Lichens are symbiotic partnerships between a fungus partner, the mycobiont, and an algal partner, the phycobiont. Lichens are widely distributed, they are found in harsh environments, such as the Polar Regions, as well in temperate regions where they can be observed growing on stone or trees.
Lichens are classified according to the fungal component, which determines the form of the lichen: most lichenized fungi are members of the ascomycota, and rarely, basidiomycota; the algal partner can be eukaryotic or, less frequently, a cyanobacterium. The photosynthetic sugars produced by the algal partner provide a carbon substrate for both partners. In turn, the fungus protects the algal partner by helping it to retain water, and provides it with nutrients acquired through adsorptive nutrition. Many lichens are highly sensitive to air pollution, particularly SO2, which makes them good biological indicators of pollution levels.
Thin lichen cross-section stained with methylene blue
Images of lichen beneath dissecting microscope