By Jay T. Cullen

The purpose of this post is to review how the background dose of ionizing radiation has changed through geologic time until the present. I was motivated to write this by questions and misinformed statements made to me regarding the likelihood that the low levels of ionizing radiation now added to the Pacific Ocean might harm marine microbes and effectively kill the base of the oceanic food chain – given levels being measured this is for all intents and purposes impossible. This post is part of an ongoing series that summarizes the results of scientific research into the impact of the Fukushima Dai-ichi nuclear disaster on the health of the marine environment and residents of the west coast of North America. Life on Earth has been exposed to ionizing radiation since the first organisms began leaving chemical signs of their existence almost 4 billion years ago. In a paper published in 1999 Karam and Leslie calculated how the dose experienced by organisms from naturally radioactive geological and biological materials has changed over time. They find that overall the annual beta and gamma dose experienced by organisms has dropped from about 7 millisievert (mSv = 0.001 Sv) 4 billion years ago to about 1.4 mSv today. Given the similarity of repair mechanisms that organisms use to cope with damage from ionizing radiation it is likely that these mechanisms evolved early in Earth’s history which may explain why organisms are capable of dealing with higher than background doses in the environment today.

Mighty Microbes

Ionizing radiation can affect organisms at the molecular level and induce changes to genetic material (Deoxyribonucleic acid DNA) leading to mutations that can range from harmless to lethal. Generally speaking, microorganisms tend to be more resistant to the harmful effects of ionizing radiation than animals with members of marine photosynthetic algae being well represented (e.g. Singh and Gabani 2011 Journal of Applied Microbiology). Recently, as an extreme example, a eukaryotic algae (link, link) was isolated from the spent fuel pool of a research reactor in France that can resist a dose of ~20,000 Sv or more than 2,000 times the lethal dose to human beings. Basically, these amazing organisms live in distilled water and withstand whithering amounts of ionizing radiation. Given that maximum offshore activities of Fukushima derived isotopes like cesium (¹³⁷Cs half life ~30 yr and ¹³⁴Cs half life ~2 yr) are ~20 Bq m^-3 in seawater (Kumamoto et al. 2014 and diary) dose rates experienced by marine microbes are not likely to approach levels known to induce phytoplankton mortality. Why are these organisms so radiation tolerant? One, but not the only reason, may be that these evolutionarily deeper branching organisms evolved on a young planet where ionizing radiation doses were much more intense than today.

Estimating Ionizing Radiation Doses in the Past

In their 1999 paper Karam and Leslie consider ionizing radiation from primarily two sources – geologic and biologic materials. The dose to organisms on the planet has changed over time due to the evolution of the continental crust, and changes in the ratios of and abundance of radioactive elements like Uranium isotopes 235 and 238 (²³⁵U half life 703.8 x 10⁶ years, ²³⁸U half life 4.47 x 10⁹ years), Thorium isotopes and Potassium-40 (⁴⁰K half life 1.25 x 10⁹ years) over time. To calculate the dose experience by aquatic organisms over time Karam and Leslie made the following assumptions:

1. exposure to aquatic organisms was considered as life on land is a relatively recent phenomenon
2. beta and gamma radiation dose was considered as alpha exposure would be low in an aquatic setting
3. radiation dose from radon and daughters was not considered given that the evolution of lungs and gills occurred only ~400 million years ago
4. the growth of the continental crust, which is enriched in uranium, thorium and potassium, was assumed to be more or less complete by 2.0 billion years before present

Dose From Geologic Materials

The dose rates to organisms owing to radionuclides in the crust and their dissolution in seawater diminishes through geologic time owing to the progressive decay of the long lived isotopes. The figure below shows the change over time of the dose from geologic emitters in milliGray per year which is functionally similar to the Sv.

Dose From Internalized Radionuclides

Karam and Leslie estimated how the progressive decay of the primordial isotope ⁴⁰K would impact internal dose to organisms who maintained intracellular potassium concentrations similar to the average bacteria cell (250 mmol L^-1). Even though ⁴⁰K has a very long half life (~1.25 billion years) life is so ancient that roughly three half lives have passed since the first cell evolved on our planet. The figure below shows how the dose from internal ⁴⁰K has changed over geologic time.

The dose from primordial ⁴⁰K diminishes from about 5.5 to 0.7 mGy year^-1 from the origin of life to the present.

From both sources the beta and gamma dose experienced by aquatic organisms has dropped from about 7.0 on the early Earth to about 1.4 mGy year^-1 today. Organisms that evolved the ionizing radiation damage and DNA repair mechanisms early in Earth history needed to cope with much higher doses of ionizing radiation than they tend to experience today.

Comparing Natural Background to Man-made Ionizing Radiation in the Environment

A more comprehensive accounting of doses from natural and artificial radionuclides in the environment was presented by Thorne in the peer-reviewed Journal of Radiological Protection in 2003. This review found an overall, worldwide dose rate from natural background of about 2.4 mSv year^-1 when alpha, beta and gamma and other forms including cosmogenic radiation were considered. Compared to this natural background dose the exposure owing to man-made radionuclides released to the environment were small. Worldwide average doses from weapons fallout peaked in 1963 at ~0.11 mSv year^-1 (about 5% of natural dose) and have since fallen to 0.006 mSv year^-1 (about 0.2% of natural background). Releases and resulting fallout from Fukushima in the North Pacific and on the North American continent are unlikely to cause biological harm given the very low doses measured in the aftermath of the meltdowns either from atmospheric or ocean transport. Indeed, while radioisotopes from Fukushima were detected in rain and air in the days and weeks following the disaster in Canada their levels were not high enough to cause a measurable change in doses experienced by residents of Canada as evidenced by Health Canada’s fixed point monitoring station data. To quote the Health Canada website:

Measurements from these networks have confirmed, and continue to confirm, that the quantities of radioactive materials that reached Canada as a result of the Fukushima Daiichi nuclear accident were very small and did not pose any health risk to Canadians. The very slight increases in radiation across the country, observed during the first few weeks following the onset of the incident, were smaller than the normal day to day fluctuations from background radiation.

Bold is mine.

We can be confident in concluding that the levels of artificial radionuclides present in our marine environment from Fukushima are very unlikely to have any negative impacts on marine microbes and the base of the food chain. What should concern us are large scale changes in productivity of the North Pacific owing to El Niño conditions and the warm water anomaly that is likely to have effects throughout the ocean ecosystem.