Could supercritical co2 act as a biosolvent?

Question

Let me break it down:

Having failed to terraform venus, I am now trying to the process of pantropy to inhabit this world, (pantropy = to adapt humans/Terran life to other planetary environs through genetic engineering. I don’t know whether pantropy can be considered a verb, but hey-ho).

In an absence of surface water I have decided to create life forms that use supercritical carbon dioxide as a solvent. However, beyond this, I am uncertain as to the specifics of this biochemistry. How would using supercritical co2 instead of water as a solvent work?

Answer 1

Probably wouldn't work

The problem is that water is an unusually good solvent, dissolving a wide range of substances. It also allows acid base chemistry, has easy to synthesize biological substances that are hydrophobic and hydrophilic, producing one of the mechanisms of protein folding, and life has evolved using it for a few billion years.

Pretty much any biological substitution will be easier than this. We have extremophile organisms that live in boiling acid, temperatures well below zero, and pressures that would crush a human many times over.

By the time you're at the tech level needed to achieve this, it'll probably be much easier to terraform the whole planet

Edit to add in some chemistry ideas:

Venus is generally pretty close to hell, with surface temperatures that would melt lead, crazy pressure etc. There's no real point in trying to engineer life for the surface. Instead, I'd aim for the "habitable zone" - this is 48-70km off the ground, with earth like pressure, and only clouds of boiling sulphuric acid to deal with. Organisms on earth already deal with these kind of conditions, so bioengineering would be much simpler - it's not like a good environment, but at least within biological tolerances

Answer 2

Basically there are two main problems with using CO2 as a solvent for modified Earth-biology.

The first is that CO2 isn't a polar solvent and won't form hydrogen bonds in the same way that water does. This means, I'm fairly certain, that proteins immersed in supercritical CO2 will not fold, or if they do fold they'll fold in completely different ways to the way they fold in water. This means, most likely, that you'd have to find a whole other class of molecules to perform all the roles that proteins play in the cell, which include catalysing all of the cell's reactions as well as structural roles. Swapping out proteins for something else would require changing virtually every molecule in the cell.

The different properties of CO2 will also cause big issues with the cell's membrane. The membrane is made of lipids, which have a hydrophobic and a hydrophilic end. This is possible because of the polar nature of water, so it will be difficult if not impossible to form a similar kind of membrane in supercritical CO2.

The other issue you face is that water doesn't just play the role of a solvent in biochemistry, it's also a very important reagent. Many of the most important biochemical reactions either produce water as a side-product or use it up. So if you were using CO2 as a solvent but still using chemistry similar to Earth-life, you would still need a source of water. This means there would need to be a lot of water around anyway, presumably dissolved in the supercritical CO2.

Although nobody knows for sure, I like to think that life is possible in solvents other than water. But in order to exist it would surely need to be very, very different from the life we know. In your case we would not be talking about a simple case of changing a few genes, but about completely re-designing the molecular architecture of the cell from the bottom up. This might not be impossible, but it's a long way beyond what we're currently capable of.