Radiotrophic Organisms: How Microbes Turn Hazard into Food

On a cold morning in 1950, a truck carrying a stack of oil drums reversed onto a jetty. Soldiers dismounted and rolled the drums into the ocean. Despite their efforts, a handful of drums retained some air and refused to sink. The soldiers opened fire on the drums, allowing the seawater to fill the few stubborn drums. Unbeknownst to the bystanders, the drums contained radioactive waste from the nearby nuclear plant. From the period between 1946 and 1993, an estimated two-hundred thousand tons of radioactive waste was dumped into the ocean in a similar manner across the world. The operations went relatively unnoticed by people until decades later when citizens of countries like Russia and the United States discovered empty barrels washing up on beaches. Inquiries revealed the prior contents of the barrels, but the empty barrelss spelled a more alarming implication – the nuclear waste must have diffused into the ocean. As of today, the environmental impacts of these operations remain ambiguous at best, but records of these operations still cause considerable panic among people around the world, especially given the possibility that ocean currents may transport these radioactive contaminants across the world. That salmon sushi on the table may contain trace elements of radioactive waste from a Soviet nuclear plant already decommissioned decades ago.

As people are worried about trace radioactive components in their food, some microbes actually “eat” nuclear reactions as their source of sustenance. The two main kinds of energy source categories of organisms are phototrophs and chemotrophs. Phototrophs are organisms that obtain their energy from sunlight. They use light energy to convert free carbon into organic compounds through a process called photosynthesis. In photosynthesis, light energy from photons is captured by pigments in the leaves and used to synthesize carbohydrates from carbon dioxide and water. Plants, algae, and some bacteria such as cyanobacteria, are examples of phototrophic organisms. Chemotrophs are organisms that obtain their energy by breaking chemical bonds in organic or inorganic compounds. In this category, there are chemoautotrophs that derive energy from inorganic compounds, such as electric bacteria that consume and excrete electrons, and chemoheterotrophs that derive energy from organic compounds. Humans are a type of chemoheterotroph.

In the early twenty-first century, a few researchers at the University of Toronto were curious about microbial life deep underground. They began their investigations with fieldwork. By examining water isolated from beneath the South African and Canadian crust, they found high concentrations of hydrogen. It was impossible for light to reach the water, so this eliminates the possibility of them being phototrophs. The question then arose: if not relying on light, how were these microbes sourcing their energy? Fortunately, previous literature on bacteria energy sources suggests that hydrogen is a common source for bacteria, serving as a rudimentary electron donor, similar to the role NADH plays in human metabolism. The remaining puzzle centered on the origin of these hydrogen molecules. While there was an abundance of water below the surface, the hydrogen supply was limited. However, water offered a potential precursor to hydrogen through processes like electrolysis—where electricity splits water (H2O) into hydrogen (H) and oxygen (O)—and radiolysis, a process where ionizing radiation splits water into hydrogen and oxygen. Further investigations supported the hypothesis that microbial communities in these deep environments sustain themselves through radiolytic reactions of water, utilizing the produced hydrogen as their source of energy. In 2019, they demonstrated that bacteria from deep aquifers utilized hydrogen, acting as electron donors, and sulfate, acting as electron acceptors, produced through radiolysis for their energy, creating a self-sustained system lasting billions of years and is projected to continue for billions of years.

Such life is not dependent on the minor fluctuations in global temperatures, atmospheric composition, or even the presence of sunlight. These bacterias can still survive nevertheless using small nuclear reactions deep underground. These bacteria can persist by simply harnessing small-scale nuclear reactions deep underground. Furthermore, these bacteria offer a potential new model for the origin and the emergence of early life on Earth. Specifically during the transition from the Archean eon to the Proterozoic eon, life dependent on sunlight for basic sustenance faced constant threats of extinction from various natural events like volcanic eruptions and changes in atmospheric composition above ground. In the present, the resilient radiation-eating bacteria underground may be evolving rapidly and undisturbed. The discovery of bacteria that use molecular-scale nuclear reactions as fuel also prompts a reevaluation of the prerequisites for life throughout the observable universe. Light may no longer be a crucial factor; a simple combination of water and slightly radioactive rocks may be sufficient to sustain life deep underground on many planets and moons.

Sources:

“The yield and isotopic composition of radiolytic H2, a potential energy source for the deep subsurface biosphere” Geochimica et Cosmochimica Acta, Volume 69, Issue 4, 2005, Pages 893-903,

Waczewski, James. Legal, Political, and Scientific Response to Ocean Dumping and Sub-Seabed Disposal of Nuclear Waste, 1998.

Photo Credits:

CDC/Dr. Leanor Haley