Carr, E. C. | bioRxiv Science

bioRxiv2025

Crusty-Pinky: A Novel Polyextremotolerant Fungus and its Methylobacterium Symbionts Could be an Essential Symbiosis for the Biological Soil Crust Consortium

JF2 08-2F Crusty is a novel melanized polyextremotolerant fungus isolated from a biological soil crust, which we believe harbors Methylobacterium spp. endosymbionts, called Pinky. Crusty is capable of utilizing many sources of carbon and nitrogen and is resistant to multiple metals and UV-C due to its melanized cell wall. We were unable to recover a Pinky-free culture of Crusty via usage of antibiotics. However, when exposed to antibiotics that kill or stop the growth of the Pinky, growth of Crusty is significantly stunted, implying that actively growing Pinky symbionts are needed for Crustys optimal growth. The Crusty-Pinky symbiosis also seems to be able to perform active metabolism in carbonless and nitrogenless medium, which we believe is due to Pinkys ability to perform aerobic anoxygenic photosynthesis. Finally, Pinky was identified as being capable of growth stimulation of the algae Chlorella sorokiniana, indicating that Pinky likely produces cytokinins or auxins which Methylobacterium are known for. Features of this symbiosis provide us insight into the ecological roles of these microbes within the biological soil crust.

microbiology↗

bioRxiv2021

Deciphering the potential niche of novel black yeast fungal isolates in a biological soil crust based on genomes, phenotyping, and melanin regulation

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 have been suggested to contain 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 yeasts from the genus Exophiala that were recovered from dryland biological soil crusts. Despite notable differences in colony and cellular morphology, both fungi appear to be members of the same species, which has been named Exophiala viscosa (i.e., E. viscosa JF 03-3 Goopy and E. viscosa JF 03-4F Slimy). A combination of whole genome sequencing, phenotypic experiments, and melanin regulation experiments have been performed on these isolates to fully characterize these fungi and help decipher their fundamental niches within the biological soil crust consortium. Our results reveal that E. viscosa is capable of utilizing a wide variety of carbon and nitrogen sources potentially derived from symbiotic microbes, can withstand many forms of abiotic stresses, and excrete melanin that can potentially provide UV resistance to the biological soil crust community. Besides the identification of a novel species within the genus Exophiala, our study also provides new insight into the regulation of melanin production in polyextremotolerant fungi.

microbiology↗