Lichens

A lichen is the symbiotic partnership between an alga and a fungus: the algal partner is called the phycobiont (or photobiont) and the fungal partner the mycobiont. The alga, being photosynthetic, supplies nutrition to the fungal partner, while the fungus in turn provides physical protection and anchors the whole thallus to its substrate using root-like rhizinae. Asexually, the lichen as a whole reproduces by fragmentation, or by specialised propagules called soredia and isidia; within that partnership, the phycobiont can additionally reproduce on its own by akinetes, hormogonia or aplanospores, while the mycobiont can undergo true sexual reproduction and produce ascocarps. Lichens are classified in three independent ways. By habitat: corticolous (on bark), lignicolous (on wood), saxicolous (on rocks), terricolous (on ground), marine (on siliceous sea rocks) and fresh-water (on siliceous rocks in fresh water). By thallus morphology: leprose (lacking a distinct fungal layer), crustose (a thin crust), foliose (flat and leaf-like) and fruticose (a branched, pendulous, shrub-like form). By the distribution of the algal cells within the thallus: homoiomerous, where the algal cells are scattered evenly throughout the thallus with no separate layering, versus heteromerous, where the thallus is organised into distinct layers, with the algal cells confined to a separate band sandwiched within the fungal tissue. If the fungal partner belongs to the Ascomycetes the lichen is called an ascolichen, and if it belongs to the Basidiomycetes it is called a basidiolichen. Economically and ecologically, lichens secrete organic acids such as oxalic acid that corrode rock surfaces, contributing to rock weathering and making lichens pioneer colonisers in a xerosere (a dry-habitat succession); usnic acid extracted from lichens shows antibiotic activity …