Could biofilms float and survive in the sulfuric acid clouds of Venus?
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?