Aspergillus niger mutants affected in conidial pigmentation do not have an increased susceptibility to water stress during growth at low water activity.

Aspergillus niger forms conidia that contain melanin in their cell wall. This black pigment has been shown to protect fungi against UV radiation, and experimental evidence has indicated that it also protects against drought and high salt concentrations. In this study, growth of A. niger was evaluated at low water activity (a(w)) and after changes in relative humidity (RH). In addition, deletion strains of A. niger affected in the melanin synthesis pathway were compared. Germination of conidia of the wild-type and deletion strains was observed at 081 a(w) and germ tubes continued growth at a(w) >= 083. Conidia and microcolonies of the different strains were incubated for 1week at lowered RH (33-84%). Conidia of all strains germinated and formed colonies after exposure to RH >= 33% when transferred back to malt extract medium at a(w) 098. Conidia germinated and showed limited growth at 84% RH. Microcolonies of all strains did not survive an incubation of 1week at RH <= 75%, but continued growth after exposure to 84% RH. Together, this is the first genetic evidence that melanin does not play a role during germination and radial extension of fungi at low water conditions. Significance and Impact of the StudyAspergillus niger, a cosmopolitan fungus with melanized conidia, is used here as a model system for fungal growth at low water activity (a(w)) and humidity dynamics. From this study it becomes clear that melanin, contrary to what has been suggested before, is not a key factor in survival and growth during situations that mimic indoor conditions. Indoor fungal growth can lead to cosmetic damage to building materials and health problems. This knowledge makes clear that novel ways to limit indoor fungal growth have to be based on interference with other cellular traits of fungi.