[tarice]

A bit of cyanobacteria[1] could theoretically thrive on Mars. All it takes is CO2, sunlight, and water.

[1]http://en.wikipedia.org/wiki/Cyanobacteria

[ars]

They also need nitrogen and trace minerals.

The minerals they could get from soil, but without nitrogen they can't make proteins. The atmosphere of mars is 2.7% nitrogen, but I'm not convinced that that is enough for nitrogen fixation to work.