Ok .. so looks like they've tested a few earth-based extremophiles in trying to understand how they might fare under martian conditions.
This article says they tested one called
'Psychrobacter cryohalolentis', which thrives under extremely dry conditions and at temperatures as low as minus 10 degrees C. However, it didn't fare too well under the UV radiation to which it would be exposed on Mars. It normally lives in salt brine mixtures .. like what they've recently hypothesised might exist on Mars where trace liquid water might also survive for brief spells. (We had a thread about this just recently
here).
It seems the conclusion to this one, is that if such an extremophile did exist on Mars, it might only be possible at depths greater than the top 10 to 20 centimetres.
Then there's the good old
'Bacillus subtilis' which survived on NASA's Long Duration Exposure Facility spacecraft for six years. It lived exposed to UV radiation of up to 3 times that of its original ancestors (it got its resistance courtesy of a DNA mutation). They sent some dormant spores of this up to the International Space station for 18 months, exposed it to hard UV and cosmic ray radiation. They came back in March of this year (not sure of the results yet, though).
So, on one hand they're saying that neither of these would be likely to survive on Mars' surface, through native hardiness or DNA mutation mechanisms. And it also looks like deliberate transplantation of our own hardiest extremophiles, is also not viable for colonisation purposes, (should anyone be thinking along those lines).
The conclusion here would be that they are eliminating panspermia mechanisms (from accidental probe contamination) which would be necessary, in the case where Curiosity
does find 'life indicators' on Mars.
Which means they definitely
are looking for Martians which emit earth-based-life-form biosignatures, even though they are expecting the life forms to be different from any earth-originating extremophiles. The model is very much centred on detecting organic based, purely martian DNA.
Very intriguing .. to say the least !
Cheers