atmosphere plant sustaining a breathable atmosphere but does find the next best thing that helps to keep him alive. The Martian surface does have organisms that had not been detectable from Earth. The tumble-lichen has to crack both water vapor and oxygen out of the limonite sands in which it grows, then has to maintain both inside itself. Thus, when Dolph squeezes water from the plants back in his cabin, he also gets some oxygen - and can crack more from the water by electrolysis. Further, the resin from the lichen is edible and slows the metabolism.
A far cry from the ERBian Barsoom, to which Dolph refers for contrast. But each fictional Mars must be considered in the light of all the others. From SM Stirling's ...Crimson Kings, the readers mind may move either to other works by the same author or to other works set on Mars. Versions of the fourth planet recede away from us sideways in fictional time. Even CS Lewis, the theological sf writer, has a Mars with a humanly breathable atmosphere in a novel and a more realistic Mars in a short story.
Poul Anderson is not noted as a writer about Mars but does present several races of Martians and it makes sense to view his inhabitants of the red planet alongside everyone else's.