Sclerotia formed by citric acid producing strains of Aspergillus niger: Induction and morphological analysis

Some strains of Aspergillus niger have been previously reported to produce sclerotia under certain conditions. Sclerotia are aggregations of hyphae which can act either as survival or as sexual structures in species related to A. niger. In this study, we were able to induce the formation of sclerotia in the progenitor of the industrial citric acid producing strains of A. niger, ATCC 1015, and in pyrG mutants derived from it. Sclerotia can be stably formed by ATCC 1015 on malt extract agar medium supplemented with raisins, showing a spatial differentiation of the fungus dependent on the addition and on the position of the fruits into the medium. On other media, including malt extract agar, pyrG auxotrophs also form abundant sclerotia, while the complementation of this gene reverses this phenotype. Additionally, a macro- and microscopical analysis of the sclerotia is reported. Our results show that the sclerotia formed by A. niger are similar to those formed by other fungi, not only in their morphology but also in their ability to germinate and regenerate the organism.