Fungal Diversity in Lichens: From Extremotolerance to Interactions with Algae

Abstract

Lichen symbioses develop long-living thallus structures even in the harshest environments on Earth. These structures are also habitats for many other microscopic organisms, including other fungi, which vary in their specificity and interaction with the whole symbiotic system. This contribution reviews the recent progress regarding the understanding of the lichen-inhabiting fungi that are achieved by multiphasic approaches (culturing, microscopy, and sequencing). The lichen mycobiome comprises a more or less specific pool of species that can develop symptoms on their hosts, a generalist environmental pool, and a pool of transient species. Typically, the fungal classes Dothideomycetes, Eurotiomycetes, Leotiomycetes, Sordariomycetes, and Tremellomycetes predominate the associated fungal communities. While symptomatic lichenicolous fungi belong to lichen-forming lineages, many of the other fungi that are found have close relatives that are known from different ecological niches, including both plant and animal pathogens, and rock colonizers. A significant fraction of yet unnamed melanized (‘black’) fungi belong to the classes Chaethothyriomycetes and Dothideomycetes. These lineages tolerate the stressful conditions and harsh environments that affect their hosts, and therefore are interpreted as extremotolerant fungi. Some of these taxa can also form lichen-like associations with the algae of the lichen system when they are enforced to symbiosis by co-culturing assays.

Interior of saxicolous lichens on different types of rocks as a habitat for microfungal communities in Upper Galilee, Israel

“…By contrast, typical soil-borne fungi, such as fast reproducing Penicillium and Aspergillus species, as well as very fast-growing zygomycetes and mycoparasitic Trichoderma and Clonostachys spp., were extremely rare components of the studied endolichenic communities. Penicillii and aspergilli were considered as epithalline, unspecific, and ubiquitous contaminants which could be isolated from lichen thalli after insufficient surface sterilization [9,43]. At the same time, representatives of Aspergillus, Penicillium, and Trichoderma were recorded from epiphytic and terricolous macrolichens characterized by leaf-like or fruticose thalli [44].…”

Phytohormone release by three isolated lichen mycobionts and the effects of indole-3-acetic acid on their compatible photobionts

“…Lichens are complex symbioses between a fungus, the "mycobiont", with one or more species of green algae and/or cyanobacteria, the "photobiont", also hosting a complex microbiota of bacteria and other microfungi (Honegger 1991;Grube and Berg 2009;Lutzoni and Miadlikowska 2009;Spribille et al 2016;Cernava et al 2017;Muggia and Grube 2018;Hawksworth and Grube 2020). The "signalling hypothesis" presented by Joneson and Lutzoni (2009) postulates that the transition to the symbiotic state involves chemical signals, which trigger metabolome re-arrangement prior to morphological changes.…”

What is in a lichen? A metagenomic approach to reconstruct the holo-genome of Umbilicaria pustulata

“…The bacterial microbiome in particular may contribute to auxin and vitamin production, nitrogen fixation, and stress protection (Erlacher, et al 2015;Sigurbjornsdottir, et al 2016). Lichenized fungi are well known for synthesizing diverse, bioactive natural products (reviewed in (Muggia and Grube 2018)), which has recently stimulated research into biosynthetic pathways and gene clusters of these fungi (Bertrand and Sorensen 2018;Wang, et al 2018;Calchera, et al 2019). The estimated 17,500-20,000 species of lichens (Kirk, et al 2008) are distributed across nearly all ecosystems (Ahmadjian 1993).…”