Assessment of radiotropic behavior in isolates of Aspergillus niger, Aspergillus calidoustus, and Paecilomyces variotii.

Pigmented species of filamentous fungi isolated from high-radiation environments have been reported to exhibit radiotropism, characterized by their directed growth toward a source of ionizing radiation. However, inconsistencies in the experimental approaches and results have impeded our understanding of the key factors influencing radiotropism. In the present study, we assessed radiotropic behavior in four fungal isolates: Aspergillus niger, A. calidoustus JC-1043, Paecilomyces variotii SNL-1, and P. variotii IMV-00236. Of these fungi, only P. variotii IMV-00236 was previously reported to exhibit radiotropic behavior. Plates of each fungus were placed for 7 day at equivalent distances from a137Cs source, which emits a primary gamma photon at 662 keV. Differences in the rate and direction of mycelial growth over this period were assessed. Our results revealed no significant differences in the relative rate and direction of mycelial growth based on exposure to gamma radiation, indicating a lack of measurable radiotropism in these experiments. Additional studies varying parameters such as gamma emission rates and energies, as well as other types of ionizing radiation (e.g., alpha and beta particles, neutrons) are necessary to gain further insight into the factors critical to the expression of radiotropic behavior in filamentous fungi.