The influence of ionizing radiation on spore germination and emergent hyphal growth response reactions of microfungi

Abstract

The accident at the Chernobyl Atomic Energy Station resulted in radiation contamination of large tracts of land and particularly the reactor building itself. Sustained exposure of microfungi to radiation appears to have resulted in formerly unknown adaptive features, such as directed growth of fungi to sources of ionizing radiation. We evaluate here spore germination and subsequent emergent hyphal growth of microfungi in the presence of pure gamma or mixed beta and gamma radiation of fungi isolated from a range of long term background radiation levels. Conidiospore suspensions were exposed to collimated beams of radiation and percent spore germination and length of emergent hyphae were measured. All fungal species isolated from background radiation showed inhibition or no response in germination when irradiated. Isolates from sites with elevated radiation showed a stimulation in spore germination (69% mixed radiation and 46% for gamma irradiation). Most isolates from low background radiation sites showed a significant reduced or no response to exposure to either source of radiation, whereas the stimulatory effect of experimental exposure to radiation appeared to increase in magnitude as prior exposure to radiation increased. We propose that the enhanced spore germination and hyphal growth seen in the exposure trials is induced by prior long term exposure to radiation and these factors could be important in controlling the decomposition of radionuclide-bearing resources in the environment.

Ionizing radiation: how fungi cope, adapt, and exploit with the help of melanin

“…They concluded that inducible MHMR pathway could be a potential mechanism of adaptive evolution in eukaryotes. These observations might explain the radioadaptive response in fungi described by Zhdanova group (18–20), but are an unlikely explanation for the enhanced growth effects of irradiated melanized organisms, which responded within hours.…”

“…They also observed the same results for responses of fungi from contaminated areas to light (20). However, though the presence of adaptive properties in fungi exposed in the long term to elevated radiation levels are very likely, the limitations of the experimental work reported in (18–20) might interfere with the authors’ ability to observe the radiostimulation for fungi from the clean areas as well. For example, the activity of the radioactive sources used in the later studies (18–20) was approximately 1,000 lower than used in earlier work (17) which might have been insufficient to promote the hyphal growth.…”

“…However, though the presence of adaptive properties in fungi exposed in the long term to elevated radiation levels are very likely, the limitations of the experimental work reported in (18–20) might interfere with the authors’ ability to observe the radiostimulation for fungi from the clean areas as well. For example, the activity of the radioactive sources used in the later studies (18–20) was approximately 1,000 lower than used in earlier work (17) which might have been insufficient to promote the hyphal growth. This may have been the case especially for low energy 121 Sn; also, the beta particles from 137 Cs might have been absorbed by the material of the Petri dish with the fungi as the collimated beam was coming from beneath and thus have not contributed to the actual radiation doses which could have been overestimated.…”

“…In their later work, published in 2006–2007, the same group investigated the influence of external radiation from 121 Sn (low energy gamma-emitter) and 137 Cs (high energy mixed beta- and gamma-emitter) not only on hyphal growth of fungi from radioactively contaminated Chernobyl regions versus controls but also on their spore germination (18, 19). They observed that radiation promoted spore germination in species from contaminated regions, which they called “radiostimulation”.…”

Sunlight-Exposed Biofilm Microbial Communities Are Naturally Resistant to Chernobyl Ionizing-Radiation Levels

“…In addition, several micro-fungi from irradiated areas are not only radiotolerant but positively attracted by radionuclides (positive radiotropism), being able to grow upon “hot particles” and degrade them [6]. Furthermore, in some cases ionizing radiation has a positive stimulation on spore germination [7]. By contrast, positive radiotropism and stimulation of spore germination by ionizing radiation are not observed in fungi isolated from radioactively clean locations [5].…”