Frog
    Sep 8 at 1:46
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It sounds like you are talking about aeroplankton, a general term for a wide range of tiny life-forms borne on the wind.

While some of these are essentially passive while airborne (e.g., pollen, spores), others like arthropods and microbes are quite capable of being active in the air as well. In essence, anything that can be easily swept up by the wind can be swept up as high as the wind will take it, often all the way to the jet-stream, and kept aloft for a potentially long time before being deposited elsewhere in the world.

For many of these species, this is a key part of their life cycle and dispersal strategy, and as such they are well adapted to survival at altitude.

The density of available resources at altitude, however, is extremely low, and thus, to the best of my knowledge, there has not yet been any species discovered that primarily lives high in the air, as opposed to merely transiting at altitude before landing elsewhere.

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    $\begingroup$ Pollen, spores, and their like do not "fly" and being "swept up by the wind" isn't like "flying…" particularly not "freely…" How could "flying" - even "gliding" - not involve wings and how could "wings" not rule out anything "unicellular"? $\endgroup$ Sep 7 at 3:59
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    $\begingroup$ @RobbieGoodwin: The OP explicitly suggested “unicellular” in the question, so they clearly meant “fly” in a very general sense. $\endgroup$
    – PLL
    Sep 7 at 8:51
  • $\begingroup$ @PLL That's a big leap of faith and if you have that strong a belief in your powers of assumption, leap away! $\endgroup$ Sep 7 at 22:56
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Apparently yes. Caveat: the experimenters are are potentially biased because they believe in panspermia (the idea that life came to Earth from outer space), which is clearly fringe. However, I don't think that invalidates the detection of the three new species, only the interpretation.

(PhysOrg.com) -- Three new species of bacteria, which are not found on Earth and which are highly resistant to ultra-violet radiation, have been discovered in the upper stratosphere by Indian scientists ... In all, 12 bacterial and six fungal colonies were detected, nine of which, based on 16S RNA gene sequence, showed greater than 98% similarity with reported known species on earth. Three bacterial colonies, namely, PVAS-1, B3 W22 and B8 W22 were, however, totally new species. All the three newly identified species had significantly higher UV resistance compared to their nearest phylogenetic neighbours.

The samples were collected at 20km to 41km altitude.

The article doesn't say if the bacteria land. Most probably, we don't know.

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  • $\begingroup$ Is panspermia "clearly fringe"? I am not an astrobiologist, but my (admittedly limited) understanding is that it is a perfectly valid, if unproven, hypothesis that is taken seriously as a possibility in the field. I see 103 results for "panspermia" in the International Journal of Astrobiology, a perfectly respectable peer reviewed journal published by the Cambridge University Press. $\endgroup$
    – terdon
    Sep 8 at 11:22
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    $\begingroup$ @terdon as far as I know, it's fringe compared to abiogenesis. Some of the scientists involved (Chandra Wickramasinghe & Jayant Narlikar) also have a rather spotty reputation, being behind theories such as the quasi-steady state cosmology and the COVID-came-from-space hypothesis which are also fringe. $\endgroup$
    – Allure
    Sep 8 at 11:50

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