Lichens: Hybrid organisms
PDFLichens are considered to be “cousins” of fungi because, traditionally, they are made up of a combination of a fungus and either a green algae or a cyanobacterium or both. This association is called symbiosis, the fungus is a mycosymbiote, and the algae a photosymbiote.
Lichenized fungi belong mainly to the Ascomycetes (Ascomycota). Only a few lichens are formed from Basidiomycetes (Basidiomycota). As for algae, 90% of them are green algae, mainly Trebouxia (50 to 70% of lichens) and Trentepohlia. The rest is represented by cyanobacteria, of which the genus Nostoc is the most common. But this association is much more complex (see Lichens, surprising pioneering organisms).
1. How is a lichen built?
In gelatinous lichens such as Lathagrium, the fungus and cyanobacteria are homogeneously mixed : we speak of a homeomerous structure (Figure 1). In other lichens, the two partners are distributed differently to form a so-called heteromerous structure. In foliose lichens, for example, the structure shows layers (heteromerous stratified structure).
On a vertical section of the thallus, we recognize from top to bottom (Figure 2):
- an upper cortex formed only by fungi;
- an algal zone where fugi filaments and algae cells are intertwined;
- a medulla or medullary layer [1] where there are only fungi filaments;
- a lower cortex formed only by fungi from which escape filaments or rhizinae used to fix the thallus on the substrate.
2. Birth of a lichen
Two modes of reproduction can take place: vegetative and sexual reproduction.
At maturity, the spores are violently expelled from the ascus then germinate on the substrate forming mycelial filaments. For a lichen to recover, the mycelium thus formed must meet a single algae. A new thallus can therefore be rebuilt and give back an adult thallus. The symbiosis then gradually recovers.
Notes and references
Cover image. Xanthoparmelia conspersa. [Source: © J. Asta]
[1] In crustose lichens, the medulla consists of loosely packed hyphae that are inseparable from the substrate.