Exposure to elevated relative humidity in laboratory chambers alters fungal gene expression in dust from the International Space Station (ISS).

Microorganisms are present in all occupied indoor environments, including homes on Earth and within specialized systems like the International Space Station (ISS). Microbes when exposed to excess moisture, such as from an unexpected ventilation system failure, can undergo growth that is associated with material degradation and negative health effects. However, we do not yet understand how exposure of these microbes to excess moisture alters their function. A de novo metatranscriptomic study was performed using dust collected from the US air filtration system of the ISS and incubated in laboratory chambers on Earth at different equilibrium relative humidity (ERH) levels. Changes in fungal function (gene expression) were significantly associated with moisture (adonis2 p = 0.0001). Secondary metabolism and fungal growth genes were upregulated (FDR-adjusted p ≤ 0.001, log2FC ≥ 2) at elevated ERH compared to 50% ERH. Elevated moisture conditions showed upregulation of aflatoxin and fungal allergen genes such as Asp f 4 (log2FC = 26.4, upregulated at 85% ERH compared to 50%) and Alt a 7 (log2FC = 2.98, upregulated at 100% ERH compared to 50%). Our results demonstrate that understanding microbial functional changes in response to elevated moisture will help develop more robust microbial monitoring standards for spacecraft environments to protect astronaut health and spacecraft integrity in low-Earth orbit and beyond.