Here is a really cool thread on soap scum in hard water and about the possible origins of life in a hot springs.
Imma gonna riff on that and add in a little extra here.
(Thread) https://twitter.com/JeffHavig/status/1356103393794785283

So at molecular level, those metal cations (looking at you calcium) are grabbing onto the negative carboxylate anion end of the fatty acid chain. Screws up soap micelle structure. Makes scum. Bad.
But aren't cells micelles? Will calcium ions screw them up too?
Yeah, it does!

Imagine the surface of a cell. It's a got a semi-permeable "wall" (membrane, not super structural, think if it like Gore-Tex liner - you Pacific NW people know what I'm talking about.) of two layers of molecules with long greasy (fatty) section inside, polar part (acid) outside.

In a cell, these things aren't fatty acids like in soaps, but are called phospholipids. They have fattyacid-glycerol-phosphate. Phosphate is more polar than carboxylic acid.*

(Shout out to all my hardcore lab synthetic organic chemistry tweeps. Got a bad emulsion during an acid work up? Try adding some phosphoric acid. Clears up. Dishbanger magic.)

But that phosphate end (and any other stuff on outer wall of cell membrane) can get cross linked with calcium ions or other cations if the outer ion concentration is too high. That's bad.

You want your cell membrane moving, shaking, stirring things up. Letting some molecules go past. Stopping others. Keeping proteins, receptors all in right shape and place. Cation-locked up membranes are bad!

So if you are a microbe, but you live in a salty place, how do you deal with that?
One way, is to modify the fatty part, make everything on the "back side" less likely to pack up neat and tidy during calcium complexation.

You can throw in a cis double bond to put a severe kink in that long-chain fatty part. Now that fatty part is going to be sweeping around and making a chaotic mess. Prevents cross-linking, or at least keeps it a little more chaotic, more liquid-y at the molecular level.

Other tricks include branching near the end, or putting cyclopropyl ((triangles!) groups on the end of the fatty chain. Those also help keep it fluid. Can also use shorter fatty acid chains (makes a few less hydrophobic contacts with neighboring fatty chains).

All these keep that membrane fluid. The neat part is that you can add a double bond to a pre-existing fatty acid. The cell first have to build all those molecules up again from scratch. Modify what you got with a desaturase (des) enzyme.

Extremopihiles that live in super salty places use these tricks to deal with that environment. Prevent membrane lock-up. These same membrane tricks can also help with cold (low T things want to lock up) and pressure (things solidify under pressure).

So microbes developing strategies for salt can also give solutions for pressure and temperature. This microbes that live in cold places like briny microhabitats between ice grains, also have to deal with any salt that got squeezed out of ice into their little pocket home.

Cold places on Earth (Deep inside ice sheets and glaciers) and maybe deep Mars, and the warm deep ice and oceans like Europa and Enceladus.
(Those oceans are very cold, btw. I hadn't appreciated it before. Like the deep Hadal zones here on Earth.)

So yes, watching detergent cloud up in hard water tells you about life in rest of solar system!
(End thread)