Black yeast

“Black yeasts”, sometimes also black fungi, dematiaceous fungi, microcolonial fungi or meristematic fungi is a diverse group of slow-growing microfungi which reproduce mostly asexually (fungi imperfecti). Only few genera reproduce by budding cells, while in others hyphal or meristematic (isodiametric) reproduction is preponderant. Black yeasts share some distinctive characteristics, in particular melanisation of their cell wall. Morphological plasticity, incrustation of the cell wall with melanins and presence of other protective substances like carotenoids and mycosporines represent passive physiological adaptations which enable black fungi to be highly resistant against environmental stresses. The term "polyextremotolerance" has been introduced to describe this phenotype, a good example of which is the species Aureobasidium pullulans. Presence of 1,8-dihydroxynaphtalene melanin in the cell wall confers to the microfungi their characteristic olivaceous to dark brown/black colour.

The consortium comprises two phylogenetically very different fungal groups. Many are found in the orders Capnodiales, Dothideales, and Pleosporales (class Dothideomycetes). These black fungi mostly have an extremotolerant life style. Many representatives of this group can colonize bare rocks e.g. in the Mediterranean basin or in hot and cold dry deserts and are therefore referred to as rock-inhabiting fungi, or occur in salterns. These black yeasts are believed to be the most resistant eukaryotic organisms known to-date. They were firstly described in the early 80s by three almost concomitant seminal research articles. Members of Chaetothyriales (class Eurotiomycetes) are found in hydrocarbon-rich environments or in nutrient poor, moist indoor environments, and may occur as opportunistic pathogens of vertebrate hosts, such as Exophiala (Wangiella) dermatitidis. Several species are associated with lichens as well as other phototrophs and sometimes with ants in specific ant-fungi associations.

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