What is the optimum level of ionizing radiation exposure for Life?

By Jay T. Cullen

High energy cosmic rays from deep space lead to a cascade of energetic particles and ionizing radiation in our atmosphere that contribute to the dose experienced by living organisms on Earth. (Swordy, UChicago/NASA)



An interesting open access, peer-reviewed study was published earlier this year in Frontiers in Microbiology that examined how lower than background doses of ionizing radiation affected the growth of bacteria.  This post is part of an ongoing series dedicated to communicating scientifically derived information related to the impacts of ionizing radiation in the environment largely in response to the Fukushima Daiichi nuclear power plant meltdowns in 2011.  Life emerged on our planet billions of years ago when levels of environmental radioactivity were about 5-fold higher than they are today. On average living organisms experience a background ionizing radiation dose of ~1-2 milliSievert (mSv) although there is significant geographical variation across the globe given local geology (radioisotope content of rocks and minerals) and altitude (exposure to cosmic radiation).  Deviations from background occur due to proximity to medical exposure or nuclear energy or weapon related events that only act to increase the dose livings things must tolerate.  Castillo and Smith (2017) conducted experiments to understand how bacteria responded when they were grown in lower than background ionizing radiation dose conditions.  How did they do this and what did they find?

Experimental Conditions

How exactly do you get lower than background ionizing radiation dose conditions for an experiment?  Castillo and Smith were given access to the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM which you may be aware of given an accidental release of artificial radionuclides that occurred there in 2014. The low background radiation experiment (LBRE) used the geological conditions at WIPP, radiation shielding and radiation sources to test how lower than background ionizing radiation doses affected the growth and gene expression of radiation tolerant bacteria Shewanella oneidensis and Deinococcus radiodurans. The LBRE laboratory is located at a depth of 660 m (~1/3rd of mile) inside a 610 m thick salt deposit that is naturally low in naturally occurring radioisotopes and emits significantly less radiation than other rock formations. To further lower ionizing radiation exposure experiments can be conducted in a 15 cm-thick vault made from pre-World War II (and therefore not exposed to nuclear weapons testing artificial radionuclides), low-activity steel.

This pre-World War II, 15 cm thick steel chamber used to incubate the below background treated cells in the WIPP underground. (WIPP)

Castillo and Smith incubated cells inside the vault to achieve lower than background doses of ionizing radiation (WIPP formation + metal shielding) and control cells grown in the presence of 11.5 kg of a potassium-rich salt (KCl) to generate an energy field of gamma radiation close to aboveground background levels. The WIPP facility, local geology and experimental setup with radiation doses experienced by the bacteria are shown in the figure below.

All organisms on earth grow under the influence of a natural and relatively constant dose of ionizing radiation referred to as background radiation, and so cells have different mechanisms to prevent the accumulation of damage caused by its different components.
LBRE at the WIPP. Numbers in red indicate dose rate in nGy hr-1 =  nSv hr-1. (A) The WIPP site, located near Carlsbad, NM, designed for the permanent disposal of artificial radionculide wastes 660 m below ground in the middle of a 610-m-thick Psalt deposit. (B) LBRE pre-WWII steel vault showing the location of the treatment (below-background) and control (background) incubators, with their respective estimated radiation dose. (C) Side view of the LBRE underground laboratory housed in portable laboratories in the WIPP. (D) Comparison of measured and modeled ionizing radiation dose rates.

Cells were grown in the presence of lower than and at natural background radiation doses and their growth and gene expression measured.

What did they find?

The two organisms responded differently to the radiation treatments.  S. oneidensis cultures did not show a significant difference in growth in response to the reduced radiation dose while D. radiodurans growth was inhibited at the beginning of its exponential growth phase and remained significantly lower than the control with normal background radiation levels. When D. radiodurans was taken from lower than normal radiation and returned to normal background ionizing radiation doses its growth returned to normal again.

Lines indicate bacterial growth under radiation-sufficient (background) and radiation-deprived (below background) conditions with p-values indicating whether differences are significant or not above the datapoints for (A) S. oneidensis and (B) D. radiodurans. The dotted line is the reciprocal control where D. radiodurans was placed back in background from below background conditions where growth returned and was identical to the background treatment.

So D. radiodurans was not able to grow as fast in low radiation conditions while S. oneidensis grew equally well at lower than background and background levels of ionizing radiation.  The authors found that gene expression between the two species was significantly different as well. During mid-exponential phase (8 h in S. oneidensis), six genes related to oxidative stress response, DNA repai, protein folding, and a putative efflux pump that pushes metals out of the cells were turned on (blue bars in graph above).  Poor growth under low radiation for D. radiodurans became clear (p < 0.05) at 34 h . The difference in gene expression for D. radiodurans was that genes related to DNA repair and protein folding activities were turned on, while genes necessary for dealing with oxidative stress and energy production were turned off. The regulation of these genes by D. radiodurans was reversed when the cells were returned to normal levels of radiation suggesting that difference was driven by the reduced amount of ionizing radiation they were exposed to in the lower than normal treatment.

What is the explanation and what does this mean?

The authors thought that S. oneidensis responded to the lack of ionizing radiation as an environmental stress and mounted a classic stress-response to the reduction of natural levels of environmental radiation allowing it to grow at its maximum rate. In contrast, D. radiodurans did not sense this stress, did not mount a stress response, and was therefore limited in its ability to grow (was less fit). Specifically, under radiation-reduced conditions, S. oneidensis increased its ability to deal with oxidative damage, repair DNA damage, and repair damaged proteins, which allowed it to continue to grow normally. In the case of D. radiodurans, it did not respond by expressing enough of these critical genes and suffered as a consequence. The authors are continuing their work to test:

  1. why radiation deprivation may increase oxidative stress and levels of reactive oxygen species (hydrogen peroxide, hydroxyl radical and superoxide) inside the cells.
  2. whether or not the ability to sense and respond to the absence of normal levels of radiation is a trait that both prokaryotic (bacteria and archaea) and eukaryotic (e.g. plants and animals) possess.

This study adds to a growing body of scientific literature that suggest that some level of ionizing radiation may be required for cells to appropriately regulate their internal function and be maximally fit.



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