The bacterium at the end of the Earth

If there’s one thing we can all agree on, it’s that SPAM is definitely not alive. So it must have been surprising when someone yelled, “there’s something alive in my SPAM!” 

To be fair (and accurate), it wasn’t exactly SPAM, but rather a tin of corned beef that had been irradiated with x-rays to prevent microbial spoilage. In 1953, researchers accidentally discovered living in this tin of meat the amazing bacterium Deinococcus radiodurans, the star of today's installment in the ongoing "Life, uh, finds a way" series. 

D. radiodurans– which we’ll call rad for short–has an insanely high resistance to ionizing radiation. It can grow under chronic gamma (γ) radiation of 50 grays (Gy)/hour and can survive doses greater than 10 kGy, which explains why it was able to thrive in that irradiated can of beef. But you may be wondering, should I be impressed? 

Before we address this question, let’s cover some basics about radiation. The unit gray (Gy) describes a dose of radiation: the amount of absorbed energy per unit mass of tissue (1 gray = 1 Joule/kilogram). The type of radiation we are talking about is γ radiation: electromagnetic radiation of very high energy, with wavelengths on the order of 10 picometers, which is a little smaller than the diameter a hydrogen atom. Now let’s put 10 kGy into context. A PET scan delivers a whole-body dose of about 0.01 Gy. Two-hundred times that amount (2.0-3.5 Gy) causes severe nausea and hemorrhaging, and –without medical treatment- has about a 25% chance of killing you. This dose is still 20-times less than that which rad can survive... per hour

Why are γ rays so life destroying to most organisms? γ rays easily penetrate biological materials (like your skin) to produce intracellular reactive oxygen species (ROS). ROS are highly reactive, oxygen-based radicals that destroy proteins and DNA through oxidative reactions. While cells have repair mechanisms to deal with the slow burn of oxidative damage that arises from life on our oxygen-rich planet, the onslaught of chemical degradation that happens with a dose of γ rays is often too much for a cell to handle, resulting in cell death.

Over the years, scientists have pieced together the molecular mechanisms by which rad can survive 10 kGy of radiation. While some researchers probably believed they would stumble upon a fundamentally new strategy for handling DNA damage, the mechanism that rad uses to recover from the chemical destruction of γ rays is actually not so different from the known strategies. It’s just much, much more efficient. While most organisms can repair about ten double-strand DNA breaks per cell, rad can repair hundreds. Rad does this by keeping ROS levels low through the use of multiple anti-oxidant small molecules. Lower ROS means less protein degradation through oxidative reactions. More intact proteins means more enzymes to repair messed up DNA. Those enzymes repair DNA through the same mechanism as in other bacterial cells, via synthesis-dependent strand annealing.

Since the discovery of rad’s ridiculous propensity to survive radiation, researchers have sought to understand the ecological role of this trait. What evolutionary pressure lead to rad’s extreme resistance? Since there's no natural environment that has the kinds of levels of radiation rad can survive (and could thus apply some pressure to evolve this trait), some turned to outer space for ideas. One paper, entitled “Was earth ever infected by martian biota? Clues from radioresistant bacteria” concluded that, while the evolution of highly radioresistant bacteria couldn’t take place on Earth due to low levels of background radiation,

the “process could take place in a subsurface layers of polar regions of Mars. This is the only place of the Solar System, where this process could take place. A few number of discovered high radioresistance terrestrial bacteria can be transferred from Mars on martian meteorites.”

While we love any hypothesis that invokes space-traveling microbes, there is substantial, countervailing evidence that rad are related to terrestrial bacteria and did not originate in the polar regions of Mars.

The most convincing explanation for rad’s radiation resistance came from a paper published in 1996 in the Journal of Bacteriology. Their work grew from the fact that radiation-resistant bacteria can be isolated from natural microbial populations by selecting for resistance to desiccation. When bacteria become extremely dry, double-strand breaks and other DNA mutations are created, which is similar to what happens when cells are exposed to γ rays. To explicitly connect desiccation and radiation resistance, the authors evaluated 41 radiation-sensitive rad mutants, and found that these mutants were significantly less viable than the wild-type strains after desiccation.

But why would rad evolve resistance to extreme dryness? Rad is found in diverse environments all over the world and it gets to all of these exotic locations using the wind. Carried on dust particles, rad reaches great heights up in the stratosphere. Here, it mingles with other dust-borne bacteria and is exposed to short-wave UV light 100-1000 times more intense than at the surface. When it rains or snows, rad becomes rehydrated and falls back to Earth. Since hanging out on dust particles miles above the Earth's surface is a major part of rad's ecology, it follows that it has evolved resistance to desiccation. 

From a chance discovery in some questionable meat, scientists were given the gift of a biological oddity, which has provided fodder for basic research for decades. After reading so much about rad, the thing that strikes me the most is my newfound–though strange– sense of optimism. When humans have destroyed the environment and each other using nuclear weapons, I'm confident that rad will survive. I imagine that, after floating carefree with some dust above the clouds, it will serenely fall back to Earth on radioactive snow to live another day. 

-CAB