While I cannot say that such an organism is impossible, here are a couple obstacles that it would need to overcome:
Sparse nutrient availability—In the ocean, phytoplankton growth is primarily gated by the available nutrients in the water column (particularly phosphates, nitrates and iron compounds, in addition to oxygen iirc). Air has significantly less capacity to transport nutrients compared to how nutrients in the ocean can be both dissolved in water and present in particulate matter.
Sub-optimal temperatures—Even at the equator, the atmospheric temperature rapidly drops with altitude, with averages quickly falling below those favorable to most algae.
What biological mechanism would it use for efficiently staying permanently aloft?
...Aww hell. Am I starting to write like an LLM or do LLMs these days write like me?
Spanish moss is able to scavenge sufficient nutrients from the air/water without needing direct contact with soil, but it is also useful to note that the water it gets is able to dissolve more nutrients as it comes in contact with tree branches and airborne particles, which are more abundant closer to the ground. I predict it would struggle to do the same even one or two kilometers higher, even if it was warm enough up there (it’s not).
It is also notable that many species of algae and moss do use airborne spores to successfully spread themselves around the planet. Spores are typically dormant until certain conditions are met & I do not know of any that grow and actively metabolize while airborne[1].
If air, rain, and light were the sole factors at play I would expect to see more things like Spanish moss growing from the ledges of lofty skyscrapers.
Though a few niche plants have adapted to extremes in elevation and temperature they are the exception rather than the rule and are far less numerous than their more down-to-earth counterparts.
TL;DR: While it is technically possible for highly specialized plants to survive in some of these conditions, it is an unforgiving environment that is less favorable for photosynthetic life.
Rather than the proverbial low-hanging fruit left untouched, the high-floating fruit has been tasted and found rather cold and bitter for most tastes.
I am not an expert, but I have a general familiarity with algae and plant life cycles.
- ^
I would love to be wrong here, if they did exist I would still expect them to fly over the radar for a while before humans look closely enough to discover them
I would love to see it happen. It’d be nice to have more stuff in the air removing Co2 and absorbing sunlight.
I’m curious, what got you thinking of floating algae?
I would estimate the relative difficulty of
[colonizing Himalayan mountain slopes vs free-floating (pelagic?) life at a similar altitude] to that of
[adapting to the salinity of the Great Salt Lake, vs that of the Dead Sea]. The former can support brine shrimp and microorganisms, the latter only microorganisms. Equivalently, the slopes can support simple multicellular life on down, while in the atmosphere we’ve found bacteria and little else so far.
We know there are particular points at which it is ~impossible for life as we know it to survive e.g. inside the sun, but less extreme absolute lines seem to tempt evolution.
I wonder if sufficient intelligence could distill a formula for estimating likelihood of life adapting to arbitrary parameters in a particular time frame?
Like: “given certain resources and conditions, viable adaptations might form in x [millon/billion] years.”[1]
Then it would simply be a question of “will these conditions last long enough for the adaptation to happen with a high probability?”
But then again, would that require it to brute-force simulate ~all possible mutations for a certain number of steps? And at what point is the simulated life behaviorally indistinguishable from the physical?
Obviously I’m out of my depth and far from my expertise here but it sure is fun to speculate