Figure 5.

Zone model cartoon illustrating mineral weathering occurring in zones that are impacted by microbes to different degrees and in different ways. Zone 4 includes unweathered rock and rock incipiently weathered by inorganic reactions. Zone 3 is where reactions are accelerated by dissolved organic molecules (predominantly acids) but cells are in direct contact with reacting mineral surfaces. Zone 2 is the area of direct contact between microbes, organic products, including polymers, and mineral surfaces. Zone 1 is where photosynthetic members of the symbiosis generate fixed carbon and where crystalline lichen acids precipitate.