Effect of Gamma Radiation on Native Endolithic Microorganisms from a Radioactive Waste Deposit Site

Abstract

A time-course experiment was conducted to evaluate the effects of gamma radiation on the indigenous microbiota present in rock obtained from Yucca Mountain, Nevada Test Site. Microcosms were constructed by placing pulverized Yucca Mountain rock in polystyrene cylinders. Continuous exposure (96 h) at a dose rate of 1.63 Gy/min was used to mimic the near-field environment surrounding waste canisters. The expected maximum surface dose rate from one unbreached canister designed to contain spent nuclear fuels is 0.06 Gy/min. Considering the current repository packing design, multiple canisters within one vault, the cumulative dose rate may well approach that used in this experiment. The microbial communities were characterized after receiving cumulative doses of 0, 0.098, 0. 58, 2.33, 4.67, 7.01 and 9.34 kGy. Radiation-resistant microorganisms in the pulverized rock became viable but nonculturable (VBNC) after a cumulative dose of 2.33 kGy. VBNC microorganisms lose the ability to grow on media on which they have routinely been cultured in response to the environmental stress imposed (i.e. radiation) but can be detected throughout the time course using direct fluorescence microscopy techniques. Two representative exopolysaccharide-producing isolates from Yucca Mountain were exposed to the same radiation regimen in sand microcosms. One isolate was much more radiation-resistant than the other, but both had greater resistance than the general microbial community based on culturable counts. However, when respiring cell counts (VBNC) were compared after irradiation, the results would indicate much more radiation resistance of the individual isolates and the microbial community in general. These results have significant implications for underground storage of nuclear waste as they indicate that indigenous microorganisms are capable of surviving gamma irradiation in a VBNC state.

Review of Microbial Responses to Abiotic Environmental Factors in the Context of the Proposed Yucca Mountain Repository

“…Because the natural population of microbes at Yucca Mountain is expected to be adapted to desiccation, some naturally occurring microorganisms at YM may also be radioresistant to a certain degree. The radioresistance of microbes in rock samples from the YM site was studied by Pitonzo et al (1999a). Radiation-resistant microorganisms in the rock samples became viable but not culturable (VBNC) after a cumulative dose of 2.33 kGy and their metabolic capability was reduced considerably.…”

“…For a YM repository this could imply that radiation to a certain cumulative dose (2.3 kGy as reported by Pitonzo et al 1999a) will likely curtail activity and metabolic capability of microbes (indigenous or otherwise) present, but that part of this activity can be restored once radiation fields have decayed away to lower values.…”

“…Microbes at Yucca Mountain could potentially be affected by ionizing radiation from the radioactive waste in the repository and, depending on waste package design, exposure could vary from negligible to 0.06 Gy/min. (Pitonzo et al 1999a). Any influence of non-ionizing (UV) radiation may occur only as long as the repository is open and illuminated by electric light, which could, for instance on wet surfaces, enhance the growth of phototrophic organisms and as such introduce organic material into tunnels and drifts of the repository.…”

“…studied almost 50 endolithic aerobic isolates from the Nevada test site and found that 89% were Gram-negative and 65% were pigmented, which may afford some protection against radiation damage (Section 3.5.2). Pitonzo et al (1999a) studied the radiation resistance of native endolithic microorganisms present in rock obtained from YM, using γ-radiation and pulverized but otherwise unamended rock samples. Cumulative radiation doses ranged from 0 to 9.34 kGy.…”

Influence of Various Factors on the Lactococcus Lactis Cells

“…The most effective among the various factors whose impact can lead to a change in the properties of bacteria or a whole cell population are gamma or UVirradiation [1]- [4]. It is known that sensitivity to UV is not the same for different bacterial species and strains.…”