Symbiosis

The symbiotic relationship between the host lichen which retains water for the photosynthetic algae is visible on this damp, north surfacing bridge. There are at least five separate colours visible including the algae trentepohlia, with contains carotenoid to make it orange, and the lichen Lepraria which is crusty white. The correct site, with good air quality and very little disturbance means these plants have an excellent chance of seeing it through the next hundreds years at least.