Lichens not implausible.
Remember lichens are symbiotic communities of either algae and fungi or cyanobacteria and fungi, all microscopic organisms, and both algae and cyanobacteria are photosynthetic.
IF microbes do survive on Mars, it would be conceivable lichens do as well.
IS THERE LIFE ON MARS?
http://www.bradford.ac.uk/admin/pr/october/mars.htm
Bob Clark
