Carbon and nitrogen availability affects biofilm growth and morphology of the extremotolerant fungus Knufia petricola

ABSTRACT Rock-inhabiting fungi thrive in subaerial oligotrophic environments, such as desert rocks, solar panels, and marble monuments, where organic carbon and nitrogen are scarce. We tested whether the rock-inhabiting fungus Knufia petricola showed a preference regarding nitrogen ( N O 3 - or N H 4 + ) and carbon (glucose or sucrose) sources, and whether it was sensitive toward carbon and nitrogen limitation. As this fungus produces the carbon-rich, nitrogen-free 1,8-dihydroxynaphthalene (DHN) melanin, we tested whether a melanin-deficient mutant would be less sensitive to carbon limitation. The carbon and nitrogen concentrations were the primary predictors of growth, with a broad optimum partially explained by an optimal fungal C:N ratio. Limiting carbon or nitrogen supply decreased biomass formation, C O 2 production, and biofilm thickness, but promoted substratum penetration through filamentous growth. The nitrogen content of the biomass was flexible within limits, increasing with increasing nitrogen supply or decreasing carbon supply. The carbon use efficiency was fairly constant, whereas melanization correlated with a higher nitrogen content of the biomass, despite melanin being nitrogen-free. In conclusion, in vitro , K. petricola switches to explorative growth under nutrient limitations, like fast-growing fungi, revealing universal fungal resource-acquisition patterns. IMPORTANCE Since their discovery in the 1980s, extremotolerant fungi have been shown to inhabit desert rocks and coastal salterns, deteriorate marble monuments, weather minerals, colonize solar panels, and positively interact with cyanobacteria, algae, and plant microbiomes. These fungi endure sudden shifts in temperature and water availability, as well as intense sunlight, while persisting on very low nutrient levels. Despite their ecological relevance, their biology remains underexplored. The black fungus Knufia petricola offers a rare opportunity to study these organisms in depth because it can be genetically engineered and represents the wider group of resilient surface colonizers. Here, we compared a wild-type strain with a melanin-deficient mutant and generated quantitative data suitable for mathematical modeling of growth. This work provides a foundation for predicting how K. petricola behaves on natural and human-made materials, informing future efforts to harness extremotolerant fungi in sustainable technologies, agriculture, and environmental resilience.