They're not sterile; their brilliant colors are produced by halophile archaea and in some cases bacteria and algae that flourish in environments that would kill anything else.
If a piece of industrial environment is so fucked that only extremophile bacteria can live there than it is sterile for all societal purposes.
I don't know what you mean by "sterile for all societal purposes". A salt pond is not a good place to carry out a surgery, even though the archaea (more common than extremophile bacteria, and also more extremophilic) aren't pathogenic. It isn't poisonous, just dry and salty.