What is a Lichen?

Overview

• Lichens are not single organisms but symbiotic associations between a fungus and algae and/or cyanobacteria.
• Lichen fungi require carbon, provided by photosynthetic algae or cyanobacteria.
• The symbiosis is generally mutualistic, benefiting both partners.

Fungi in Lichens

• Many fungi form lichens; notably, 98% are ascomycetes (cup-fungi).
• Lichenisation is a common nutritional mode among unrelated fungi.

Lichen Photobionts

• Photobionts are green algae or cyanobacteria providing sugars to fungi.
• 90% of lichens partner with green-algal photobionts.
• Fungi are selective and may associate flexibly based on environmental factors.

Lichen Symbiosis

• Lichens consist of fungi and photobionts: algae/cyanobacteria.
• Symbiosis may be mutualistic or parasitic, with evidence supporting both views.
• Fungi benefit from sugars; photobionts get stable environment and nutrients.
• Fungi may also offer protection from UV radiation, desiccation, and herbivores.
• Recent findings suggest additional complexity, including yeasts and non-photosynthetic bacteria.

Evolution of Lichen Fungi

• Initially not recognized as fungi, lichens are now understood to share a evolutionary history with non-lichen fungi.
• Lichen fungi evolved in various lineages, mainly from ascomycetes (cup fungi).
• Lecanoromycetes, a major class, evolved in the Carboniferous period.
• Evolution shows repeated patterns, making identification tricky.

Photobiont Details

• Fungi are heterotrophic, needing carbon from photosynthetic cells within.
• Green algae or cyanobacteria serve as photobionts.
• Lichens with both algae types gain additional benefits, like nitrogen fixation.
• A small number of species make up most photobionts, mainly from Trebouxia and Nostoc.
• Photobionts often change morphology within lichens, complicating identification.
• Lichen fungi can flexibly associate with different photobionts as needed.
• Some lichens contain multiple photobiont strains, possibly regulated by fungi for maximum benefit.