Deep Underground Long Term Evolution Experiments, a key to understand the impact of low doses on living organisms

Deep Underground laboratories are unique environments for exploring the impact of ultralow radioactivity on living organisms. They also provide unique features for running long term controlled low-dose experiments.

Evolution is an on-going process, and it can be studied experimentally in organisms with rapid generations. The E. coli Long-Term Evolution Experiment (LTEE) is an ongoing study in experimental evolution begun by Richard Lenski at the University of California, which has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988 on more than 60.000 generations.

A first evolution experiment conducted at Modane Underground Laboratory with the same E. Coli strain and the same growth medium used by Richard Lensky and collaborators has shown no change in the fitness trajectory over 500 generations when radiative background was reduced by a factor 6 from 150 to 26 nGy/hr. Monte-Carlo simulation of the experimental set-up showed that 40K in the E. Coli culture medium (Davis Medium) was the almost exclusive source of radioactivity to the bacterial strains, representing 99% of the dose received.

Potassium has three naturally occurring isotopes: 39K (93.258%) and 41K (6.730%) are stable, while 40K (0.012%) is radioactive, with a half-life of 1.25 billion years. As 40K in the nutritive medium was the main obstacle to the reduction of the dose received by the bacterial strains during this experiment, depleting 40K in the potassium used to feed the bacteria would reduce significantly the dose received and allow exploring further the ultralow radioactivity frontier. Reciprocally, enriching the potassium in 40K would increase the dose absorbed by the bacteria without changing any other physico-chemical parameters.

We therefore propose to compare the fitness trajectories over 1000 generations of the same E. Coli strain using Davis Medium (DM) nutritive media either enriched or depleted in 40K. Although the isotopic composition of natural Potassium is very stable, potassium enriched in 39K and depleted 10 times in 40K can be purchased from commercial vendors for less than 10 € per milligram.  To enrich natural potassium in 40K, a promising approach is through neutron irradiation.

Repeating the same experiment using DM nutritive media that differ only by the isotopic composition of the potassium allows isolating the sole impact of radiation on the evolutionary path of the bacteria. Increasing 40K isotopic fraction increases proportionally the absorbed dose and radiation induced mutations are expected to modify the strain evolutionary paths when they exceed the spontaneous mutation rate.

These experiments could be performed in several Deep Underground Laboratories to compare the observed fitness trajectories and quantify the reproducibility of the observed evolutionary paths.