There aren't a lot of farms on Mars. And there won't be in the near future, because Mars is a dry, airless, inhospitable world.
So how will the first human settlers feed themselves?
With the help of microbes, says Charles Cockell. "We use microbes on Earth in all sorts of ways - to make food, to treat waste, to purify water, to make drugs. So when we go into space we will take microbes with us, and they will do all these things and more. They will also create oxygen for the life-support systems."
But space-travelling microbes will have to be hardy, he says. This is why scientists are studying microbes in extreme conditions - such as the outside of the Space Station, or deep down Boulby Salt Mine.
"Suppose you have a settlement on Mars and it suddenly de-pressurises," Charles says. "The humans could survive in their spacesuits for a time. But if you're using ordinary microbes for the life-support they will die. Then the oxygen will run out and the settlement will be lost.”
That won't happen if the first people on Mars take hardy microbes with them - like those that Charles is studying.