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The atmosphere of Venus is composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide.
The main cloud deck is located in the 48-70 km altitude range and is composed mainly (75-96%) of sulfuric acid droplets, with a temperature of 27$⁰$C and a pressure of 0.53 atm at 55 km.
But to put the acidity into perspective, the mass loading of the droplets is 10 mg/m$³$, while the density of CO$_2$ at that altitude is about 950 g/m$³$.

Despite these harsh conditions for life, could biofilms float and survive there when certain bacteria species producing hydrogen could live within them ?

Nostoc commune for instance,is a colonial species of cyanobacterium which forms a gelatinous mass for protection and can withstand acidic exposure and other extreme conditions like desiccation and high and low temperatures.
In addition the cells contain pigments that absorb ultraviolet radiation, which enables it to survive high levels of it, and in some cells nitrogen-fixing occurs.

There are also bacteria, like Enterobacter aerogenis, which produce hydrogen, and when it could live inside the gelatinous mass it could provide the necessary buoyancy to float within the clouds of Venus.

Assuming the biofilms could be provided with the necessary trace elements, could they be airtight to some degree, to even float in the air with gas vesicles for buoyancy ?

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I have pondered life on Venus. I think the biggest problem would be lack of free water. The atmosphere is thick and under high pressure, and polar CO2 and H2SO4 like to bind with water. There is no water there. Mass action would pull water out of any concentration of water (like a Nostoc blob) and into the waterless density of the adjacent atmosphere. It would be like putting a Nostoc in a tub of salt grains.

Dessication would be the biggest problem. The other problem related to lack of water is its requirement for photosynthesis. If you had a photosynthetic organism there would be CO2 in abundance, but green algae type photosynthesis also requires water as a raw material: CO2+H2O+ light -> O2 + CHO (carbohydrate). If you have only the water you brought with to Venus, with no prospects of replenishing it, you will run out quick.

You would need an organism that is not only durable, but had the metabolic chops to either pry H2O off of H2SO4 or use sulfur as its reducing agent instead of water. Purple sulfur bacteria can do sulfur chemistry like this but they like starting with reduced sulfur, not H2SO4. Purple sulfur bacteria need water as a solvent inside the cells just like we do. But maybe something that can handle H2SO4 can make water out of it also?

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  • $\begingroup$ Although not much, there's some water in the atmosphere of Venus, 20 ppm, and the acidic droplets contain up to 25% water. Indeed, dessication would still be a big problem, but the main question for me is if a biofilm could really float in the atmosphere with hydrogen producing bacteria within it ? $\endgroup$ – Conelisinspace Dec 20 '19 at 10:33
  • $\begingroup$ @Conelisinspace - such a feat would be easier on Venus than Earth. CO2 is a big molecule and atmospheric pressure aside, CO2 is denser than our N2+O2 atmosphere. If a critter could produce an H2 bubble inside its shell (made of elemental polymeric sulfur?) it might be buoyant enough to float! We are converging on WorldBuilding Stack here which suits me fine. $\endgroup$ – Willk Dec 23 '19 at 0:37
  • $\begingroup$ Where, I might add, your speculations would get a lot more enthusiasm than is apparently the case on Biology stack. $\endgroup$ – Willk Dec 23 '19 at 0:47
  • $\begingroup$ Lucky me that you appear enthusiastic enough ! It could be that my speculations could already have been tried out in the laboratory, i like to know if it's really possible. So there are bacteria that produce hydrogen from glucose, so there have to be also cyanobacteria to nourish them. Now i think about aerogels ( en.wikipedia.org/wiki/Aerogel ) to hold the different bacteria species and the necessary trace elements and the produced hydrogen , be made airtight outside with the extracellular, acid, sulphated polysaccharides. $\endgroup$ – Conelisinspace Dec 23 '19 at 10:00

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