Erratum: Introduction to the lichen ultrastructure series (Algal Research, (S2211926421002782), (10.1016/j.algal.2021.102459))

A lichen is a slow-growing organism that forms a body – a thallus – via scripted symbiotic/mutualist relationships between fungi, algae, and bacteria – in some cases cyanobacteria. Some 20,000 different ascomycete fungal species are lichenogenic, and every fifth species of fungus on the planet, and every second species of ascomycete, resides within a lichen. An estimated 12% of the planet's terrestrial ecosystems are lichen-dominated, including arid deserts and vast regions of arctic tundra. Lichens have been found that are 9000 years old. Humans are generally unaware of their existence since they are not common sources of food or fuel. Each thallus is constructed using the same basic body plan. A mycelium of fungal hyphae occupies the central region of the thallus, called the medulla. The medullary hyphae send out branches that differentiate to form the outer layers, including a green-algal layer, that encase the medulla. The outer layers, which also carry bacterial biofilms, mediate the attachment of the thallus to its substratum, its interactions with the environment, and its reproductive strategies. The layered tissues in turn generate the large-scale architecture of the thallus: some lichens (foliose) produce flattened lobes, some (fruticose) cylindrical stems, some (squamulose) tubular structures called podetia. The unique shapes, sizes and colors of these tissues allow the skilled lichenologist to recognize thousands of lichen species, much as a skilled ornithologist recognizes thousands of bird species.