In contrast to the majority of fungi, which live more or less comfortably inside of hosts or substrates, some lineages have specialised to thrive on exposed surfaces. With little competition on nutrient-deprived conditions, such a stressful lifestyle also requires adaptations to fluctuations of hydration, among other challenges. Fungi in these situations also need to shield their cell content from excessive radiation using various kinds of pigments in their cell walls. The symbiotic thalli of lichen-forming fungi are one example on how to cope with conditions of exposed surfaces. Lichenforming fungi develop diverse forms of characteristically compacted mycelial morphologies to filter light to sheltered photosynthetic algae. Another large group of fungi does not depend on symbiotic partners, but it is characterised by dark pigments, and unlike lichens, these fungi may tolerate much higher levels of air pollution. These
‘black’ fungi, as they are commonly called, are the focus of this chapter. We provide an overview of the current understanding of diversity and taxonomy of black fungi and review their phenotypic traits. As we previously recognised ecological and evolutionary links of black fungi with the lichen symbiosis, we also discuss first results from co-culture experiments of black fungi with algae. Finally, we also review first insights gained from -omics approaches. The phenotypes of black fungi have evolved in different lineages of ascomycetes. Under the extremes of abiotic conditions, the typical morphology is usually restricted to vegetative mycelia with insufficient diagnostic characters for species recognition. Hence it is practically impossible to identify the species directly in the environment or with a stereomicroscope. Diagnostic phenotypic characters are often expressed under controlled laboratory conditions, even though species commonly lack sexual structures. Analyses of DNA sequence data are the only way to prove the relationship of species with similar morphology and to place these fungi in a phylogenetic framework. Molecular approaches showed both that many lineages exist which have not been named so far while described taxa turned out to be polyphyletic (e.g. Ertz et al. 2014). DNA data are still of limited use to recognise species as little is known about the genetic variation within species, but they clearly showed that black fungi are phylogenetically more diverse than previously thought and that they primarily belong to two lineages of ascomycetes, the early diverging clades of Dothideomycetes and Eurotiomycetes in Ascomycota.
