I am writing a book featured above the ground of Venus in the atmosphere. In my book I imagine that human colonists would have genetically engineered species to survive floating in the atmosphere. One idea I had was to have a species with a natural sack that can fill with a type of gas that it produces which would enable it to float above the ground. I am wondering if this would be possible and if so, what basic requirements would it need in order to survive? I am planning on it eating floating bacteria in the atmosphere. The idea is that bacteria gets trapped in it's sack and then it filters it out and eats it sortof similar to some marine species that live in the deep ocean. I would also like for them to be bio-luminescent so if their are any special requirements for that please work it into your answer.
Since an atmosphere is a fluid, what you are asking is can we have animals that float in a fluid. The answer is clearly yes as seen by fish. For Venus there is no need to posit helium or hydrogen as the lifting gas as normal (for earth) atmospheric gasses are lifting gasses on Venus; with 1 earth atmosphere and similar to earth temperatures being found 50 km up. As there are sulfur bacteria on earth then the idea is somewhat plausible.
Note that as your question is regarding Venus the basic physics is the same as floating mammals or Whales the requirements can in someways be much more "normal" to a water-dwelling whale on earth due to the atmospheric density of Venus.
Something to deal with is how your creature deals with or avoids sulfuric acid clouds.
Of Venus, Space.com says "The most Earth-like atmosphere in the solar system occurs 30 to 40 miles (50 to 60 kilometers) above the surface of Venus. Both oxygen and hydrogen rise above the heavier gas layer covering the ground, and the pressures are similar to our planet.
- Carbon dioxide: 96 percent
- Nitrogen: 3.5 percent
- Carbon monoxide, argon, sulfur dioxide, and water vapor: less than 1 percent."
It seems to me that a plant form that "inhales" carbon dioxide and "exhales" oxygen might be genetically modified to capture that oxygen in a large structure. On Venus oxygen acts as a lifting gas, allowing your organism to spend its life in midair, only falling to the ground when it dies. That takes care of "off the ground."
According to the Michigan Forests Forever Teacher's Guide (http://mff.dsisd.net/Environment/TreePhys.htm), "The major chemical elements used by plants are: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, and magnesium." There are also several beneficial fungi and bacteria that help a plant convert these elements into useful forms for the plants. Perhaps the airborne bacteria consumed can be rich sources of these elements in a bio-available form?
Most problematic will be a ready source of water. If the scientists doing the engineering are humans, that means they are probably playing in the "life as we know it" playground, which means water (at least water vapor) will be non-negotiable. Where does that water come from on Venus? Perhaps the same level of terraforming that allows plentiful floating bacteria in the troposphere will have inserted enough water to the system to solve that requirement?
Scientific American (https://www.scientificamerican.com/article/how-and-why-do-fireflies/) says that fireflies luminesce "[w]hen oxygen combines with calcium, adenosine triphosphate (ATP) and the chemical luciferin in the presence of luciferase, a bioluminescent enzyme," thereby producting light. You might reasonably adapt the process to your floaters, perhaps to help them locate other members of their species, considering they would never, in life, have a home base.