Abstract
Black yeasts are polyextremotolerant fungi that contain high amounts of melanin in their cell wall and maintain a primarily yeast form. These fungi grow in xeric, nutrient deplete environments which implies that they require highly flexible metabolisms and the ability to form lichen-like mutualisms with nearby algae and bacteria. However, the exact ecological niche and interactions between these fungi and their surrounding community is not well understood. We have isolated two novel black yeast fungi of the genus Exophiala: E. viscosium and E. limosus, which are from dryland biological soil crusts. A combination of whole genome sequencing and various phenotyping experiments have been performed on these isolates to determine their fundamental niches within the biological soil crust consortium. Our results reveal that these Exophiala spp. are capable of utilizing a wide variety of carbon and nitrogen sources potentially from symbiotic microbes, they can withstand many abiotic stresses, and can potentially provide UV resistance to the crust community in the form of secreted melanin. Besides the identification of two novel species within the genus Exophiala, our study also provides new insight into the production and regulation of melanin in extremotolerant fungi.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Edit 2: Minor grammatical edits, and citation corrections. Edit 1: Lines 288-302 edited to: To determine this, we used the UVP HL-2000 HybriLinker UV crosslinker as our source of UV light, which has a UV wavelength of 254 nm. Lower wavelengths (100-280 nm) are of the UV-C range, they are considered ionizing radiation and are the most detrimental to living organisms, but are completely blocked by the ozone layer (Molina & Molina, 1986; Schreier et al., 2015). Therefore, using this wavelength we are able to push our organisms beyond the UV limits found in their natural habitat and test extreme amounts of UV exposure. Placing UV-exposed cells in complete dark after exposure is essential for preventing native cell repair mechanisms to act upon any potential mutations, allowing for only those cells that are capable of withstanding the UV exposure without the use of repair mechanisms to grow (Weber, 2005). Lines 420-42 edited to: Exact methods are as follows. 10 mL of culture was centrifuged at 3,000x g for 5 minutes, and the resulting supernatant was filter sterilized through a 2 μm filter to ensure all cells were removed. The supernatant was then transferred into a 50 mL centrifuge tube, and 40 mL of 6M HCl was added to the tube. Phylogenetic tree added (Figure 2), as well as methods for production of the tree. Lines 748-754 have additional sentence for clarification: They were chosen to be whole genome sequenced out of our 25 total isolates because of their ease of experimentation compared to other filamentous isolates, as well as their uniquely dark phenotype and melanin secretion capabilities which no other isolates exhibited.