Methane present on Mars could be created by methanogens like Methanosarcina barkeri

That's the gist of an article here on the subject. The methanogen featured in the article is kind of like an extremophile in that it can survive in harsh environments, but not at all the same in the sense that it doesn't need them to survive, and has a much larger range of environments in which it can thrive. It seems that Methanosarcina barkeri could possibly also survive on the surface of Mars.

Scientists will not be in a rush to transport life to the planet though until they have been able to make a thorough search in order to make sure once and for all that there is no life anywhere, because once life is brought over from Earth it could make subsequent discoveries suspect if they happen to be similar to the life brought over from Earth. The discovery of pre-existing life elsewhere in the Solar System is certainly more worthwhile in the beginning than transporting it from Earth.