A balloon is a remarkably energy-efficient way of moving. No wonder. A balloon floats in the air, as it were. The displaced volume of gas of air is heavier than the contents of the balloon plus the charge carried by the balloon. In theory, could there be animals or plants that can float like a balloon?
Flying takes a lot of energy, but not floating
On earth we only know animals that fly by flapping their wings, birds for example, or make a gliding flight, such as flying fish and some birds of prey. Large birds of prey such as condors and bearded vultures use thermals: rising air currents. Small animals often let themselves be carried away by the wind - the well-known autumn threads act like a kite and enable young spiders to travel great distances.
However, staying in the air by flying costs an animal a lot of energy. Much more than it costs an animal of comparable size to move across land. Then why not develop balloon sharks or floating pumpkins, which can trap the light for even the tallest plant? There are some explanations for this.
Few gases that provide sufficient lift
A lot of buoyancy is needed to keep a plant or animal of, for example, one kilo in the air. One cubic meter of air weighs about 1,300 grams. Suppose the animal were filled with hydrogen, it would in principle be possible. Unfortunately, hydrogen is quite explosive and flammable. A gas like ammonia (NH3) is then more obvious. That is about half as dense as air. However, only half of the buoyancy remains. Making ammonia from atmospheric nitrogen and hydrogen also requires a lot of energy.
Gastightness a problem
The skin of the balloon must be alive or the animal or plant cannot repair itself. This skin must be very light, gas-tight and capable of sustaining the life of the cells. A very difficult combination of properties. In theory it could be if a layer of fat is separated on the outside and inside. However, any leak is deadly.
Log and slow
The reason that air balloons and blimps are only used to transport heavy cargo in aviation is their unmanageability. If you want to get ahead, their sheer size becomes a major drawback. After all, the larger the cross-section in the direction of movement, the more air resistance when moving. So you don't have to be very afraid of a balloon shark, unless the animal strikes with its tentacles like a huge jellyfish.
No access to water and nutrients
It is not a problem for plants if they are unwieldy and slow. After all, they already are. A balloon plant would have a great advantage over a tree. The plant can harvest unlimited light, without the plant having to create a huge trunk that requires a lot of energy. The plant would thus have an enormous advantage over trees. However, there is another drawback for plants. There are hardly any nutrients in the air. All in all, there are two: carbon dioxide and nitrogen. The plant could also harvest water through clouds, condensation or at sea.
The plant would then have to store this water and here we come directly to a major disadvantage. The weight. The plant cannot take a very large part of its weight as water without becoming too heavy. Perhaps you could imagine a cycle where the plant floats and produces photosynthesis products (using the propellant as a source of hydrogen and nitrogen) and which, when the plant lands, converts it into new propellant and charges water and salts. However, the plant should not end up in stagnant air above a desert, because then the balloon plant will soon be over when it tries to land ...