Relative Humidity Influences Aureobasidium pullulans Degradation of Polyester Polyurethane Foam

Microbial-induced corrosion costs billions of dollars, including replacing plastics degraded by fungi. Fungal growth is moisture dependent, but we need to better understand how equilibrium relative humidity (ERH) affects plastic degradation. The goal of this project was to measure how ERH impacts degradation of polyester polyurethane foam by Aureobasidium pullulans and identify potential genetic pathways. We incubated three environmental strains of A. pullulans on foam at 50%, 85%, and 100% ERH and evaluated degradation through foam weight loss, Scanning Electron Microscopy (SEM), external nutrient availability, RNA sequencing, and proxy Impranil clearing. Higher ERH after 1 week of incubation corresponded to greater weight loss in foam (p = 0.002), with percent weight loss ranging from 0.11% to 5.1%. SEM foam imaging shows signs of fungal growth and degradation at high ERH while nutrient data suggests that, beyond the foam, additional carbon is not required. We identified 10 cutinases among the three strains. In one strain, two cutinases were 2.5 to 100-fold upregulated at 85% and 100% ERH compared to 50% (p < 0.05) and the cutinase with the highest upregulation demonstrated clearing of Impranil. Our results demonstrate that increased relative humidity can increase fungal degradation of polyurethane foams as relevant for biodegradation prevention or promotion.