Survival On Mars

In James Blish's Welcome To Mars (London, 1978), how do Dolph and Nanette manage to continue breathing, drinking and eating on Mars long after their imported supplies have run out? The Martian lichen, which grows all over the surface in the area where they have landed and which they manage to store in sufficient quantity during the long winter, provides oxygen, water and food. In fact, it is beneficial and invigorating. Would this single source suffice? And how plausible is it?

Writing in 1965, Blish tried not to contradict then known facts although new data were due from a Mariner probe and are noted in the Afterword. I think that such complex organisms were unlikely on Mars even then. Having got us to accept the lichen, Blish moves on to animals and even intelligence.

He wrote in the Foreword that Mars was:

 "...apparently the only other planet in the Solar System on which a brave man might just barely manage to stay alive, if he never stopped trying." (p. 10)

- but he gives his characters a lot of help and I do not think that his claim could be made of Mars now.