There's a nice blurb at Eurekalerts about microbes growing in pH of 1, which is acidic enough to corrode nails. It seems that there's a whole ecosytem, including algae and mycobacteria, living just inside porous sandstone and possibly eating mineral or thermal energy.
The Editor's summary in Nature is here.
(Image is from the Eurekalert )
The astrobiology angle is basically that these microbes' niche is an extremely narrow physical space. If the rock bed is perturbed, the microbes are literally toast, generating a lifeless "instant fossil." A robot explorer, coming to an extinct hotspring, could detect this fossil band just by scraping the rock surface.
I didn't quite manage to get this post to talk about bacteria and steel. While googling around for rust and bacteria, I came across this great description of rusticles forming on the wreck of the Titanic. These are iron stalactites, where the iron has been bio-extracted from the Titanic's rotting steel superstructure. Very odd. Ozymandias in steel.