LSU microbiologist Gary King, writing for Popular Mechanics:
If we want to grow life in the watery-subsurface of Mars, King says, the opening move is identifying the right spot to start. The scant amount of subsurface water recently discovered does not suddenly transform Mars into a fertile Eden. However, “there’s no reason to suspect that the entirety of the planet is effectively sterile—that Mars is so limiting, and so extreme, that it can’t support any microbial life anywhere,” King says. On Earth, King adds, no matter how extreme an environment (“from the dry Atacama desert to geothermal vents under the Atlantic,” he says) life almost always finds a way.
As for finding the most potentially habitable spot, “that’s a task which I think basically continues to come down to a question of water,” King says. In other words, wherever Mars’s subsurface water pools the most, that’s where we’ll want to start.
Granted, the Martian water supply seems to be scarce at best, but it’s possible that places like the recurring slope lineae might be our wettest, best bet. This is a swath of land identified in 2011 by the Mars Reconnaissance Orbiter that visibly darkens with the seasons, suggesting that subsurface water there may ebb and flow in much greater amounts than was found in the Gale crater (where the evidence of liquid water was found last month).
As the article says, using microbes to terraform Mars is a speculative idea. But it’s a worthy thought experiment. Assuming there are no ethical quandaries, the research involved with such an endeavor increases the breadth of our knowledge in geoengineering, and that may help us here on this planet as well as others.