Some lichen symbionts can survive independently on bare rocks in the absence of any external source of organic matter due to the ability of the phototrophic partner to fix CO2 from the air, and .

Some lichen symbionts can survive independently on bare rocks in the absence of any external source of organic matter due to the ability of the phototrophic partner to fix CO2 from the air, and nitrogen fixation by the cyanobacterial partner.

Explanation:

Lichen are organisms formed from the symbiotic association of a fungus with an organism capable of photosynthesis, be it an algae, a cyanobacterium, or both.

The fungus depends entirely on the photosynthetic organism in its metabolism of carbohydrates, receiving sugars or saccharine alcohols from it. For its part, the algae depends, for its metabolism, on the water and the mineral substances that the fungus provides it, at the same time that it is also protected by it from too high light intensities.