This is not a question on whether or not floating, balloon-like organisms are biomechanically viable - I already know the answer to that, which is yes. This instead deals with the plausibility of such a thing evolving, a problem I've thought about for quite some times.
So, in my alien world, there are giant colonial invertebrates, similar to siphonophores, and they form huge balloon-like sacs, floating through the sky and filter-feeding aerial plankton. They float by means of hydrogen gas, which they produce by electrolysis (using biolectricity to split water molecules in the air).
However, I've recently discussed this concept with others, and it seems as though balloon organisms are quite a difficult thing to justify, mainly because of the energy expended in producing the hydrogen gas. A friend did some research, and according to his Google searches, it takes around 4 kilowatt hours to split one litre of water into hydrogen and oxygen gases - giving you 1,200 litres of hydrogen gas. That can lift roughly 4 pounds in an Earth-like atmosphere, giving you the capacity to lift roughly 1 pound of mass per kilowatt hour. 1 kilowatt hour, I'm told, costs 3,500 kilocalories to produce.
Now, before I continue, let me say that my planet does not have conditions identical to Earth. The gravity is about 0.85 times lower, while the atmosphere is denser - perhaps a quarter of the way or a little under between Earth and Venus. Does anyone know if it's feasible to alter the calculations above to apply to these conditions?
So, even in a place with an atmosphere about 20 times denser than Earth's and a gravity 0.85 times lower, I doubt that the conversion rate of 1 Kwh = 1 lb will alter drastically. Sticking with that initial equation, even if the balloon colony was so light that it weighed only as much as a human, it would still expend about 500,000 kilocalories lifting its mass, which, let's face it, is ridiculous - it's 1 million times the daily calories spent on a male human brain.
So, I guess the first thing I need to know is how much will the altered conditions change the 1 Kwh = 1 lb equation? If it does by a lot, which I doubt, then it's a start.
Then there's the question of how can I go on to reduce the tremendous cost in energy of floating with hydrogen? Their lifestyle isn't exactly the worst imaginable for reducing energy expenditure, in fact. Here are a few things which should help by cutting calory intake in other areas of life:
- Passive filter feeding lifestyle
- They are colonial and thus do not move
- Specialized zooids: only special castes of individuals produce hydrogen, the rest perform single tasks e.g. reproduction, digestion, defense
So, here is my main question, summarized:
In an atmosphere 20x denser than Earth's and with gravity 0.85x that of Earth, could organisms which have the traits above float by means of a huge bladder of hydrogen gas, producing the hydrogen via electrolysis? Are there other methods of biological hydrogen production, real or speculative, that are more energy-efficient?
Note: the gas must be hydrogen, and I would prefer if hydrogen production methods which require photosynthesis were not used in answers.