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 (aw ) 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 0.81 aw and germ tubes continued growth at aw ≥ 0.83. Conidia and micro-colonies of the different strains were incubated for 1 week 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 aw 0.98. Conidia germinated and showed limited growth at 84% RH. Micro-colonies of all strains did not survive an incubation of 1-week 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. This article is protected by copyright. All rights reserved.