Environmental fungi from cool and warm neighborhoods in the heat island of Baltimore City show differences in thermal susceptibility and pigmentation

A major barrier for most fungal species to infect humans is their inability to grow at body temperature (37°C). Global warming and more frequent extreme heat events may impose selection pressures that allow fungal adaptation to higher temperatures. As fungi adapt to warmer environments, they may overcome the thermal barrier that limits infection of warm-blooded hosts, including humans. Cities are heat islands that are up to 8°C warmer than their suburban counterparts and may thus be an important reservoir of thermotolerant fungi that inhabit environments near humans. Here, we describe a novel and inexpensive technique to collect fungal samples from various sites in Baltimore, MD using commercially available taffy candy. Our results show that fungal isolates from warmer neighborhoods show greater thermotolerance and lighter pigmentation relative to isolates of the same species from cooler neighborhoods, suggesting local adaptation. Lighter pigmentation in fungal isolates from warmer areas is consistent with known mechanisms of pigment regulation that modulate fungal temperature. The opportunistic pathogen <i>Rhodotorula mucilaginosa</i> from warmer neighborhoods had a higher resistance to gradual exposure to extreme heat than those from cooler neighborhoods. Our results imply fungal adaptation to increased temperatures in warmer areas of cities. The acquisition of thermotolerance poses a potential risk for humans, as it is necessary for fungal survival within humans.